Note: Descriptions are shown in the official language in which they were submitted.
- -
~ The present invention relates generally to isolation
¦ apparatuc fGr mulviple antenna installations ard more particularly
¦Ifor cervain ,ns'allations such 2S portable aircral' ccntrol
l¦~acilities ha~ring many closely spaced antennas and other auxil'a~y
!1 app2ratus such as a wind sensor toge'h2r ~ith 2 plurality of re-
spect've radio transceiverC. This equipment is inciuded in a se~f
I contained unit which as a consequence o~ relatively close spac ng
normally has undesired interaction bet~Jeen the antennas, the
vertical'y disposed feed l nes, and the wind sensor convrol cable.
It is well-known that the electrical characteristics of
an antenna can be adversely affected when situated in the vicinit~
of other antennas, metal masts, metal surfaces, electrical wiring
or transm~ssion lines. For example, the antenna impedance and
I radiation c~.aracteristic may be substantially changed due to the
l parasitic excitation and reradiation by and from the other conduc-
¦~tors.
-- 1 --
... . ....... ....... ~ ... .......
1~)~7756
In certain militar~ installations, such as in aircraft
control towers where many closely spaced antennas for different
frequencies are employed, acceptable antenna performance is diffi-
cult to obtain. The various antennas interact with one anGther,
resulting in modified impedances and radiation characteristics.
When attempting to analyze the radiation characteristics
of a complicated system such as a portable aircraft traffic con-
trol facility for military use, and more particularly a facility
such as the A~/TS~-97, not only the antennas, but the entire
structure consisting of antennas, cables~ ~-ind sensor, console
ground and metal masts, have to be regarded as a complex system
for radiating and absorbing electromagnetic waves. Accordingly,
it is very difficult if not impossible to predict theoretically
the radiation characteristics of such a complex structure. It is
practical, however, to measure the antenna patterns in the hori-
zontal and vertical planes, but the experimental determination of
radiating characteristics is a time consuming procedure when the
operating bandwidth is great and therefore such a determination
m~y have to be based upon measurements made at only a few selected
frequen~ies.
One approach to the problem of improving the radiation
properties of a multi-antenna system is to locate all of the
antennas on a common vertical axis. Such an arrangement provides
a substantially omnidirectional pattern in the horizontal plane.
Radiation in the vertical plane~ however, depends upon curren~
distribution, antenna height above grounG an~ operating frequency.
Over all, such an antenna s~stem provides relatively bet~er per- ¦
formance in comparison to ~ny radiating system hav~ng an'ennas
mounted in a broadside relationship.
10~7756
Where, however, a broadside array is desirable, notwith-
standing the advantages gained by a vertical in-line array, it is
possible to reduce the strength of the induced feed line currents
by careful arrangement of radiators and avoiding resonant lengths
5 of feed lines; however, in a case of systems operat~ng over a
broad frequency ran~e, e.g., over two octaves for example, it is
very difficult to pick optimum lengths of cables, etc.
One means which is helpful in optimizing broadband
radiating systems is to insert separate high impedance broadband
10 cable chokes ir. series with the antenna feed lines. When connected
in series with feed lines, the high impedance property of the
cable choke can tend to suppress feed line current flowing on the
~utside of the feed line induced by the impressed electrical field.
Illustrative examples of such apparatus is taught for example, in
15 U.S. Patent 3,879,735 of Donn V. Campbell and James J. Arnold,
entitled "Broadband Antenna System With Isolated Independent Radi-
ators"; and U.S. Patent 3,961,331 of Donn V. Campbell, entitled
"Lossy Cable ~hoke Broadband Isolation Means for Independent
Antennas'l, both assigned to the assignee of the present invention.
20 The cable chokes illustrated therein consist of a high ind~ctance
made by winding a plurality of coaxial cables in the shape of a
helix and with ~everal separate cable chokes being provided ln
those cases in which d~fferent frequency ranges are involved. For
example, one choke may impede feed line current at frequenc~es be-
25 tween 30 and 80 M~z while another choke may be designed for the~requency range of 200-40~ MHz. At VHF frea~uencies, the cho~e
¦would normally be wound on a magnetic core such 2S ferr~te in order
to maximize the inductance of the cable choke wh~le at ~-HF fre-
encies, the magnetic core is usuall~ deleted.
