Coaxial dipole antenna with extended effective aperture

A coaxial dipole antenna includes a first radiator which is approximately one quarter wavelength long. A second radiator exhibits length less than one quarter wave length and is coupled to the feed port by a reactive element which has an electrical reactance which is insufficient to increase the electrical length of the second radiator to one quarter of the wavelength. The length of a dipole antenna is substantially shortened while an effective aperture of one half wavelength is maintained by causing a portion of the transceiver housing to radiate in phase with the antenna.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of dipole antennas and more 
particularly to dipole antennas which are designed for use with small 
portable transceivers where it is desirable to shorten the overall length 
of the antenna while retaining acceptable electrical performance. 
2. Background of the Invention 
As improved integrated circuit technology allows portable transceivers to 
be reduced in size, it is also desirable to reduce the overall length of 
the antenna structures used with such radios. Not only is reduction of the 
size of the antenna appealing from the point of view of aesthetics and 
marketability, it is also vital to the improved portability and 
inconspicuousness of such two-way transceivers. For example, such 
miniature transceivers are often utilized for security and surveillance 
applications where the size of the antenna is a limiting feature in the 
user's ability to conceal the transceiver and thereby attain maximum 
strategic effectiveness of the communication system. 
One of the smallest antenna structures frequently used with portable 
transceivers is the quarter wavelength whip antenna. However, as one 
skilled in the art will readily appreciate, the quarterwave whip antenna 
requires an extensive ground plane or a large counterpoise at its base in 
order to radiate effectively and predictably. Since this is not generally 
the case with a portable transceiver, the radiation patterns and other 
electrical parameters are somewhat unpredictable and indeed vary 
drastically as a function of the manner in which the user holds, carries 
or uses the radio. A half-wave dipole antenna requires no such extensive 
ground plane and produces much more desirable and predictable electrical 
performance although it is considerably larger. 
FIG. 1 shows a typical half-wave coaxial dipole antenna structure as is 
commonly used with portable transceivers. The prime disadvantage of this 
structure is that its length L is significantly longer than twice the 
length of a quarter-wave whip antenna and may even be substantially longer 
than the transceiver itself. It does, however, have excellent radiation 
characteristics. 
In FIG. 1 a wire radiator 20, which is approximately one quarter of a 
wavelength in air, is fed by the inner conductor 25 of a coaxial 
transmission line 30. A dielectric insulator 32 separates inner conductor 
25 from an outer conductor 35. The outer conductor 35 of coaxial 
transmission line 30 is electrically coupled to feed a metallic sleeve 40 
which is also approximately one quarter of a wavelength in air. In order 
to improve the compactness of this antenna structure, metallic sleeve 40 
is normally disposed about of a portion of coaxial transmission line 30, 
with a uniform dielectric spacer 45 positioned to maintain the proper 
physical relationship between the coaxial line 30 and the metallic sleeve 
40. Dielectric spacer 45 is generally cylindrical in shape and serves to 
establish an outer transmission line 47 wherein the outer conductor is 
metallic sleeve 40 and the inner conductor is the outer conductor 35 of 
coaxial transmission line 30. This outer transmission line is 
approximately one quarter of a wavelength in the dielectric material of 
spacer 45. Outer transmission line 47 serves to choke off radiating 
currents in transmission line 30 and prevent excitation of the radio 
housing in order to properly control the electrical parameters of the 
dipole antenna. 
FIG. 2 is a combined perspective view and current as a function of length 
diagram showing the relative magnitude of the antenna current I along the 
length of this half-wave dipole structure when the antenna is mounted to a 
transceiver housing. In this figure the length axis is not scaled but 
rather a perspective view of a transceiver with antenna is shown adjacent 
the graph to indicate where the relative current is present on a 
particular portion of the structure. The distribution of current I for 
this structure is consistent with that of a properly functioning half-wave 
dipole antenna of overall length L1. In operation, the outer coaxial 
transmission line effectively chokes off nearly all currents from the 
transceiver housing and only a small quantity of out-of-phase radiating 
currents are radiated by the transeiver housing. These currents cause only 
a slight deviation from the radiating pattern of an ideal dipole antenna. 
Although this antenna structure is an effective radiator, its overall 
length L1 is approximately 200 mm for transceiver operation in the 860 MHz 
frequency range. As the size of modern transceivers decreases this is an 
unacceptably long antenna structure. 
In a U.S. copending application, Ser. No. 452,166, filed Dec. 22, 1982, 
having the same Assignee as the present invention, a coaxial dipole 
antenna is disclosed which utilizes series inductance in a coaxial sleeve 
and a resonant tank on the wire radiator to obtain two sharp and distinct 
narrow resonant peaks. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved antenna for 
a portable transceiver. 
It is another object of the present invention to provide a shortened 
coaxial dipole antenna structure for a portable transceiver which excites 
the transceiver's housing in order to extend the effective radiating 
aperture of the antenna structure. 
