Note: Descriptions are shown in the official language in which they were submitted.
13~3~
This inve~tion relates to an apparatus employing a single
antenna to transmit and receive at low loss and without
mutual interference, signals in different frequency bans,
such as mobile telephone signals and radio broadcasting, and
is p~eferably mounted to a car.
Reference is now made to the accompanying drawings, in which:
Fig. 1 is a longitudinal sectional view of a conventional
whip antenna 1 in an extended state;
Fig. 2 is an equivalent circuit diagram in which whip antenna
1 is used for the reception of frequency-modulated broadcast;
Fig. 3 is a equivalent circuit diagram in which whip antenna
1 is used for the reception of amplitude-modulated
broadcasts;
Fig. 4 is a longitudinal sectional view of another
conventional whip antenna 31 in an extended state;
Fig. 5 is a block diagram of a conventional transmission and
reception apparatus;
Fig. 6 is an electric circuit diagram showing the equivalent
of an antenna 53 and a low pass filter 52 of a transmission
and reception apparatus 50;
Fig~ 7~is an equivalent circuit diagram in a frequency band
of AM broadcast in a conventional antenna 61 and a cable 62;
Fig. 8 is an overall schematic of a mobile transmission and
reception apparatus according to the present invention;
. '~
-- 2
13~ 3~
Fig. 9 is a sectional ~lie~ of one embodiment of a multi-band
r~hip antenna according to the present invention as shown in
an extended state;
Fig. 10 is a sectional view taken along line A-A in Fig. 9;
Fig. 11 is a sectional view taken along line B-B in Fig. 9;
Fig. 12 is a sectional view of another embodiment of a multi-
band whip antenna according to the present invention as shown
in an extended state;
Fig. 13 is a sectional view taken along line C-C in Fig. 12;
Fig. 14 is a sectional view of a further embodiment of an
multi-band whip antenna according to the present invention as
shown in an extended state;
Fig. 15 is an electrical circuit diagram of an embodiment of
a branching filter according to the present invention;
Fig. 16 is a graph showing frequency characteristics of a
band inhibiting filter;
Fig. 17 is a schematic of an embodiment of an antenna circuit
according to the present invention;
Fig. 18 is an equivalent circuit diagram of an antenna
circuit for explaining the principle of the present
invention.
Fig. 19 is an equivalent circuit diagram for explaining the
principle under consideration with respect to the capacity of
CF in the equivalent circuit shown in Fig. 18;
~3~L3~
Fig. 20 is a graph showing -the relation bet~een reception
frequenc~ f and output ~oltage leve] V41 in the equivalent
circuit shotin in r ig. 15;
Fig. 21 ls an equivalent circuit diagram in an AM radio
signal frequency band f2a of an antenna circuit; and
Fig. 22 is a schematic drawing of a further embodiment of an
antenna circuit.
Fig. 1 is a sectional view of a typical conventional car-
mount whip antenna 1 in its extended state. This whip
antenna 1 is mounted, for example, near the rear trunk of an
automobile car body 2, and is used commonly of the
transmission and reception of signals for a mobile telephone
and the reception of radio broadcasts. An antenna element 3
of this whip antenna 1 comprises a first antenna element part
lS 4 having a round tubular shape, and a second antenna element
part 5 telescopically disposed within the first antenna
element part 4. The antenna element 3 is accommodated in a
housing tube 6 fitted in a mounting hole 14 formed in the car
body 2. The housing tube 6 is composed of a tubular body 7
made of electric insulating material such as resin, an outer
conductor 8 and an inner conductor 9 made of conductive
materials.
The first antenna element part 4 is composed of a sequential
connection of a first conductor 15, a phase shifting coil 18,
a second conductor 16, a band separating coil 19, and a third
conductor 17. These conductors 15 to 17 and coils 18 and 19
have identical outside diameters. The phase shifting coil 18
functions as a phase shifter on frequency fl of a mobile
telephone, so that the current distribution in reverse phase
may be suppressed low, while the normal phase portion is
emphasized in the current distribution profile. The band
separating coil 19 has a high impedance against frequency fl
- 4
1~3~
of a mobile telephone, and a low impedance against frequency
f2 of a radio broadcast
Therefore, a colinear array antenna is constituted by
conductors 15 and 16 and the phase shifting coil 18, which
may be used for the transmission and reception of mobile
telephone signals. The overall length of the antenna element
3 is used in the reception of radio broadcasts.
A leaf spring 28 is fixed at a lower end part 15a of the
antenna element 3. By this leaf spring 28 the antenna
element 3 is supported so as to be slidable in the axial
direction, while it is electrically connected with the inner
conductor 9. At an upper end part 6a of the housing tube 6,
the outer conductor 8 is fixed to the car body 2 by way of
metallic fixing tubes 21 and 22 and fixing plate 23, and
thereby connected electrically. The connections of the
housing tube 6, fixing tubes 21, 22 and the fixing plate 23
are filled with sealing resin 24, and a nut 25 is screwed
thereover.
Beneath the housing tube 6, a connection hole 26 is formed
near the lower end part 9a of the inner conductor 9. In the
connection hole 26, an inner conductor 12 of a coaxial cable
11 is connected to the inner conductor 9, and an outer
conductor 13 of the coaxial cable 11 is connected to the
outer conductor 8. The coaxial cable 11 is supported by a
cable support member 30 fitted to the outer conductor 8.
This coaxial cable 11 is connected to a branching filter 27,
and this branching filter 27 is connected to the
transmitter/receptor of the mobile telephone and the radio
set by the coaxial cable 29a and 29b.
This whip antenna 1 is erected, for example, near the rear
trunk of the car body 2. Therefore, the are a large number
of restrictions imposed due to the shape of the car body 2,
1 3 ~
such as on the width of the rear fender, and the size of the
mounting hole 14 for mounting the housing tube 6. Besides,
if the outer diameter of the antenna element 3 is reduced too
much in order to resist the wind pressure while traveling,
the tubular body 7 made of electric insulation material
becomes thin, and the spacing between the inner conductor g
and the outer conductor 8 becomes small.
Therefore, as mentioned below, the characteristic impedance
Z2 from the upper end part of the housing tube 6 to the lower
end part 9a of the inner conductor 9, that is, in the section
2 up to the current feed point P is lowered. On the other
hand, if the mobile telephone is used in a state in which the
impedance at the current feed point P is mismatched, the
signal sent out from the he transmitter is reflected, so that
the coil in the transmitter may be burnt.
Therefore, by forming the length of this section ~ 2 at about
15 cm or half of the wavelength ~ 1 of the mobile telephone
the impedance matching is achieved. Therefore, the current
feed point P cannot be set at an arbitrary position. Such
construction of the whip antenna 1 in accordance with the
above-mentioned restriction causes the following problems.
Fig. 2 is a equivalent circuit diagram in which the whip
antenna 1 is used for the reception of frequency modulated
(FM) broadcasts. In this antenna element 3, supposing the
characteristic impedance of the section ~ 1 projecting from
the upper end part 6a of the housing tube 6 to be Z1, and the
characteristic impedance of the section ~ 3 of the coaxial
cable 11 to be Z3, the characteristic impedance Z1 of section
1 is nearly equal to the characteristic impedance Z3 of
section ~ 3, and is, for example, about 50 ohms. Moreover,
the characteristic impedance Z2 of the section ~ 2 is
expressed as follows, assuming the outside diameter of the
inner conductor 9 to be d, the inside diameter of the outer
-- 6
~3~3~
conduc-tor 8 to be D and the specific dielectric constant of
the tubular body 7 to be r r:
Z2 = ~,_ log~ d [.`~ .................. (1)
On the other hand, because of the restrictions imposed by the
S shape of the car body 2 as mentioned above, there is not a
large difference between the outside diameter d of the inner
conductor 9 and the inside diameter of the outer conductor 8,
and therefore as is clear from eg. (1), the characteristic
impedance Z2 in the section ~ 2 is lowered, and the impedance
matching between the section ~ 1 or antenna element 3 and the
section ~ 3 or the coaxial cable 11 is worsened, whereby
transmission loss lncreases. Accordingly, the length of the
section ~ 2 becomes too long to be ignored with respect to
the wavelength ~ 2 of FM broadcast, and the band width is
consequently narrowed.