97756
SUMMARY
Instead of employlng a separate choke for each feed
line, it has been found practical to utilize one or more multiport
cable chokes in the feed line in accordance with the teaching of
the sub~ect invention. Briefly, the subject invention is directed
5 to the improvement comprising the winding of a plurality of coaxial
cables and any other shielded multiconductor cable(s) respectively
adapted for connection to a plurality of antennas and auxiliary
apparatus, in the same direction on a common core with the same
number of turns and having the shields or outer conductors of all
lO the cables commonly connected together at each end of the winding
ad~acent to the core. Additionally, the choke including the core
and windings may be included in a non-magnetic, non-conducting
housin~ having end walls including connector means respecti~ely
for each of the coaxial cables and shielded multi-conductor
15 cables(s). These chokes preferably are inserted a quarter wave
length from the antennas for maximum effect. When a broad band of
frequencies, or a plurality of separate frequencles, is lnvolved,
one can establish the quarter wave spacing on the basis of the
center of the band or the arithmetic mean freauency, as the case
20 may be. The chokes are each tuned to the geometrical mean fre-
quency when the fre~uency bandwidth is of the order of an octave or
less. For example, if the bandwidth extends between frequencies
fl or f2, or if the two separate frequencies fl and f2 are sepa~ate
by not more than an octave, the chokes ~lould be tuned to approx-
25 imately f~ = ~ . If two separate widely spaced frequencies areinvolved, one multiport choke could be tuned to frequency fl and
t other multiport cnoke tuned to frequency ~2
~0~7756
B~I~F D~SC~IPTION OF THE ~RAWINGS
Figure 1 ls illustrative of prior art practice whereby
separate cable chokes are utilized for each antenna of an array,
Figure 2 is a diagram illustrating relative amplitude of
surface current as measured along the length of a typical mast or
5 line connected to a half wave dipole antenna;
Figure 3 is a diagram illustrating the measured current
amplitude along the same structure as shown in Figure 2 when cable
chokes are positioned at quarter wave length intervals;
Figure 4 is a schematic diagram broadly illustrative of
10 the concept of the subject invention;
Figure 5 is a plan view of one embodiment of the subject
invention;
Figure 6 is a plan vlew of another embodiment of the
subject inventlon equivalent to the one shown in Figure 5;
Figure 7 is an electrical schematic view further illus-
trative of the concept of the sub~ect invention;
Figure 8 is a plan view of yet another embodiment of the
subject invention;
Figure 9 is a plan view of an embodiment of the sub;ect
20 invention equivalent to the embodiment shown in Figure 8;
Figure 10 is a perspective view of the housing for the
sub~ect inventionj
Figure 11 is a perspective view of an embodiment of the
subject invention which is adapted to be located ~n the houslng
25 shown in Figure 10;
Figure 12 is an electrical equivalent circu~t diagram
~f the subject invention;
. ~ I
lQ~7756
Flgure 13 is a diagram i11ustrat'~e Or the reactance vs.
frequency characteristic of the sub~ect inventionj
Figure 14 is a diagram illustrative of a radio system
for a plurality of radio apparatus operating on different frequency
5 bands and employing more than one multiport cable choke according
to the sub~ect invention; and
Figure i5 is a diagram illustrative of a retransmission
station employing the sub~ect invention for improving isolation be-
tween receiving and sending radio apparatus.
DESCRIPTION OF THE PREFERRED EMBODIME~TTS
An antenna is a conductor so constructed as to either
radiate electromagnetic energy, to collect electromagnetlc energy,
or both. A transmlttlng antenna converts electrical energy into
15 electromagnetic waves called radio waves ~Jhich radiate away from
the antenna at speeds near the velocity of light. A receiving
antenna converts electromagnetic waves which it intercepts into
electrical energy and applies this energy to electronic circuits
for interpretation. Some antennas are adapted to serve both func-
20 tions and accordingly the electrical and physical features are de-
termined by the use to which they are put. Such ~eatures will
vary with operating fre~uency, power handling capability, plane of
polarization, and desired radiation field pattern. The physical
size of an antenna is determined by its operatin~ frequency and
25 power handlin~ capability whlle its shape and height are deter-
mined by the desired radiation field pattern. Such apparatus is
well known to those skilled in the art and is well documented in
all the literature dealing with fundamentals of radio trans~.ission.
