Note: Descriptions are shown in the official language in which they were submitted.
CA 02525995 2005-11-08
BALANCED DIPOLE ANTENNA
Background Of The Invention
The present invention relates a balanced dipole antenna, and more
particularly, is
directed to a symmetric balun used with a coaxial cable and dipole antenna.
Fig. 1 shows dipole antenna 10 as having coaxial cable 5 having outer coaxial
conductor 1 S and inner coaxial conductor 16 used with a dipole antenna having
dipole left
blade 11 and dipole right blade 12. Coaxial outer conductor 15 is connected to
dipole
right blade 12. Coaxial inner conductor 16 is connected to dipole left blade
11 via wire
17.
As used herein and in the claims, "coupling" includes a radiative connection
and a
direct electrical connection.
Since an isotropic antenna is physically impossible, antenna gain is measured
against a standard dipole antenna, and the results are indicated as decibels
vs. dipole
(dBd).
Common mode current flows on the outside of the coaxial line, reducing the
efficiency of a pure dipole radiation pattern. Additionally, common mode
current is
caused by radiative coupling between the dipole antenna and an external
coaxial cable.
The majority of the distortion of the dipole antenna pattern is due to common
mode
current flow caused by the conducting imbalance of the structure, and a
smaller amount of
the distortion is due to radiative coupling.
To reduce the common mode current flow, a balun is used. A balun acts as a
transformer, connecting a balanced two-conductor line to an unbalanced coaxial
line.
Fig. 2 shows dipole antenna 30 as having coaxial cable S connected to a dipole
antenna using Roberts balun 40. The dipole antenna forms a balanced load (or
source).
Coaxial cable 5 connects to an unbalanced source (or load) and is connected to
Roberts
balun 40 at connection 41 which may be a threaded screw-type connection.
Roberts
balun 40 has a main coaxial segment having outer coaxial conductor 45 and
inner coaxial
conductor 46. Coaxial outer conductor 45 is connected to dipole right blade
32. Roberts
balun 40 also has a short coaxial cable segment having outer conductor 35 and
inner
conductor 36. Roberts balun 40 is a quarter wavelength current choke. Coaxial
outer
conductor 35 is connected to dipole left blade 31. Coaxial inner conductors 35
and 45 are
connected at their top ends via wire 37 and coupled to dipole left blade 31.
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Sliding bar 38 connects the bottom end of coaxial outer conductor 35 to
coaxial
outer conductor 45. Sliding bar 38 creates a short circuit, providing an
infinite impedance
across the terminals of dipole left arm 31 and dipole right arm 32.
Fig. 3 shows dipole antenna 50 as having coaxial cable 5 coupled to a dipole
antenna using IEEE-type balun 50, sometimes referred to as a Type III balun.
The dipole
antenna forms a balanced load (or source). Coaxial cable S connects to an
unbalanced
source (or load) and is connected to IEEE-type balun 50 at connection 51 which
may be a
threaded screw-type connection. IEEE-type balun 50 has a main coaxial segment
having
outer coaxial conductor 65 and inner coaxial conductor 66. Coaxial outer
conductor 65 is
electrically connected to dipole right blade 52. Coaxial inner conductor 66 is
electrically
connected to dipole left blade 51 via wire 57. IEEE-type balun 50 also has
conductor SS
electrically connected to dipole left blade 51. IEEE-balun 50 is a quarter
wavelength
current choke. Conductor 55 is located generally parallel to the coaxial
cable. Sliding bar
58 connects the bottom end of conductor 55 to coaxial outer conductor 65.
Sliding bar 58 creates a short circuit, providing an infinite impedance across
the
terminals of dipole left arm 51 and dipole right arm 52.
The quarter wavelength current choke in each of Figs. 2 and 3 serves to reduce
common mode current. However, conventional baluns used with dipole antennas do
not
prevent radiative coupling between a coaxial cable and the dipole antenna and
do not
completely eliminate common mode current. Accordingly, there is room for an
improved
coupling between a coaxial cable and a dipole antenna.
Summary Of The Invention
In accordance with an aspect of this invention, there is provided a balanced
dipole
antenna, comprising a left dipole arm having a center end, a right dipole arm
having a
center end, a coaxial cable having an outer conductor and a single inner
conductor and a
top end electrically located between the center ends of the left and right
dipole arms, a left
stub coupling the left dipole arm and the coaxial cable, and a right stub
coupling the right
dipole arm and the coaxial cable.