Fig. 3 is an equivalent circuit diagram in which the whip
antenna 1 is used for the reception of amplitude-modulated
(AM) broadcasts. The length of the antenna element 3 is
formed in accordance with the mobile telephone and FM
broadcast, so that it is extremely short for the wavelength
of AM broadcasts, and the radiation resistance almost becomes
null, and the characteristic impedance Zl becomes
capacitative.
Supposing the capacity of section Q 1 to be Cl, that
~ 3 ~
o~ section R 2 to be C2, and that of section ,f 3 to be C3,
the relation bet~.~een a voltage V1 induced in the antenna
element 3 and a voltage V2 at the powder receiving end
obtained by way of the coaxial cable 11 is shown in the
following equation:
v2 Cl + C2 + C3 vl ......... ....... ,....... , (2)
where the capacitance C1 of section 1 and the capacitance c3
of section 3 are constant, and the power receiving end
voltage V2 may be raised by reducing the capacitance c2 of
section ~ 2. However, the capacitance C2 of section ~2 is,
supposing the specific dielectric constant in a vacuum to be
Eo, expressed as follows
C2 = 2 ~D~/2-Q2 iF]............. ........ ....... (3)
and the ratio of the inside diameter D of the outer conductor
8 to the outside diameter d of the inner conductor 9 cannot
be increased too much as stated above, and therefore the
power receiving end voltage V2 cannot be increased too much.
Fig. 4 is a sectional view of another conventional whip
antenna 31 in an extended state. This long bar-shaped whip
antenna 31 is mounted near the rear trunk of an automobile
car body 32, and is commonly used for the reception of ratio
broadcasts and the transmission and reception of mobile
telephone signals. An antenna element 33 of this whip
antenna 31 is composed in a sequential connection of a firs
conductor 34, a phase shifting coil 38, a second conductor
3S, a band separating
~3~3~$~
coil 39 ~ ri conci~_t~r 36, and a ~o~ conduct~r 37.
The fi-st conductor 3! and the second co)ld~ctor 35 ila;e
-i~ a round cylindricalshape, and the third conductor 36 is
formed like a cap.
~ 1ithin a space 43 formed by the ~irst conductor 34,
the phase shifting coil 38, the second conductor 35 and the
band separating coil 39, the fourth conductor 3/ is accom~
modated. The outside diametersof the first to third con-
ductors 34 to 36, and coils 38 and 39 are identical,
and are kept in a housing tube 40 provided in the car body
32.
The housing tube 40 is composed of an electric
insulating tube body 40a, an outer conductor 4Cb, and an
inner conductor 40c. An outer conductor 44a of a coaxlal
cable 44 is connected to the outer conductor 40b, and a
inner eonductor 44b of the coaxial eable 44 is conneeted to
the inner conduetor 40c.
At the high frequeney fl of a mobile telephone or
the like, the phase shifting eoil 38 functions as a phase
shifter, and the normal phase portion is emphasi~ed by
suppressing the current distribution in the reverse phase,
while the band separating coil 39 has a high impedance,
thereby forming a colinear array antenna by the first
conductor 34, the phase shifting coil 38, and the second
cndUctr 35 to be used for the transmission and reception of
.L~ r~ f ~L ~t ~r t}l~
r~ti~ L ,~ - lv~ p~la~ nd
'.il `; .' S' '.~ vll~''UCt')L_. .,; ':' 3 / 1 ._ :,~iia lG ~1-'8 '?
~L-~ arlt~lln~ ~or i~ L~ ttl-~ r~
l c ;~
~ ,irlc~ trl~ ?ortiorls of COl ' S 3G dll'i 3? ~ llOit low
strellgth, -he~ are li~:ely to be brok~n, a~ tne~ are
reinEorce~l b~ molding resins 41 and 42 .hereto. The resin
portions 41 and 42 have the same outside diameters as those
of first to third conductors 3-~ to 36 so as not to form an
oostruction ~whell the antenna eJement 33 is put into the
housing tube 40.
In the thus composed whip antenna 31, the resin
portions 41 and 42 are bulged out, inward in the raaial
direction of coils 38 and 39, in order to obtain a desired
strength. Therefore, such bulaing would interfere with the
displacement of the fourth conductor 37 into the space 43,
and it is difficult to provide resin portions 41, 42 with a
thickness sufficient to obtain a desired strength. Besides,
after the coils 38 and 39 are once molded with resins 41 and
42, it is difficult to adjust the length of the coils 38 and
39. Furthermore, since the first to third conductors 34 to
36 are metallic, thus being of material different from thc
resin 41 and 42, the antenna is deemed to be unaesthetic.
Fig. 5 is a block diagram of a conven~ional
transmission/reception apparatus 50 tor a mobile telephone.
--10--
1 3 ~L tJ .~ ~J ~
'.;r)~dca-ats i, or1er to z.,lar~ ~he ~: .:.'.~!iil-^. in ~IllS '~Ja';, ~h"
.l'Ji~ t ::tlr` !nooil~ t~ n~ a _ ~n!lr~r`~ 'ti~l th~
.-jL-lllli lina 5. ~he radio set. Ther-: e, wilen a radio
broadcast ~ L-ceived ;rhile using ~ne mobile tele~hone, the
,o-called b-at noise is mi~:e-d in the sound reproduced b~f the
radio set. To prevent the generation o~ such beat noise,
the elelnellts sho~/n in Fig. 5 have been us-d r.itherto.
The frequency band f2 of radio broadcasts is, in ~il
broadcasts, frequency band E2a, that is, 500 to 1620 kHz,
and, in F~t broadcasts, frequency band f2b, tAat is, 76 to 90
~IHz. In the mobile telephone, on the other hand, for radio
communication ~./ith the ground station connected with the
telephone line, a frequenc1f band fla of 870 to 890 ~Hz is
used in receiving, and a frequenc,~ band Elb of 920 to 9~0
~1~z is used in sending. The prior art sho~ln in Fig. ~ makes
use of such a diEference in frequency band.
In other words, a radio set Sl is connected to an
antenna 53 by way of a low pass filter ~2, and the mooile
telephone 5~ is connected to the antenna 53 by way of a high
pass filter 5;. The signal line connected to the mobile
teLephone ~ is joined to the signal line connected to the
131~
radio set ~ cas~ radio cOn~unicatialls b,s the mobile
telephor~e 5~!, since thc freauenc-~ band ~l of the ai~,~.als .~ns.~i--e,.
or receive~l bv ~he mobile telephone 54 is relativel~ hignj
the ra~io set 51 ~ill not generate beat noise hy the 1nterfer-
ence wlth the signal ln the frequency band ~2 used in the
mobile telephone 54 o~lng to the low ?ass -il er 52A
The equivalent circuit of the antenna 53 and the
typical circuit composition of the low pass filter 52 are
shown in Fig. 6. A capacitor Cll is connected in series to
a signal source 56, and coils Lll and Ll2 are connected in
series to this capacitor Cll. The contact point 57 of coils
Lll and Ll2 is grounded by way of another capacitor Cl2.