The sub~ec' invention is directed to an im~roved means
3o for reducing the interaction of closely spaced antenn2s, auxiliary
apparatus, and the respective feed lines therefor.
ll 10.-7756
Referring now to the drawings, reference is first made
to Figure 1 whlch is ~llustrative of prior art practice wherein
a plurality of radio apparatus 10, 12, and 14 are coupled to re-
spective dipole antennas 16, 18 and 20, mounted in a broadside
5 array by means of the feed lines 22, 24 and 26. As shown in
Figure 1 single port chokes 17, 19 and 21, constructed in the man-
ner shown in Figure 6 of the earlier-mentioned patent #3,879,73~,
preferably should be located as close as possible to the point of
connectlon of the feed lines to the corresponding antenna radiating
10 elements (for example, the dipole ar~s in a center-fed dipole
antenna) to create a high impedance point at said point of con-
nection and establish the correct electrical length of the radi-
ating elements. The single port choke could be replaced by a
quarter wave coaxial sleeve choke with the sleeve surrounding the
1~ feed llne and the end of the sleeve remote from the antenna con-
nected to the shield of the feed line. In addition, each of the
feed lines 22, 24 and 26 includes a separate series connected cable
choke 28, 30, 32. The feed lines 22, 24 and 26 are comprised of
coaxial transmission lines and the cable chokes 28, 30 ~nd 32 are
20 formed fro~ a portion of the coaxial cable wound in the shape of a
helix as shown for example in the above referenced U.S, Patent
3,879,735, and are configured for the operating frequencies of the
respective radio app~ratus 10, 12 and 14 with which they are
util,zed.
F~gure 2 illustrates the undesirable build up of surface
currents along a typical single coaxia~ antenna feed line 11 con-
nected between a dipole antenna 13 and ground and including a
single cable choke 23 indicated by a cross in Figure 2 (corre-
sponding to chokes 17, 19 and 21 of Figure 1) located at one end
3o of the dipole to define the half wave length ol the dipole. In
Il l~g7756
dditlon to the dipole radiating current, spurious current peaks
~ccur at half wave length intervals along the feed line 11.
By positioning a plurality of single port cable chokes
25 along feed line 11 at quarter wave intervals, starting at a
5 quarter wave length from the antenna choke 23, as indicated in
Figure 3 the measured antenna feed line current is substantially
¦eliminated, except for a negligible current peak ~ust below the
~ipole. It has been found that, at a distance of about one wave
length from the dipole, further chokes usually are not required.
10 In some instances, one single port choke 23 can provide adequate
reduction of feed line current. The larger the inductance of the
choke, the greater is the reduction of shield currents along the
feed line.
Referring now to Figure 4, there is disclosed in ~lock
15 diagrammatic form the basic concept of the sub~ect invention.
Single port chokes 17, 19 and 21 are connected ad~acent the re-
spective antennas, 16, 18 and 20, as shown in Figure 1. The
plurality o~ radlo apparatus 10, 12 and 14 of ~igure 4 are coupled
to their respective antennas 16, 18 and 20 through unitary multi-
20 port cable chokes 34A and 34B, two embodiments of which are shownin Figures ~ and 6. Figure 5 is illustrative of one embodlment of
the sub~ect invention wherein the three feed lines 22, 24 and 26
are wound side by side in the same dlrection with the same number
of turns on a common core 33 comprised preferab'y of a cylinder of
25 ferrous material such as ferrite, but, when desired3 can be made
from non-~errous non-conducting material. Each of the multiport
cable cho~es 34 are inserted a quarter wave ~ength from the cor-
respon~ing single port chokes 17, 19 and 21 at the respecti~e an-
tennas 16, 18 and 20, as sho~n in ~igure 4 and the interval be-
3o tween chokes 34A an~ 34B is a ~uarter wave length. In some cases,one multiport choke along the feed lines may not suffice. Usually,
1~ 10~7756
one can reduce spurious feed line currents to a negligible amount
¦after proceeding about one wave length along the feed line; in
¦other words, no more than four mult$port chokes 34 are normally
. ¦recuired.
5 ¦ The shields or outer conductors of the coaxial cables
¦22, 24 and 26 are electrically connected together at both ends of
¦the respective windings such as at points A and B as shown in
¦Figure 5. When so connected, the radio frequency potential of
¦all three windings is the same at end A and likewise the potential
10 ¦of the three windings is the same at point B, although the poten-
¦tial at point A may differ from the potential at point B. The
¦configuration shown in Figure 6 is similar in all respects except
¦that a toroidal core 35 is depicted; as in Figure 5, all coaxial
¦cable windings are wound in the same direction with the same num-
15 ber of turns and the shields are interconnected at the ends of the
windings.