It is another object of the present invention to provide an antenna 
structure which is substantially shorter than a half-wave dipole antenna 
yet provides approximately the same performance as a half-wave dipole. 
It is a further object of the present invention to provide a coaxial dipole 
antenna structure exhibiting broad bandwidth and half-wave dipole 
performance in a considerably shorter configuration. 
In one embodiment of the present invention a shortened dipole antenna for 
use with portable transceivers, includes a feed port having a first and a 
second input terminal and a first radiator element coupled at one end to 
the first input terminal. This first radiator element exhibits an 
electrical length approximately one quarter of a predetermined wavelength 
and extends outward from the feed port in a first direction. A second 
radiator element exhibits a length less than one quarter of the 
predetermined wavelength and extends outward from the feed port in a 
direction which is substantially diametrically opposed to the first 
direction. A reactive element couples the second radiator at the end 
closest to the feed port with the second input terminal and has an 
electrical reactance insufficient to increase the electrical length of the 
second radiator to one quarter of the predetermined wavelength. 
The features of the invention believed to be novel are set forth with 
particularity in the appended claims. The invention itself however, both 
as to organization and method of operation, together with further objects 
and advantages thereof, may be best understood by reference to the 
following description taken in conjunction with the accompanying drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to FIG. 3, a wire radiator 100 having length of approximately 
one quarter of a wavelength in air at the predetermined frequency of 
interest is electrically coupled to be fed by the inner conductor 105 of a 
coaxial transmission line 110. The junction of the coaxial transmission 
line 110 and wire radiator 100 forms one circuit node or terminal 114 of 
feed port 115. A metallic sleeve radiator 120 is disposed about coaxial 
transmission line 110 and is substantially less than one quarter of the 
predetermined wavelength in air. In the preferred embodiment the length of 
the sleeve radiator 120 is approximately 0.084 wavelengths long in air at 
860 MHz. 
At a second circuit node or terminal 116 of feed port 115, the outer 
conductor 125 of coaxial transmission line 110 is coupled to one end of an 
inductor 130. The other end of inductor 130 is connected to metallic 
sleeve 120. The inductance value of inductor 130 is such that when placed 
in series with metallic sleeve 120 the equivalent electrical length of the 
series combination is still significantly less than one quarter of the 
predetermined wavelength in air. In the preferred embodiment, an inductor 
130 has 1.2 turns of conductor, wound with the same diameter as the sleeve 
radiator and having a total length of 0.017 wavelengths has been found 
acceptable for operation at 860 MHz. A dielectric spacer 135 substantially 
cylindrical in shape maintains the proper physical relationship between 
metallic sleeve 120 and coaxial transmission line 110. The end of coaxial 
transmission line 110 is terminated in an appropriate connector 140 for 
connection to the transceiver. 
FIG. 4 is a cross-sectional view along line 4--4 of FIG. 3 which more 
clearly shows the relative location of each of the elements within 
metallic sleeve 120 of the present invention. It is readily seen that 
coaxial transmission line 110 is made of an inner conductor 105 surrounded 
by a dielectric material 145 which is then covered with an outer conductor 
125. In the preferred embodiment a 93 ohm coaxial transmission line, 
commercially available as RG 180, is used. Coaxial transmission line 110 
is surrounded by dielectric spacer 135, which is preferrably made of 
Polytetraflourethylene such as Dupont Teflon.RTM. or similar substances 
with a dielectric constant of approximately 2.2, and is covered by 
metallic sleeve 120. As with the prior art dipole antenna a second 
transmission line is formed by the combination of outer conductor 125, 
dielectric spacer 135 and metallic sleeve 120. Unlike the prior art 
half-wave coaxial dipole, this second transmission line only attenuates or 
partially chokes off electro-magnetic energy from being transferred from 
the antenna to the transceiver housing. This partial attenuation is 
desired with the present invention to excite a portion of the radio 
housing electro-magnetically in order to produce in-phase radiation of 
energy therefrom. The sleeve is coupled, for example by stray capacitance, 
to a transceiver housing or other structure and excites it as if it were 
part of the antenna structure. This results in an effective radiating 
aperture of one half wavelength. The overall length of the resulting 
antenna structure L2 is substantially shorter than the length L1 of the 
prior art sleeve dipole. In fact, in the preferred embodiment of the 
present invention a 25% reduction in overall length was attained while 
obtaining superior performance between 820 MHz and 900 MHz. 
FIG. 5 shows the critical details and dimensions for an embodiment of the 
present invention which is designed to operate in the range from 
approximately 820 to 900 MHz with a reflection coefficient of less than 
0.3 throughout the designated frequency band. In this embodiment, the 
quarter wave wire radiator 100 is formed from the inner conductor 105 of 
coaxial transmission line 110 shown in phantom. The dielectric insulator 
145 of the coaxial transmission line 110 is left in place along the entire 
length to enhance the structural rigidity of wire radiator 100. Due to the 
asymmetry in the structure at feed port 115 (more clearly shown in FIG. 