The relation between voltage Vll generated in
signal source 56 and output voltage Vl2 of the low pass
filter 52 due to electrostatic capacity of capacitors Cll
and Cl7 is as follows:
V12 ~ Cll + cl2'Vll ......... ,,..... ,,,,,
That is, in the low pass filter 52, since the capacitor C12
is provided between the signal line and the ground, the
output voltage V12 of the low pass filter 52 unfavorably
becomes smaller than the generated voltage Vll in the signal
source 56. In e~. 4, since it is supposed that radio
broadcasts are to be received the attenuation of signals by coils
Lll, Ll2 is
-12-
~3.~v~
ass~me~i to be sllfficio~tl~ small.
~ is. 7 is all equiialent circuit ~ii.gram in the
frequenci baaa t2a of A;i broad_ast of an aatenna 61 an~ a
cahle 62 in a different prior device.
In a car-mountedradio set, it will ~e very convenient
if FM radio signa~, ~1 radiosignals, and mobile telephone
signa~ can be r^ceived by one antenna. In a construction in
which the antenna is expanded or contracted by a motor or the
like, a signal cable cannot be attached to the lower end of
the antenna, and it is difficult to shorten the signal cable.
Accordingly, the cable capacity of the signal cable increases,
and the impedance derived from the cable capacity becomes
high. In parti^ular, in radio signals of a relatively low
frequency band such as AM radio signa~, the effect of cable
capacity bscomes larger. Therefore, in a car-mounted antenna,
signals in a wide frequency band must be sent out to the
radio set while suppressing the loss Dy the signal cable.
The antenna 61 can be represented by
antenna effective capacity Ce and antenna reactive
capacity Ca, and the ~M radiosignals receive~ by this antenna
61can be represented by an alternating-current power source V21.
The cable 62 can be shown as a line ~ll between
terminals Al and Bl, and this line Qll is groundsd by way
of cable capacity Cb. The signal at the terminal Bl is fed
into a radio set. The voltage V22 at this terminal Bl is
-13-
1 3 ~
e~?-essed as follo/s
~ + ~a + Cb ~21 ........................ .
As e~:pressed in eq. 5, supposing that the cable capacity Cb
is large, the gain of the AM radio signal of relatively low
frequency received by the ant2nna 61 is ~owered so
that the cable capacity Cb makes the receiving sensitivity
and the ratio of signal to noise (S/N ratio) drop.
To prevent such a drop in receiving sensitivity and
S/N ratio, an amplifier is placed between the antenna 61 and
the cable 62, that is, at the position of terminal Al, so
that the receiving sensitivity and S/N ratio are improved.
In such an antenna,
since active e].ements are used, they give rise to an increase
of cost, and also involve other problems such as maintaining a
circuit characteristic of suppressing only the distortion
of signa~ at the time of input of a strong electric field.
In addition, new problems may be also experienced, such as
loss due to impedance conversion in the amplifier, and
insufficient matching of impedance.
3 ~
~ A~ t ~1r,l~,ar, o~ ct ~ t:t ~ 5.~ _5
:prz.,zn-: a r~ , im?ro;ed ~ransm ssi,n a?.~ '-'Ce?i- 5n
~:~?r~ra-Us GL- ~tt~m30ilcs ',lhl-i-l sol ;cs ' ''i'' -~3~;z-~ii.,~,lls-iz~
prsoicms.
J t is a~lother ob~ec~ o~ this in~;en~3n . 3 ?reseQt a
multi-band whl? antenna havinG relati;zl, low .-ansmis.sion
Loss, capable cf matching ~hz impec~ancc fa~-orabLy, while
COnfOrmillq t3 restrictions im?osed bv the car bod-~ sha?e.
To achieve the above objects, in an multi-band whi?
antenna of the present invention, having a housing tube
which is connected and fi~ed to a car body of an automobile,
an antenna element which is disposed in the housing
tube, is electrically insulated from the housinq tu~e, and
c~n be e:ctended and retracted like a telescope upward from
the housing tube, and
a cable which is electrically connected to the lower
end part of the antenna element, in a state where the
antenna element is drawn upward and e~tended from the
housing tube, the improvement comprising:
a lower end part in the housing tube has a first lower
end part which is smaller in diameter than the ?ortion of
the antenna that extends above the housing tube when the
antenna element is ectended, and a second lower end part
which is larger in diameter than the
J i ~
c~r ~ r ~ ~ _ r r~
se ~ r~ iCi. i J ~_r~tl ! -~ 'i,,~cent _ 'r~ r~ ~' ~~ ~ ~
io:e- end -ar~ -~~ i5 lar~er .. diame~e~: -han ~ rr--
lo/er eni ?a-- Ti.e housing tu~e nas a irst oll~er - -e
part sur-ound~l.- the firs~ lcler en~i par~ b~,/a; 5, an
elecL-ic insulaclcn tube bod~, and a second outer tu3e part
. .
surrounding the second lower end part b~ ~./ay o tr.e elect-lc
insuia~ion tube bod-y, the second Guter tube part being
disposed adjacent the first ou~er tube ?art and naving ,
larger inside diameter than the first outer tube part.
The outside diameter of the first lower end part and
insi~e diameter of the first outer tube part, and the
outside diameter of the second lot~er end part and inside
diameter of the second outer tube part are selected so that
the characteristic impedance due to the first io~ler end part
and first outer tube part, the characteristic impedance due
to the second lot~er end part and second outer tube part, and
the characteristic impedance due to the antenna elemen~ and
cable may be nearl~/ equal to each other.
Thus, according to this invention, if the antenna
element is used for the transmission and reception of mobile
telsphcne signals and for the reception of FM broadcasting,
the impedance matching of antenna element and cable may be
achieved
- 16 -
' D ~ ` O '1, i !' .'~ u o r~ Eirs- ~o;~r ~.~
~ar- ~;n -n , maller ln diamecer th_n C,`'' D5r-'On 0.' ''.^"
-~nten.;a -`ie'-le!l_ ?r_lec~in~ ;-om cn-- ;.olsill~ c~ke, and ~
se~onc iowe: ''!lC ?art ~nicn i, directi, adiacen_ c~e f -s.
iower ena ~ar~ -nd is larger ~n clamerer .han ~he r,e
lower end part. Tne housing tube has a .irst outer .~be
oart surrounding the f irst losler end ~art by way of an
elec rlc insula~isn .ube body, -snd a second outer ~ke ~art
surrounain~ the second losfer end part oy way of the electrlc
insulation tube body, the second outer tube parl oeinc
disposed adjacent the first outer tube part and navina a
larger inside diameter tnan the first outer tube part.
The outside diameter of the first lower end part and
insiae diameter of the Eirst outer tube part, and the
outside diameter of the second lower end part and inside
diameter of the second outer tube part are selected so that
the characteristic impedance due to the first iower end part
and first outer tube part, the characceristic impedance due
to the second lower end part and second outer tube part, and
the charac_eristic impedance due to the antenna element and
cable may be nearly equal to each other.
Thus, according to this invention, if the antenna
element is used or the transmission and reception of mobile
telephone signals and for the reception of F~l broadcasting,
the impedance matching of antenna element and cable may be
achieved
-17-
r.~.inlsa ~ ?~;lcn-~?. ~.-, '3r
~::am?l?, .~ S ~n~?.~.~a ei~?ln~nr ~ ,e~ E~ ?~e?tl3n
'Od'i-ls-i, Cne capac t-,- oi~ .,s;e -3-- -r. ma~ ee
- ?dl!C'?'i, ~O ~_~d~ the Jol~aqe -~t i-.- ? ? ' -?C:''l' _oie- -e5e~iln~
end ma~ i~e ~ sed. ;lore3:er, ~he an~?n.n,~ c~n _c~or~oda~e
for restricsions imposed thereon due .o ~he c;r bod~ shape.
In a prererred embodiment, an lnsertion hole pi~ces the
first and second lower end parts i; communication, and d
wire for driving the antenna elemen[ ~s set in .his
insertion hole.