Referring now to the configuration shown in Figure 7,
there is contemplated in addition to a pair of radio apparatus 36
and 38 coupled to respective antennas 40 and 42, the use of auxil-
20 ~ary apparatus 44 which may be, for example, a wind sensor orother devices associated with a portable aircraft traffic control
~acility coupled to respecti~e control and/or metering apparatus
46 through a shielded multi-conductor cable 48.
Owing to the proximity of the feed line 48 for the aux-
25 iliary apparatus 44 of Figure 7 to the feed lines 50 and ~2 forantennas 40 and 42, the possibility exists that, in the absence of
choke means, in the feed lines 50 and 52, spurious currents could
appear on feed lines 50 and 52 and induce spurious sh~eld currents
in feed line 48. Moreover, spurious currents could also
3o be induced in all three ~eed lines 48, 50, and 52 ol ~igure 7 owing
7756
~o the presence of additional radiation sources in the vicinity.
hese undesirable shield currents in feed line 48 could then be
oupled lnto auxlllary apparatus 44 or device 46, thereby adversely
nfluencing the operation of devices 44 and 46. Similarly, when no
hoke means is used in the feed lines, it is possible that spurious
urrents appearing alon6 feed line 48 could be induced into antenna
eed lines 50 and 52.
As in the case of Figure 4, and for reasons pointed out
n the description of Figure 4, single port chokes 17' and 19' are
nserted adjacent the antennas 40 and 42; in addition, a multiport
hoke 49 is inserted at the point of connection of the feed line 48
o the auxiliary apparatus. The multiport cable choke 34' of
?igure 7 (in the case illustrated, a three-port choke) is positionet L
quarter wave length from the chokes 17'~ 19', and 49'. The feed
ine 48 ~s illustrated in Flgure 7 as including three separate con-
~uctors; however, the number of such conductors is not restricted
to three.
Although only one multiport cable choke 34' is shown in
the feed lines of Flgure 7, two or more such multiport cable chokes
20 can be used, as indlcated ln Figure 4, and such cable chokes would
~e spaced apart from one another by a ~uarter wave length, and the
~ultiport cable choke nearest cable chokes 17', 19', and 49' would
be spaced one quarter wave length ~rom each of the latter three
cho~es.
Schematically, the multiport cable choke 34' is shown
in two forms in Figures 8 and 9 wherein the two coaxial feed lines
50 and ~2 are wound together with the shielded multiconductor cable
48 on a common core com~osed of ferrous or non-ferrous,
non-conducting material. In Figure 8 there is disclosed a cylin-
3o ~rical core 5~ whereas in Figure 9 a toroidal core 55 is shown.
. l
Il
I
11 1~977S6
11 `
The cables 48, 50 and 52 are wound in the same direction a~out the
core 55 and are located ad~acent one another; furthermore, each of
the cables has the same number of turns on the core. It should
also be pointed out that the cables 48, 50 and 52 are covered with
5 suitable insulation so as to prevent short circuits between ad~a-
cent turns. As in the case for the other embodiments shown in
~igu~es 5 and 6, at the be~inning and end of the winding, i.e., at
points A and B, the shields or outer conductors are electrically
connected together for the purposes set forth above. Accordingly,
10 since the three cables 48, 50 and 52 are wound in the same direc-
tion on a common core 54 with the same number of turns, it follows
that the impedance characteristic of the entire assembly will be
unitary.
The single multiport cable choke concept is a dlstinct
15 improvement over the prior art due to the fact that the following
disadvantages accrue with the use of separate cable chokes as
shown in Figure 1. First, the capacitive coupling between ad~a-
cent shields, i.e., outer conductors, and the fact that the RF
potential of the shields at the points where they are connected to
20 the separate chokes wlll in general cause external feed line cur-
rents to be induced on the shield which in turn will interfere
with the normal operation of the separate antennas. ~econdly, the
fact that the chokes are wound on separate cores the effective
resonant frequency o~ the respective chokes will tend to be dlf-
25 ferent and will be a function of the spacing and placement of thecho~es with respect to each Gther, whereas the multiport caole
choke of the subject invention will have a well established uni-
tary resonant frequency.