The structure of the balanced dipole antenna substantially eliminates
radiative
coupling between the coaxial cable and the left and right dipole arms, and
substantially
eliminates common mode current between the coaxial cable and the left and
right dipole
arms.
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CA 02525995 2005-11-08
In a further aspect of this invention, the left and right stubs are formed of
respective lengths of coaxial cable. In this case, one of the left and right
stubs has an inner
conductor that electrically connects to the inner conductor of the coaxial
cable, and the
other of the left and right stubs has an inner conductor that electrically
connects to the
outer conductor of the coaxial cable.
In yet a further aspect of this invention, the left and right stubs are formed
of
metallic material. In this case, the inner conductor of the coaxial cable is
connected to one
of the left and right dipole arms, and the outer conductor of the coaxial
cable is connected
to the other of the left and right dipole arms.
In accordance with another aspect of this invention, a dipole antenna
comprises a
left dipole arm, a right dipole arm, a coaxial cable, and means for coupling
the coaxial
cable to the left and right dipole arms to substantially eliminate common mode
current and
radiative coupling between the coaxial cable and the left and right dipole
arms.
In accordance with yet another aspect of this invention, there is provided a
symmetric balun, comprising a left stub for coupling to a left arm of a dipole
antenna, a
right stub for coupling to a right arm of a dipole antenna, and a center
branch for
connecting to a coaxial cable, the center branch having an inner conductor and
an outer
conductor.
It is not intended that the invention be summarized here in its entirety.
Rather,
further features, aspects and advantages of the invention are set forth in or
are apparent
from the following description and drawings.
Brief Description Of The Drawings
Fig. 1 is a diagram showing a coaxial cable coupled directly to a prior art
dipole
antenna;
Fig. 2 is a diagram showing a coaxial cable coupled to a dipole antenna using
a
prior art Roberts balun;
Fig. 3 is a diagram showing a coaxial cable coupled to a dipole antenna using
a
prior art IEEE-type balun;
Fig. 4 is a diagram showing a prior art candelabra balun and transformer;
Fig. 5 is a diagram showing a coaxial cable coupled to a dipole antenna using
a
symmetric balun;
Fig. 6 is a diagram showing a telescoping dipole blade;
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Fig. 7 is a diagram showing a sliding short circuit bar; and
Fig. 8 is a diagram showing a coaxial cable coupled to a dipole antenna using
another embodiment of a symmetric balun.
Detailed Description
Fig. 4 shows a prior art candelabra balun and transformer 70 fed by a special
twin
lead cable having outer conductor 85 and two inner conductors. The twin lead
cable forms
the center branch of a candelabra structure, which also has a left branch
having left outer
conductor 75 and a right branch having right outer conductor 95.
Left outer conductor 75 and right outer conductor 95 are electrically
connected to
outer conductor 85 below sliding bar 78.
Sliding bar 78 creates a short circuit between outer conductors 75, 85, 95.
A central segment having outer conductor 78 is located at the center of the
candelabra structure next to the top of outer conductor 85. The bottom of
outer conductor
78 is not electrically connected to sliding bar 78.
Inner conductor 86 of the twin lead cable continues to the top of outer
conductor
85.
Inner conductor 76 of the twin lead cable feeds into the left branch of the
candelabra structure.
Conductor 79 has a U-shape and is located inside the left branch of the
candelabra
structure and inside the center branch of the candelabra structure.
The right branch of the candelabra structure has inner conductor 96.
The central segment of the candelabra structure has inner conductor 84.
Wire 77 couples inner conductors 76 and 79 of the left branch of the
candelabra
structure to inner conductor 84 of the central segment of the candelabra
structure.
Wire 87 couples inner conductors 86 and 79 of the special twin lead cable
forming
the center branch of the candelabra structure to inner conductor 96 of the
right branch of
the candelabra structure.
Candelabra balun and transformer 70 provides a transformation ratio of 4:1.
Adding more branches, namely a total of three arms on each side of the center
branch,
provides a transformation ratio of 9:1. A total of four arms on each side of
the center
branch, provides a transformation ratio of 16:1.
Embodiments of a balanced dipole antenna will now be discussed.