In ano~her preferred embodiment, a brush touches a
contac. piece connected to the cable and instailed in the
housing tube when the antenna element is extended, and
supports the antenna element in the second lower end part
while sliding on the inner wall of the housing tube durinq
the extenslon and retraction of the antenna element.
In a different preferred embodiment, the first lower
end part is covered at the outer circumference thereof with
electric insulation material so as to be nearl~y equal to the
inside diameter of the housing tube.
In other preferred embodiment, the upper end part or
the housing tube is arranged to be level with or lower than
the lower end part of the antenna element when the antenna
element is in the extended state.
-18-
~L 3 1 ~
Tn another ~/L~-er~ed embodiment, t~se housinc3 tube
comprises a tubular il~l r conductor electricalll connected
to the lower end part of the antenna elemeni-, and a tubular
outer _onductor accommodating this inner conductor by way of
a space defined therebetween.
According to this invention, the housing tube for
accommodating the antenna element comprises the tubular inner
conductor and outer conductor, and the antenna element is
stored in the inner conductor. The antenna element is
electrically connected with the cable by way of this inner
conductor. The outside diameter of the inner conductor and
the inside diameter of the outer conductor are selected so
that the charac'eristic impedance due to the transmission
line of the inner conductor and outer conductor, and the_har-
acteristic impedance due to the antenna element and cable
may be nearly equal to each other.
Thus, according to this invention, since the space
between the inner conductor and outer conductor has a small
specific inductive capacity ~r, the characteristic impedance
of the transmission line of the inner conductor and outer con-
ductor, and the characteristic impedance of the antenna element
and cable may be equalized, so that impedance matching may
be effected favorably. Besides, it is not necessary to
increase the outside diameter of the outer conductor too
much, and the antenna may accomodate for restrictions of the car
body shape.
-19-
~ 3 1 ~
In a certain preferred embodiment, the outer conductor is
fitted to the car bod~, and an electric insulating member is
disposed in the space so as to support the inner conductor.
It is a further object of this invention to present a multi-
band whip antenna exhibiting sufficient strength and an
aesthetic appearance.
According to one aspect of this invention, there is provided
a multi-band whip an antenna element including a first
antenna element part having a tubular conductor and a coil
for operating electrically connecting the tubular conductor
in the antenna, and a second antenna element part
telescopically extendable in the first antenna element part;
and a covering tube made of an electric insulation material
for covering the first antenna element part along its axial
direction.
The antenna element of this invention comprises a first
antenna element part having a tubular conductor and a coil
for operating electrically connecting this conductor in the
antenna in the axial direction to be mounted on the car body,
and a second antenna first antenna element part. The first
antenna element part is covered with a covering tube made of
an electric insulation material along is axial direction.
Thus, according to this invention the first antenna element
part having the coil exhibiting a small amount of strength
- 20 -
3 ~
is reinforced by the covering tube. Risk of breakage thereof
may be eliminate~, and ~eflection or deformation hardly
occurs, so that stable transmission and reception may be
realized. F'urther, the first antenna element part is covered
with a homogeneous covering tube, and has an aesthetic
appearance.
In a further preferred embodiment, the antenna element
comprises the first antenna element part extending from the
lower end part and the second antenna element part which can
be stowed in this antenna element part, the first antenna
element part, the first antenna element part having a plural
tubular parts composed telescopically.
In another preferred embodiment, the first antenna element
part is composed of two tubular parts which are
telescopically extendable and retractable.
In a different preferred embodiment, an end of the wire is
fixed at the lower end part of the second antenna element
part, and another end of this wire is wound on a take-up
shaft of a motor. The motor may be driven to extend and
retract the antenna element telescopically.
It is other object of this invention to provide branching
filter capable of suppressing the mutual interference of
signals between plural communication means using different
frequency bands.
According to another aspect of this invention there is
provided a branching filter comprising:
- 21 -
~ 3 ~
a first communication means for transmltting at least in a
~irst fre~uency band f 1; a second communication m~ans for
receiving at least in a second frequency band f2 which is
different from the first frequency band fl; and a band
inhibiting means possessing an electrostatic capacity which
has a larger impedance in the first frequency band fl and is
connected in series to the signal line of the second
communication means.
The branchlng filter of this invention has the signal line
from the commullication means facilitating the transmission or
reception of signal at least in the first or second frequency
band fl, f2 connected to a common antenna.
The signal line of the second communication means is provided
with band inhibiting means having an electrostatic capacity
in series with the signal line and having large impedance in
the first frequency band fl. Therefore, electrostatic
capacity does not occur between the signal line of the second
communication means and the ground, and the signal level will
not be reduced by the band inhibiting means. Besides, the
signal in the first frequency band fl at least transmitted by
the first communication means is inhibited by the band
inhibiting means, so that there is no adverse effect on the
reception signals b~ the second communication means.
~3~
Thus, according to this invention, the effect of the
transmiss.ion signal or the first communication means on the
reception signal of the second communication means can be
suppressed without lowering the level of reception by the
second communication means, and mutual interference bet~,leen
the transmlssion and reception signals of the antenna
commonly used in different frequency bands fl, f2 can be
suppressed.
In a further different preferred embodiment, the band
inhibitlng means is a parallel resonance circuit connected to
the signal line, and its resonance frequency is selected in
the first frequency band fl.
` r~
In another preferred embodiment, the first communication
means transmits and receives signals for a mobile telephone,
while the second communication means is a radio set for
receiving signals in the frequency band f2 lower than the
frequency band fl of the first communication means, and the
band inhibiting means is designed to inhibit signal within
the transmission and reception frequency band fl of the firsL
communication means.
In a further different preferred embodiment, the band
inhibiting means is a connection of parallel resonance
circuits for resonating in the reception frequency band fla
and the transmission frequency band flb of the first
communication means.
- 24 -
13~ 3~9~
In another preferred embodiment, a bypass filter for allowiny
signals in the first frequency band fl to pass and blocking
signals in the second frequency band f2 is provided in the
signal llne connecting the flrt communicating means and the
antenna.
It is an advantage of this invention that it provides an
antenna circult capable of enhancing the reception
sensitivity and S/N ratio in a wide frequency band.
According to another aspect of this invention there is
provided an antenna circuit which is provided between the
antenna and an antenna input circuit of a radio set for
receiving a first radio signal in a first frequency band f2a
and a second radio signal in a second frequency band f2b
which is a higher frequency band than the first frequency
band f2a, the improvement comprises:
a signal cable;
a first impedance conversion circuit connected between the
signal cable and the antenna for converting the impedance in
the first frequency band f2a from high impedance to low
impedance;
a first filter circuit connected between the signal cable and
the antenna for allowing signals in the second frequency band
f2b to pass;
a second impedance conversion circuit connected between the
signal cable and the antenna input circuit for converting the
impedance in the first frequency band f2a
- 26 -
~ 3 ~
from low i~pedance to high impedance; and
a second filter circuit connected between the signal cable
and the antenna input circuit for allowing signal in the
second frequency band f2b. Between the antenna and the
signal cable may be disposed means for adjusting the
impedance, said means being composed of a first filter
circuit for allowing the first radio signals in the first
frequency band f2a, and a first impedance conversion circuit
for converting the impedance in the second frequency band f2b
from high impedance to low impedance. And between the signal
cable and the antenna input circuit of the radio set is
disposed means for adjusting the impedance, said means being
composed of a second filter circuit for allowing the second
radio signals in the second frequency band f2b, to and a
second impendance conversion circuit for converting the
impedance in the first frequency band from low impedance to
high impedance.