~ 10_17~6
Referring now to Figures 10 and 11, there is disclosed
a physical embodiment of the present invention. Reference numeral
~6 des~gnates a generally cylindrical dielectric housing having a
pair of end walls 58 and 60. A first plurality of coar.ial connec-
5 tors 62, 64 and 66 and a multipin connector 6~ are mounted onthe end wall 58 while a like number of coaxial connectors 70, 72
and 74 as well as a corresponding multipin connector 75 are
.~ounted on the other end wall 60. Three coaxial cables 76, 78 and
80 as well as a set o electrical conductors 82 inside of a
10 braided shield 83 (shown partially cut-away) are wound ad~acent
one another on a toroidal ferrite core 84 and being held in posi-
tion on the core by means of a piece of electrical tape 86. Thus,
for example, the coaxial cable 76 terminates in opposing coaxial
connectors 62 and 7~, the coaxial cable 78 in opposing coaxial
15 connectors 64 and 70 and coaxial cable 80 in coaxial connectors
66 and 72. The se' of electrical conductors 82 accordingly ter-
minates in the pair of multipin connectors 68 and 75. The fact
that a~l of the coaxial cables and electrical conductors have
the same number of turns and the coaxial cables and the shielded
20 cable have their outer conductors commonly connected together as
by connections 88 and 87, ad~acent their respecti~e connectors
exhibits an equi~alent circuit as shown in Figure 12 and having a
reactance vs. frequency characteristic such as sh~wn in ~igure 13,
wherein a single resonant frequency fO is established.
The number of turns used in a given choke depends on
such factors as the core permeability, operating frequency and
required bandwidth. In general, bandwidth is inversely propor-
tional to the self-capacitance C (figure 12) of the ~-ir.ding.
5reatest bandwidth is obtained by minimiz ng the self-capacitance.
.
77S6
Another ~actor to be consldered ~n the design Or a
cable choke according to the sub~ect invention is the power loss
in the magnetic core and surrounding dielectric material. When
connected in series with the transmission line, it is possible
5 that high RF voltages can develop across the choke and consider-
able power could be dissipated in the choke.
For operation in the frequency range of 30-70 MHz, the
parallel resistance ~ of the multiport cable choke is in the order
of 10,000 ohms; however, as long as the radlo ~requency voltage
10 across the choke is less than 100 volts, rms power loss is less
than one watt. The reactance of the multiport cable choke, as
is well known, is a function of frequency and varies in the same
way as the reactance of a parallel tank circuit being positive ~or
frequencies below resonance and negative for frequencies above
15 resonance.
~ andwidth ls deflned somewhat arbitrarily by the
frequency range within which the cable choke's reactance exceeds
a certain minimum value. For example, suppose that the minimum
acceptable choke reactance is 1,000 ohms in a particular appli-
20 catlon. With such a specification, the VHF multiport cable chokeaccording to the sub~ect invention has a broadband width extending
from approximately 37 MHz to above 70 ~Hz.
Figures 14 and 1~ are included to indicate the use of
multiport cable cho~e tuned to different resonant requencies to
25 reduce antenna interaction and feed line radiation ~here different
~requency bands are employed by separate radio apparatus. More
particularly, a VHF-AM radio 88, a VHF-FM radio 90, and a UHF-
radio g2 are coupled ~o respective antennas 94, 96 and 98 through
two multiport cable chokes 100 and 102 and which would be con- !
3o~ ~igured, for example, in the manner shown in Figures ~ and 6.
ll lQ97756
14
As indicated in connection with the embodiments o~ ~igures 4 and
7, high impedance chokes 17", 19", and 21" are inserted ad~acent
antennas 94, 96, and 98, respectively.
Referring now to Figure 15, there is disclosed another
5 application for the sub~ect invention wherein one or more multi-
port cable chokes 104a to 104c and lC6a to 106c spaced at quarter
wave intervals are used to improve electrical isolation between
radio apparatus 108 and 110 included in a retransmission station
which ~or example receives signals on antenna 112 and re-transmits
10 radio signals from antenna 114. Reference numeral 116 denotes a
harness including various audio/control wiring/radio frequency
cables lnterconnecting the radio apparatus 108 and 110.
Thus what has been shown and described ls a multiport
cable choke which by virtue of its high impedance property mini-
15 mizes feed line radiation while at the same time providing connec-
tions as may be required for remote control power supply or sensing
apparatus and radio frequency signal transmission while at the
same tlme reducing weight and complexity of the radio system.
Addltionally, when desired, such apparatus ls adapted to improve
20 electrical isolation between radio apparatus irrespective of their
~ tl~n ~ c~n~ h ~A~nA- ~e~ n- , ~d o r~r~h