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A balanced dipole antenna has a coaxial cable connected between a load or
source
and the left and right dipole arms to substantially eliminate common mode
current and
radiative coupling between the coaxial cable and the left and right dipole
arms. The
connection between the source/load coaxial cable and the left and right dipole
arms is a
symmetric balun having a center branch that is an extension of the source/load
coaxial
cable, and left and right stubs.
When the stubs are segments of coaxial cable, the outer conductors of the left
and
right stubs of the symmetric balun are respectively coupled to the left and
right dipole
arms, and one of the inner conductors of the left and right stubs is connected
to the inner
conductor of the center branch, while the other of the inner conductor of the
left and right
stubs is connected to the outer conductor of the center branch.
When the stubs are metallic, the inner conductor of the center branch is
electrically
connected to one of the left and right dipole arms, while the outer conductor
of the center
branch is electrically connected to the other of the left and right dipole
arms. A sliding bar
at the base of the stubs electrically connects the outer conductors of the
left and right stubs
and the center branch.
A dipole antenna using a first embodiment of a symmetric balun according to
the
present invention will now be discussed.
Fig. 5 shows balanced dipole antenna 100 having coaxial cable 5 electrically
connected to a dipole antenna using symmetric balun 110. Balanced dipole
antenna 100
can be tuned for use over various frequencies, for instance over the 300 MHz -
1 GHz
range.
The dipole antenna forms a balanced load (or source). Left dipole arm 101 and
right dipole arm 102 each have a length slightly less than 7J4, where ~, is
the free space
wavelength of a center frequency of a bandwidth of signals being received or
transmitted.
Accordingly, the total length of balanced dipole antenna 100, including the
width of
symmetric balun 110, is about ~J2. Left and right dipole arms 101, 102 are
adjustable to
the correct wavelength.
Fig. 6 is a diagram showing a telescoping dipole blade.
Coaxial cable 5 connects to an unbalanced source (or load) and is connected to
symmetric balun 110 at connection 111 which may be a threaded screw-type
connection,
as shown in the circular inset of Fig. 5.
CA 02525995 2005-11-08
Symmetric balun 110 has a left stub having outer coaxial conductor lOS and
inner
coaxial conductor 106, a center feeding branch that is a coaxial cable having
outer coaxial
conductor 11 S and inner coaxial conductor 116, and a right stub having outer
coaxial
conductor 12S and inner coaxial conductor 126.
Sliding bar 108 is located at the base of the left and right stubs and the
center
branch and electrically connects outer coaxial conductors l OS, 11 S, 125,
creating a short
circuit, to provide an infinite impedance across the terminals of dipole left
arm 101 and
dipole right arm 102. Sliding bar 108 is adjusted so that the height of the
stubs between
dipole left and right arms 101, 102 and sliding bar 108 is about a/4.
Fig. 7 is a diagram showing a sliding short circuit bar.
Inner coaxial conductor 116 extends from the top of the center branch to the
bottom of the center branch. Connection 111 is located at the bottom of the
center branch
and serves to electrically connect inner coaxial conductor 116 of the center
branch to inner
coaxial conductor 16 of coaxial cable S, and to electrically connect outer
coaxial
conductor 11 S of the center branch to outer coaxial conductor 1 S of coaxial
cable S.
Inner coaxial conductors 106, 126 extend from the top of the left and right
stubs
downwards to somewhat above the location of sliding bar 108. The height of
inner
coaxial conductors 106, 126 is about 7~g/4, where ~,g is the wavelength of a
center
frequency of a signal being received or transmitted inside the coaxial
segments of the left
and right branches.
Wire 107 electrically connects inner coaxial conductor 106 of the left branch
to
inner coaxial conductor 116 of the center branch. Inner coaxial conductor 106
is
electrically coupled to left dipole arm 101.
Wire 117 electrically connects inner coaxial conductor 126 of the right branch
to
outer coaxial conductor 11 S of the center branch. Inner coaxial conductor 126
is
electrically coupled to right dipole arm 102.
At the top exposed end of inner coaxial conductor 116, outer coaxial conductor
lOS of the left stub is electrically connected to left dipole arm 101, and
outer coaxial
conductor 12S of the right stub is electrically connected to right dipole arm
102.
Symmetric balun 110 comprises the left and right stubs and center branch and
sliding bar 108. Symmetric balun 110 is a quarter wavelength current choke.
A dipole antenna using a second embodiment of a symmetric balun according to
the present invention will now be discussed.