The second radio signals are sent out to the radio from the
antenna by way of the first filter circuit, while the first
radio signals are converted with respect to impedance by the
first impedance conversion circuit. Thus, loss due to cable
capacity in the signal cable is reduced, and the signal is
transmitted to the radio set. The second radio signals are
then transmitted to the antenna input circuit of the radio
set through the second filter circuit, while the first radio
~3~
signals are converted into an impedance matched ~Jith the
antenna input circuit of the radio set by the second
impedance conversion circuit, and are transmitted to the
antenna input circuit of the radio set. Therefore, radio
signals over a wide frequency band can be transmitted to the
radio set without increasing loss in the antenna and signal
cable.
In this way, according to this invention, when radio signals
are received by the antenna, the loss of reception signal due
to capacitative impedance of the signals cable may be
reduced. Therefore, the reception sensitivity and S/N ratio
in a wide frequency band can be outstandingly enhanced.
In a preferred embodiment, the first and second filter
circuits are series circuits of a coil and a capacitor.
In a different preferred embodiment, the first and second
impedance conversion circuits are transformers .
In a still further preferred embodiment, at least one of the
primary and secondary windings of the transformer is
connected in series with a coil for reducing the loss due to
the stray capacity of the transformer.
- 2~ -
Preferred embodiments of this invention are described in
detail below.
Fig. 8 is over all schematic drawing of a mobile transmission
and reception apparatus 101 according to the present
invention.
On an automobile car body 102 is erected a multiband whip
antenna 103 which is used commonly in transmission and
reception of signals for a mobile telephone and for the
reception of radio broadcasts. This antenna 103 is
telescopically driven by a motor 104 installed at its lower
end part. The antenna 103 is connected to a branching filter
106 by way of a coaxial cable 105, and signals for the mobile
telephone transmitter/receiver 108 by way of coaxial cable
107 while the reception signals of a radio broadcast are
transmitted to a radio set 111 by a coaxial cable 109 through
an antenna circuit 110.
Fig. 9 is a sectional view of one embodiment, a multi-band
whip antenna of one embodiment of this invention in an
extended state. Fig. 10 is a sectional view along cut
section line A-A in Fig. 9. Fig. 11 is a sectional view
along line B-B in Fig. 9. This antenna 103 is set up, for
example, near the rear trunk of the automobile car body 102.
An antenna element 123 of this
- 29 -
3 4 ~
antenna 103 is composed of a first antenna element part
(hereinafter called first part) 124 of a roun~ tubular fro~,
and a second antenna element part (second part) 125 telescop-
ically formed within the first part 124. This antenna
element 123, in a contracted state, is stored in a housing
tube 126 disposed on the car body 102.
The first part 124 is composed in a sequential
connection of a first conductor 145, a phase shifting coil
148, a second conductor 146, a band separating coil 149, and
a third conductor 147. These conductors 145 to 147 and coils
148 and 149 are formed in an identical outside diameter.
The outer circumference of thus formed tubular first part
124 is covered with a covering tube 171, while a tube body
172 is inserted into the inner circumference, so that the
first part 124 is reinforced, thereby preventing deflection
or deformation of the coils 148 and 149. The covering tube
171 and the tube body 172 are made of electric insulating
synthetic resin such as glass fiber, which will not affect
the transmission and reception characteristics of the antenna
103.
As shown in Fig. 9, in the extended state of the
antenna element 123, a lower end part 120 of the first con-
ductor 145 positioned in the housing tube 126 is composed of
a first lower end part 120a formed in a round tubular shape
in a smaller diameter than the first part 124, and a second
- 30 -
~ 3 ~ "J ~ ~J ~
lower end part 120b formed like a cap in a ~arger diameter
than the fi~st lower end part 120a, being consecutive beneath
with the first lower end part 120a. The outer circumference
of the first lower end part 120a is molded by a resin 135 so
as to be identical with the outside diameter of the first
part 124. As a result, the antenna element 123 can be
expanded and contracted smoothly. On the outer circumference
of the second lower end part 120b, a brush 134 is mounted in
order to support the antenna element 123 and slide on a
contact piece 130 which is described later.
The housing tube 126 is composed of an inner tube
127 made of electric insulation material, for edample, resin,
and an outer tube 128 made of conductive material. The outer
tube 128 comprises a first outer tube part 128a corresponding
to the first lower end part 120a, and a second outer tube
part 128b corresponding to the second lower end part 120b.
In the extended state of antenna element 123, a
connection hole 129 is formed, communicating through the
second outer tube part 128b and inner tube 127 toward the
lower end part 120b, and the contact piece 130 contacting
with the second lower end part 120b is fixed in this con-
nection hole 129. To the contact piece 130, an inner
conductor 132 of the coaxial cable 105 is connected, and
the antenna element 123 and the inner conductor 132 are
electrically connected. An outer conductor 133 of the
~L3 ~.3~
coaxial cable 105 is connected to the outer tube 128 of the
housins t~be 126, ~nd this outer tube 128 is electrically
connected with the car body 102 as mentioned below, and the
outer conductor 133 is connected thus to the car body 102.
The vicinity of the current feed poit P where the contact
piece 130 is disposed is reinforced by resin 136.
At the upper end part of-the outer tube 128 of the
housing tube 126, a step 137 is formed, and external threads
138 are formed upward from this step137. At the upper end
part of the housing tube 127 where external threads 138 are
formed, a connecting member 140 with a metallic ring 139 is
inserted. The upper end part of the housing tube 126 where
the connecting member 140 is thus inserted is inserted in
a mounting hole 141 formed in the car body 102, and is
projected from the surface of the car body 102. In the
projected part from the surface of the car body 102 of the
housing tube 126, a resin-made seat 142 is fitted, and a nut
143 is set. The end part of the car body 102 side of the
connecting member 140 is formed in a sawtooth shape, and
therefore the outer tube 128 is electrically connected with
the car body 102, and the outer conductor 133 of the coaxial
cable 105 is grounded, while the housing tube 126 is securely
fitted to the car body 102.
Flanges 173 and 174 are formed at both ends of the
second part 125 of the antenna element 123, so that the
- 32 -
~?~
second part 125 ls prevented from slipp1ng out of the firs.
part 124 or falllng into the first part 124. At the flange
173 at the lower end part side of the second par, 125, one
end of a flexible wire 175 telescopically driven by the motor
104 is fixed. The other end of this wire 175 is wound on a
take-up reel or the like mounted on the output shaft of the
motor 104 which is passing through an insertion hole 176 and
formed on the inner circumference of the tubular first lower
end part 120a and the cap-shaped second lower end part 120b,
so that the antenna element 12~ is extended or contracted by
the driving of the motor 104 in the normal or reverse
direction, and may be stored in the housing tube 126.
The signal transmitted nad received by thus
composed antenna element 123 is led into the branching filter
106 from the coaxial cable 105, and the frequency band is
separated. The separated signal is led into the transmitter/
receiver 108 of the mobile telephone through the coaxial
cable 107, and is also led into the receiver 111 of radio
broadcast from the coaxial cable 109 through the antenna
cicuit 110.
In the antenna element 123, supposing the wavelength
of mobile telephone to be ~1, the first conductor 145 is
formed in a length of 3 x ~1/8 (approx. 11 cm), while the
developing length of the phase shifting coil 148 is formed
in ~1/4 (about 9 cm), and the second conductor 146 is formed
1 3 3~
in 5 x ~1/8 (about 20 cm). Thus, a colinear antenna array is
composed by first, ~econ~ conducto~s 145 and 146, and the
phase shifting coil 148.
The overall length in the state of developing the
phase shifting coil 148 of this colinear array antenna is
about 40 cm, and in other words it is seclected at 5/4 times
the wavelength Al in the frequency band 860 to 940 MHz of
mobile telephone in Japan. The phase shifting coil 148
functions as a phase shifter for the wavelength of ~1, and
suppresses the current distribution in the reverse phase at
a low level, so that a current distribution possessing an
amplitude largely emphasized in the normal phase portion is
obtained. The band separating coil 149 has a high impedance
against the short wavelength ~1 for mobile telephone, and a
low impedance for a long wavelength ~2 for radio broadcast-
ing. Thus, transmission and reception of mobile telephone
can be effected by using a colinear array antenna.
The winding length of the phase shifting coil 148
is about 4 cm, and therefore the overall length of the
colinear array antenna is about 35 cm. The length from the
lower end part of the band separating coil 149 to the upper
end part of the second part 125 is selected at about 38 cm,
and therefore the overall length of this antenna element
123 is about 73 cm, and in order words it is selected at a
length of 1/4 of the wavelength ~2 in the frequency band 76
- 34 -
to 90 MHz of FM broadcasting in Japan. Thus, at a relatively
long wavelength ~2 of radio broadcasting, the radio broadcast
is received by using the overall length of the antenna element
123.
In this whip antenna 103, supposing the section of
the projected portion from the upper end part of the housing
tube 126 of the antenna element 123 to be Q31, the section
from the upper end part of the housing tube 126 to the current
feed point P to be Q32, and the section of the coaxial cable
to be Q33, the outside diameter dl of the first lower end
part 120a of ther lower end part 120 may be set sufficiently
smaller than the inside diameter Dl of the first outer tube
part 128a of the outer tube 128. Besides, corresponding to
the outside diameter dla of the second lower end part 120b
sliding on the contact piece 130, the inside diameter Dlb of
the second outer tube part 128b may be formed largely. Thus,
from eq. 1, the characteristic impedance Z2 in the section Q32
may be increased.
Therefore, when transmitting or receiving mobile
telephone and receiving FM broadcast, from eq. 1, a favorable
impedance matching may be obtained by properly selecting the
ratio of the inside diameters Dl and Dla of the outer tube
parts 128a and 128b to the outside diameters dl and dla of
the lower end parts 120a and 120~, so that the characteristic
impedance Z2 in the section Q32 may be nearly equal to the
- 35 -
1 3 ~
characteristic impedance Zl and z3 in the sections ~31 and ~33.
As a result, t~e transmission loss ma~ be reduced, and the
reception frequency band may be prevented being narrowed.
Besides, when receiving AM broadcast, as stated
above, since the ratio of the inside diameters Dl and Dla of
the outer tube parts 128a and 128b to the outside diameters
dl and dla of the lower end parts 120a and 120b may be set
larger, the capacity C2 in the section Q32 may be reduced
as indicated in eq. 3 and eq. 2, so that the power receiving
end voltage V2 may be increased.
Furthermore, the outside diameter of the first
outer tube part 128a of the outer tube 128 will not enlarged,
and the current feed point P may be set at an arbitrary posi-
tion, so that it may be suited to restrictions by the shape
of the car body 102, and to any model of the autmobile.
In addition, since the first part 124 of the antenna
element l23 is reinforced by the covering tube 171 and the tube
body 172, breakage of the antenna element 123 may be prevented,
while deflection or deformation may be also avoided, so that
stable transmission and reception may be realized.
Moreover, a favorable appearance is attained by
covering the first part 124 comprising the coils 148 and 149
with a covering tube 171 made of a homogeneous material, and
it can be smoothly put into the housing tube 126. By mounting
by inserting the tube body 172, the second part 125 may be
36
~L 3 ~ s ~ v ~
; lo c n l~r . . c . i~ 5 ~ ! ^,; ~ r i r. ~J
-~ ! J !.; ~ a ~ d~ * ~ ~ i l; .
~ iiL lur~:~er~ b~i Lorinir~:l ~ne if:l~.r elld Dar~ Oa dS a
r~ '.~'{~ or dnd Corliling dn ins~-L~ OlQ i/'~
second lcwer end ~-~aL- i20b, rain~a~er oeno~ra.lng ,âS~ ~he
~ir,t part 12~ may be discharqed, and the im~edance matchinq
may be furthcr enhanced.
Fig. 12 ia a sectional Vie~J of allother embodiment of a
mu1ti-band ~hip dntellna 201 according .o the present
in~ention as shown in an extended sta~e, and rig. ;3 is a
sectional vie~,~ ta~:en along line c-c in ~ig. 12. ~his
embodiment is similar to the foregoing embodiment, and the
corresponding parts are identified ~ h same reference
numbers.
In this embodiment, a nousing tube 202 ccmprises an
inner conductor 203 having a round c~lindrical shape, an
outer conductor 20~ having a round cylindrical shape with a
larger inside didmeter D2 than an outside diame~er d2 of the
inner conductor '03, and support members 205 and 206 made of
electric insulation material and interposed bet~een the
conductors 203 and 204 at both ends of the inner conductor
203.
A brush 134 fitted to a lo~ler end part 120 of the
antenna element 123 slides on the inner circumference of the
inner conductor 203. And an inner conductor 132 of a
~L 3 ~
coaxial cable 105 is connected at a current feed poirt P on
an outer circumference. At the current feed pont P, a c~n-
necting hole 129 is formed in the outer condcutor 20~, and
in this connecting hole 129, an outer conductor 133 of the
coaxial cable 105 is connected to the outer conductor 204.
Thus, in the housing tube 202, by forming a space
207 ketween the inner conductor 203 and outer conductor 204,
the specific dielectric constant Er in eq. 1 may be reduced
to the value of air, that is, nearly 1.0, and the character-
istic impedance Z2 in a section Q41 can be increased while a
capacity C2 can be reduced without enlarging the outside
diameter of the housing tube 202, so that the same effects
as in the foregoing embodiment may be obtained.
Fig. 14 is a sectional view of a multi-band whip
antenna 301 in a further different embodiment of this
invention in an extended state, and this embodiment is
similar to the foregoing embodiments, and the corresponding
parts areidentified with same reference numbers. In this
embodiment, an antenna element 302 is composed in three
stayes, and a second conductor 146 between a phase shifting
coil 148 and a band separting coil 149 is divided into a
lower side part 146a and an upper side 146b. The outer
circumferences of the conductors 145 to 147 and coils 148
and 149 are covered with a covering tube 171 and 172.
By thus dividing into three stages, the size of
- 38 -
~ 3~J~i~
the antenna element 302 ln the contracted state can be reduced,
and the length of the housing tube 202 may be shortened.
Fig. 15 is an electric circuit diagram OI a branch-
ing filter 106 in a certain embodiment of this invention.
The antenna 103 mounted on an automobile is connected to a
band inhibiting filter 413 by way of a cable 105 which is
a signal line. The output of the band inhibiting filter ~13
is applied to a radio set 111 which is second communication
means. The coaxial cable 105 is connected with a transmitter/
receiver 108 for mobile telephone, which is first communica-
tion means, by way of a high pass filter 415.
The transmitter/receiver 108 for mobile telephone
performs radio communications with the ground station con-
nected in the telephone line network in a first frequency
band fl, that is, in a frequency band fla of 870 to 890 MHz
in receiving, and in a frequency band flb of 920 to 940 MHz
in transmitting. On the other hand, the radio broadcast
received in a radio set 111 using a second frequency band
f2, that is, a frequency band f2a of 500 to 1620 kHz for
AM broadcast, and a frequency band f2b of 76 to 90 MHz for
FM broadcast. Therefore, in reception of radio broadcast
by radio set 111, if a mobile telephone is used, it is
enough when the signals in the frequency bands fla and flb
in reception and transmission be inhibited by the band
inhibiting filter 413.
- 39 -
~3~
The high pass fllter 415 intervening between the
coaxiai cable 10~ and the transmitter/receiver 108 fo~ mobil~
telephone is connected in series to capacitors c23 and C24,
and a connecting point 417 of these capacitors C23 and C24
is grounded through a coil L23, thereby allowing to pass the
signal in the freuqency band fl of mobile telephone and cutting
off the signal in the frequency band f2 of radio broadcast.