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Fig. 8 is a diagram showing coaxial cable 5 electrically connected to dipole
antenna 200 using symmetric balun 210. Symmetric balun 210 is similar to
symmetric
balun 110, except that balun 210 uses metallic stubs on either side of its
center coaxial
branch, whereas balun 110 uses coaxial stubs on either side of its center
coaxial branch.
The dipole antenna forms a balanced load (or source). Left dipole arm 211 and
right dipole arm 212 each have a length slightly less than a.14, where ~, is
the free space
wavelength of a center frequency of a bandwidth of signals being received or
transmitted.
Accordingly, the total length of balanced dipole antenna 200, including the
width of
symmetric balun 210, is about a.J2. Left and right dipole arms 211, 212 are
adjustable to
the correct wavelength.
Conductor 217 connects left dipole arm 211 to outside shield 215 of the center
branch of symmetric balun 210. Conductor 218 connects right dipole arm 212 to
feeding
center conductor 216 of the center branch of symmetric balun 210.
The center branch of symmetric balun 210 is electrically connected to coaxial
cable 5 via connector 220, which may be a threaded screw-type connection.
Center
conductor 16 of coaxial cable 5 is in electrical contact with feeding center
conductor 216
of the center branch. Outer shield 15 of coaxial cable 5 is electrically
connected to outside
shield 21 S of the center branch.
Left metallic stub 213 and right metallic stub 214 each have a length of 7,/4
and are
respectively connected to left dipole arm 211 and right dipole arm 212.
Sliding bar 219 is located at the base of the left and right stubs and the
center
branch and electrically connects conductors 213, 214, 215 creating a short
circuit, to
provide an infinite impedance across the terminals of dipole left arm 211 and
dipole right
arm 212. Sliding bar 219 is adjusted so that the height of the left and right
stubs between
dipole left and right arms 211, 212 and sliding bar 219 is about 7J4.
To measure the effectiveness of the common mode current choke, balanced dipole
antenna 200 using symmetric balun 210 with adjustable telescoping arms 211,
212 was
constructed. The length of the telescoping arms 21 l and the position of
shorting bar 219
were adjusted to minimize the common mode current of dipole antenna 200 at the
operating frequency.
Dipole antenna 200 and a commercially available dipole antenna with tunable
frequency according to Fig. 2 were tested in an anechoic test chamber. A
horizontal-
vertical measurement antenna was at one end of the chamber, at the opposite
end were a
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CA 02525995 2005-11-08
horizontal dipole antenna 200 and a vertical dipole antenna 200 arranged in a
cross-
fashion and mounted on a supporting mast. Alternatively, a horizontal
commercially
available dipole antenna according to Fig. 2 and a vertical commercially
available dipole
antenna according to Fig. 2 were arranged in cross-fashion and mounted on a
supporting
mast in the chamber. The test results are shown in Table 1. The vertical and
horizontal
numbers represent the path loss measured in dB. The difference numbers are
simply the
difference between vertical path loss and horizontal path loss.
TABLE 1
frequency
measurements are in (MHz) 730 764 799 822 915 1000
dB
Vertical -53.7-51.1-50.5-50.3-49.2-49.1
Commercially AvailableHorizontal-53.3-52.1-S -51.4-48.9-49.6
Dipole 1.7
Difference-0.4 1.0 1.2 1.1 -0.3 0.5
Vertical -53.1-51.7-50.2-50 -47.6-48.5
Balanced dipole 200 Horizontal-53.1-52.2-50.4-50.5-48.1-48.7
Difference0.0 0.5 0.2 0.5 0.5 0.2
Table 1 shows that balanced dipole antenna 200 has less path loss (higher
gain)
and less difference in vertical and horizontal path loss difference; the path
loss difference
is caused by the common mode current on the feeding cable. At a frequency of
750 MHz,
balanced dipole antenna 200 is perfectly balanced. Table 1 demonstrates that
balanced
dipole antenna 200 substantially eliminates radiative coupling and common mode
current.
Although an illustrative embodiment of the present invention, and various
modifications thereof, have been described in detail herein with reference to
the
accompanying drawings, it is to be understood that the invention is not
limited to this
precise embodiment and the described modifications, and that various changes
and further
modifications may be effected therein by one skilled in the art without
departing from the
scope or spirit of the invention as defined in the appended claims.
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