Meanwhile, the band inhibiting filter 413 is composed of a first
band inhibiting filter 418 for inhibiting the frequency band fla
of 870 to 890 MHz, and a second band inhibiting filter 419 for
inhibiting the frequency band flb of 920 to 940 MHz.
The first and second band inhibiting filters 418
and 418 are connected in series to the coaxlal cable 105,
individually. The first band inhibiting filter 418
comprises a coil L25 and a capacitor C2S, while the second
band inhibiting filter 419 comprises a coil L26 and a
capacitor C26. The inductance of coils L25 and L26, and the
electrostatic capacity of capacitors C25 and C26 are properly
selected so as to inhibit the signals in the above frequency
bands fla and flb.
Fig. 16 is a graph showing the frequency character-
istics of the band inhibiting filter 413. The band inhibit-
ing filter 413 operates during use of mobile telephone,
and inhibits the signal from the antenna 103 in reception
mode, and the signal from the transmitter/receiver 108 for
- 40 -
3L 3 ~
rnobile telephone in transmisslon :mode. In the radio set lll,
generation of its noise does not matter if less than 110 d~J
~v (~3 dBmW) at input voltage. On the other hand, the
transmission outputof transmitter/receiver 108 for mobile
telephone is 5w (+37 dsmW~ in Japan. Therefore, the band
inhlbiting filter 413 is composed so that the input signal
level may be attenuated more than 34 dB and delivered in the
frequency bands fla and flb of 870 to 890 MHz and 920 to
940 MHz. Fig. 16 shows the frequency characteristics with
respect to the input signal level VI.
Thus, in this embodiment, during use of moblle
telephone, interference of reception signal (870 to 890 MHz)
into the radio set lll is prevented by the first band
inhibiting filter 418, whereas the interference of transmis-
sion signal (920 to 940 MHz) into the radio set 111 is
prevented by the second band inhibiting filter 419. In
addition, between the signal line of the radio set 111 and
the ground, there is no intervening electrostatic capacity
such as capacitor, so that drop of voltage level induced
by antenna 103 by band inhibiting filter 413 in reception
mode of.radiobrodcast will never occur.
In this manner, without lowering the reception
signal level of the radio set 111, effects of transmission
and reception signals of mobile telephone on the reception
signal of radio broadcast may be suppresed, and mutual
. - 41 -
~ 3 ~
lnterference bet-,ieen transmission and reception signals of
the antenna co~only used in different frequency bands fl and
f2 may be suppressed.
Fig. 17 is a structural drawing of an antenna
circuit 110 in a different embodiment of this invention, and
Fig. 18 is an equivalent circuit diagram in AM radio frequency
band f2a of an antenna circuit 501 for explaining the
principle of this invention. The antenna 500 is expressed
as a composition of an antenna reactive capacity Ca existing
agaisnt the ground level, and an atenna effective capacity
Ce existing in series, and an AM radio signal which is a
first radio signal received by this antenna 500 is expressed
as an anternating-current power source V31. A coaxial cable
109 i~,s expressed to comprise a line Q61 betweern terminals
B2 and P2, and this line Q61 is grounded by way of a cable
capacity Cb. Between the antenna 500 and the coaxial cable
109 is intervening a transformer 502 for converting the
impedance. The signal at terminal P2 is transmitted to the
antenna input circuit in the radio set 111. The voltage V41
at this terminal P2 is expressed as follows, supposing the
turn ratio of input side to output side of the transformer
502 to be H:
Ce + Ca + Cb/n2 V31 --.................. (6)
As understood from eq. 6, by additionally installing the
- 42 -
transformer 502, the effect relating to the cable capacity Cb
may be reduced to l/n2 of the expla~ation ln ~ig. 7.
Therefore, the impedance derived from the cable capacit~ Cb
as seen from the terminal A2 converted to l/n2 by the
transformer 502, so that the loss at the coaxial cable 109
may be reduced.
The antenna circuit 110 is composed of an antenna
103, the coaxial cable 109, an impedance adjusting circuit
513 intervening between the antenna 103 and the coaxial
cable 109, and the impedance adjusting circuit 517 interven-
ing between the coaxial cable 109 nad the radio set 111.
In Fig. 8, meanwhile, the impedance adjustlng circuit 513
is built in the branching filter 106.
The output from the antenna 103 is applied to the
impedance adjusting circuit 513 through the branching filter
106. The impedance adjusting circuit 513 has a low impedance
in the frequency band f2b of FM radio signal, and comprises
a FM radio signal filter circuit 514 which is a first filter
circuit, and transformer 522 and others, and an impedance
conversion circuit 515 which is a first impedance conversion
circuit is connection in parallel to make up the composition.
The FM radio signal received thus by the antenna 103 is
delvired to the coa~ial cable 109 through FM radio signal
filter circuit 514.
The FM radio signal filter circuit 514 is composed,
- 43 -
.
for example, in a series connection o~ a coil 520 and a
capacitor 521, and functions as a hish pass filter with a low
impednace agalnst FM frequenc~ band f2b.
The radio signal from the coaxial cable 109 is
given to the impedance adjsuting circuit 517. The impedance
adjusting circuit 517 is composed of an FM radio signal
filter circuit 518 which filters FM radio signal and is a
second filter cirucit, and an impedance conversion circuit
519 which has an impedance conversion action on AM radio
slgnal and is a second impedance conversion circuit.
The FM radio signal filter circuit 518 is connected
in parallel to the impedance conversion circuit 519, and the
FM radio signal from the coaxial cable 109 is led out into
the antenna input circuit of the radio set 111 through the
FM radio signal filter circuit 518. The FM radio signal
filter circuit 518 is, for example, composed of a coil 523
and a capacitor 524, and functions as a high pass filter
for filtering a relatively high frequency signal such as a
FM radio signal. The impedance conversion circuit 519
comprises a transformer 525 and others, same as the first
impedance conversion circuit 522 mentioned above.
Therefore, the inductance of coils 520 and 523 in
the FM radio signal filter circuits 514 and 518, and the
electrostatic capacity of capacitors 521 and 524 are properly
selected so as to possess the resonance frequency in the FM
- 44 -
~3~3~
radio signal frequency band, respectlvely.
In the construction shown in Fig. 18, howe~er, there
is actually an effect of the capacity in the FM radio signal
filter circuit 514 in Fig,. 17. An equivalent circuit diagram
to show the principle in consideration of such capacity
component Cf is shown in Fig. 19. For the sake of simplicity,
the antenna effective capacity Ce and the antenna reactive
capacity Ca are collectively expressed as CA. Incidentally,
the transformer 502 corresponds to a transformer 522 in
Fig. 17, while the antenna 500 corresponds to the antenna 103.
A self-inductance Ll is provided at the input side, a self-
inductance L2 is provided at the output side, and there is
a mutual inductance M between the input side and the output
side. Therefore, between the alternating-current power
source V31 derived from the radio signal received by the
antenna 500, and the voltage level V41 applied to the radio
set 111, the following relation is established, assuming the
current from the antenna 500 to be il, the current flowing
in the capacity component Cf to be i2, and the current due
to cable capacity Cb to be i3:
V31 = (j C + jWLl)il + (jwM - jwL1)i2 + jwMi3 ... (7)
O = jwMil + (jwLz - jwM)i2 ~ (jwL2 + jWcb)i3 -----------~--- (8)
V31 = j C il + j Cf i2 -- j Cb i3 ................ t9)
- 45 -
?
And,
V41 = - jW1 b i3 ................... , .... (10)
Therefore, solvlng them, we obtain:
41 = ~ {~4CACf (L1L2--M2)--~2CAM}V31 . . (11)
(CACf~CACb+C~Cf) (L1L2-M2)-~2{L1(CA+Cf)+L~Cb+Cf)-2MCL-~+1
where ~ denotes the angular frequency of the received radio
signal.
At this time, when the denominator of eq. 11 is
zero, V41 reaches the maximal value. Supposing here that
the mutual inductance M be expressed as k~LI L2 (where k is
a coupling coefficiency of transformer 502), the maximal
value of V41 is expressed as follows:
f = 1 ~ Y + X ~ ................... (12)
where X = (CACf + CACb + CbCf)(l - k2)LlL2 .. (13)
Y = -{Ll(CA + Cf) ~ L2 (Cb + Cf) - 2Cf-k ~ }.. (14)
Z = 1 ................................. (15)
Thus, as shown in eq. 12, the voltage level V41
comes to possess the maximal value with respect to two values
differing in frequency f. Supposing the frequencies corre-
sponding to the maximal value of voltage level V41 to be fll,
fl2 (fll < fl2), the relation between frequency f and voltage
level Vc is expressed in Fig. 20. As understood from eq. 12
to eq. 14, as the coupling coefficient k becomes smaller,
the frequency fl2 becomes lower. Therefore, by increasing
- 46 -
~3~
the coupling coefficeint k possessed by the tr~nsformer 502,
when the A~ radio signal frequency band f2a is adjusted to
settle within frequency fll and frequency fl2, a flat reception
characteristic will be obtained in the AM radio signal
frequency band f2a. As the transformer capable of increasing
the coupling coefficient k, for example, the tranformer 502
of so-called sandwich winding or bifilar winding may be used.
Fig. 21 is an equivalent circuit diagram in an AM
radio signal frequency band f2a of the antenna circuit 110
in Fig. 17. The antenna 103 may be expressed by a capacity
CA comprising the antenna effective capacity possessing a
series electrostatlc capacity with respect to the radio
signal, and the antenna reactive capacity generated between
the radio signal and grounding level. The radio signal
received by antenna 103 may be expressed as alternating-
current power source V32.
The AM radio signal received by antenna 103 has
a high impedance in the FM radio signal filter circuit 514,
and therefore it is fed into the impedance conversion
circuit 515. In the impedance conversion circuit 515, the
turn ratio of the number of turns at the input side and the
output side of the transformer 522 is n:l. Accordingly,
the voltage of the AM radio signal is reduced to l/n and
the impedance is reduced to l/n2 by the transformer 522.
The coaxial cable 109 contains the cable capacity Cb between
- 47 -
1 3
the radio signal a~d a groundin~ level.
Relative to a high frequency signal, for exmaple, a
FM radio singal, the coaxial cable lO9 has a low impedance.
However, to a relatively low frequency signal such as a AM
radio signal, the impedance of the coaxial cable 109 due to
cable capacity Cb is large. In this embodiment, the
impedance of the AM radlo signal is reduced by the impedance
conversion circuit 515, so that the loss relating to cable
capacity Cb may be reduced.
The signal in a relat`ively low frequency band f2a
such as AM radio signal from the coaxial cable 109 is high
in impednace in the FM radio signal filter circuit 518, and
it is applied to the impedance conversion circuit 519. In
the transformer 525 of the impedance conversion circuit 519,
the ratio m against the number of turns 1 at the input side
to that at the output side is set, and the AM radio signal
fed to this transformer 525 is amplified in voltage, and ic
delivered into the antenna input circuit of the radio set
111 .
The relation between the alternating-current power
source V32 and the output voltage V42 is expressed in the
following equation.
n CA + Cb/n2 '''' ---. (16)
A capacity CTA of the antenna circuit 110 as seen from the
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~ 3 ~ ~31~
radio set lll is expressed as follows:
C CA n + Cb ............... ................. (17)
For example, this capacity CTA is defined at 80 pF in
relation with the impedance matching with the radio set,
and the capacity CA and the cable capacity Cb are determined
by the length of the antenna 103 and the coaxial cable lO9.
Therefore, the turn ratios n and m of the transformers 522
and 525 are selected so as to satisfy eq. 17 above.
The equivalent circuit of antenna circuit llO as
seen from the radio set lll may be expressed as the composi-
tion of inductance Lo/2 and capacity CTA connected in
parallel, assuming the inductance at transformers 522 and
526 to be Lo. Suppbsing the resonance frequency of such
circuit compsoition to be fp, the inductance Lo may be
expressed as follows:
(2~fP)2-CTA (18)
It is desired to flatten the frequency charactersitics in
the AM radio signal frequency band f2a by selecting the
resonance frequency fp at, for example, 250 kHz or other
outside the AM radio signal frequency band f2a. Accordingly,
the inductance Lo of the transformers 522 and 525 is
determined by eq. l8.
Thus, in the antenna circuit llO, for exmaple,
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13~3~
when receiving an A;~l radio signal and a E'M radio slgnal commonl~
by one antenna 103, the loss of the ~ radio signal at the
coa~ial cable 109 may be lowered. For instance, assuming
the antenna effective capacity Ce to be 15 pF, the antenna
reactive capacity Ca to be 5 pF, the cable capacity Cb to be
120 pF, and the turn ratios n and m to be 4, the gain is
improved by about 9 dB as calculated according to eq. 5 and
eq. 6.
In the foregoing embodiments, the loss will be
greater if a too large value is set for the turn ratios n
and m of the transformers 522 and 525, or the effect will
be smaller if a too small value is used. According to the
experlment by the present inventors, practically, favorable
results are obtained when a numerical value of 10 or less
is selected for the turn ratios n and m.
Fig. 22 is a strcutural drawing of an antenna
circuit 531 in a still different embodiment of this invention.
The parts corresponding to the foregoing antenna circuit 110
are identified with same reference numbers. In the antenna
circuit 531, in the impedance adjusting circuit 513a, the
impedance conversion circuit 515a comprises coils 532 and
533 and the transformer 522, and in the impedance adjusting
circuit 517a, the impedance conversion circuit 519a comprises
coils 534 and 535 and the transformer 525. In order to
reduce the loss due to the stray capacity possessed by the
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1 3 ~
transformers 522 and 525, coils 532 to 535 are added ~o ~he
input end and the output end of the transfor~ers 522 and 525,
respectively. As a result, the loss attributable to the stray
capacity of the transformers 522 and 525 is prevented, and
the reception sensitivity and the S/N ratio may be further
enhanced.
In the foregoing embodiments, thus, the loss in
the AM radio signal frequency band f2a due to stray capacity,
in particular, can be reduced, while the reception sensitivity
and the S/N ratio in the radio ~eceiver may be outstandingly
enhanced. Therefore, when receiving signals in a wide
frequency band by a single antenna, for exmaple, when receiv-
ing both FM and AM radio signals by a car-mount antenna, it
is particularly effective.
Besides, depending on the type of antenna, generally,
the antenna reactive capacity varies more significantly than
the antenna effective capacity. When this invention is
applied in an antenna with a large antenna reactive capacity,
its effect will be manifest. Meanwhile, the polarity of the
transformers 522 and 525 may be either normal phase or
reverse phase, but according to the experiment, a greater
effect will be obtained when transformers 522 and 525 of
normal phase are used.
In this embodiment, it is explained to receive FM
radio-signal and AM radio signal, but it may be also favorably
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embodied also in the application of recelving radio signal
and other signal such as a mobile telephone signal at the same
time.
The invention may be embodied in other specific
forms without departlng from the spirit or essential charac-
teristics thereof. The present embodiments are therefore to
be considered in all respects as illustrative and not re-
strictive, the scope of the invention being indicated by the
appended claims rather than by the foreging description and
all changes which come within the meaning and the range of
equivalency of the claims are therefore intended to be
embraced therein.
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