3), the characteristic impedance at that port was found to be 
extraordinarily high for a dipole type structure. A measured impedance of 
approximately 200 ohms has been detected at the feed port. In order to 
transform that impedance to a more useful and desirable 50 ohms, a quarter 
wave coaxial transmission line 110 having characteristic impedance of 93 
ohms is preferrably utilized and terminated in a 50 ohm SMA type 
connector. This provides impedance matching from the feed port 115 to 
connector 140. 
Inductor 130 in the structure is preferably formed by cutting metallic 
sleeve 120 in a metallic strap helix-like configuration. In many instances 
it is estimated that the inductance requirement will result in less than 2 
turns of the helix to form inductor 130. In the preferred embodiment the 
total rotational angle traversed by inductor 130 from point N to point M 
is approximately 426.degree.. Connection from outer conductor 125 to 
inductor 130 is attained by a conductive cap 150. This conductive cap 150 
is a disk or washer shaped metallic member having outer diameter 
approximately that of the dielectric spacer 135 and a hole in the center 
whose diameter is appropriate to allow passage of the wire radiator and 
dielectric insulator 145. This conductive cap 150 is electrically coupled, 
preferrably by soldering, to both inductor 130 and the outer conductor 
125. 
The principal dimensions A through K for the preferred embodiment as shown 
in FIG. 5 for this structure are tabulated below for operation between 
approximately 820 MHz and 900 MHz with a reflection coefficient of 0.3 or 
less and may be appropriately scaled for other frequency ranges: 
______________________________________ 
A 2.6 mm 
B 72.0 mm 
C 5.8 mm 
D 2.5 mm 
E 29.5 mm 
F 7.9 mm 
G 2.0 mm 
H 42.9 mm 
I .5 mm 
J 3.7 mm 
K 28.9 mm 
______________________________________ 
These dimensions should be viewed as approximate as actual dimensions will 
vary slightly due to variations in construction practices, etc. These 
dimensions may also require a slight adjustment to account for differences 
in transceiver housings although in general the parameters of the 
transceiver housing are non-critical. 
The relative magnitude of the antenna current I is shown in FIG. 6 for the 
antenna of the present invention in a graph constructed similar to that of 
FIG. 2. It is evident that the upper portion of the transceiver housing or 
other mounting structure forms a substantial portion of the effective 
half-wave radiating aperture. Thus, this invention provides an effective 
half-wave radiation aperture similar to the half-wave dipole while 
occupying 25% less overall length in the preferred embodiment. It has been 
found that the current radiating from the housing is substantially in 
phase with the current along the antenna resulting in a positive 
re-enforcement of transmitted energy rather than a cancellation. As would 
be expected some out-of-phase excitation also occurs in the lower portion 
of the ratio housing resulting in slight deviation from ideal dipole 
characteristics. 
FIG. 7 shows a plot of the magnitude of the reflection coefficient for the 
antenna of the preferred embodiment of the present invention, curve 190, 
compared with that of the prior art half-wave coaxial dipole, curve 195. 
The 0.3 reflection coefficient bandwidth of each antenna may be determined 
from this plot by reading the frequencies, from the horizontal axis, at 
which each curve intersects a horizontal line passing through the vertical 
axis at 0.3 and substracting the lower frequency from the higher 
frequency. It is evident from this plot that this invention produces an 
extremely low Q broadband antenna which is usable over a 20% broader range 
of frequencies than the prior art dipole assuming an antenna is useful for 
a reflection coefficient of less than 0.3. 
FIG. 8 shows actual radiation patterns of the antenna of the present 
invention as compared with the prior art coaxial dipole taken under 
identical conditions while individually mounted to the same transceiver 
housing. Curve 200 is for the prior art coaxial dipole while curve 210 is 
for the present invention. One skilled in the art will readily recognize 
that there is very little practical difference in the performance of these 
two antennas. In each case the butterfly wing shape of the curve is the 
result of stray out-of-phase excitation of the housing as is well known in 
the art. An ideal half-wave dipole would have a pattern that is closer to 
a figure 8 shape. 
In the preferred embodiment, the present antenna is coated with a rubber 
material to improve its appearance and structural integrity. This rubber 
material slightly changes the effective electrical length of the wire 
radiator and the metallic sleeve as is also well known in the art. These 
characteristics may be compensated for by slightly adjusting the length of 
each of these elements until proper performance is attained. The overall 
result is a slight shortening of the elements relative to the dimensions 
necessary for the uncoated antenna. 
FIGS. 9 and 10 show the relative sizes and shape factors of the resulting 
antenna complete with rubber encapsulant of the present invention 300 as 
compared with that of the prior art coaxial dipole 310. A reduction of 50 
mm in length (25%) was obtained in the preferred embodiment. The amount of 
length reduction attainable by this invention is of course dependent upon 
the frequency of operation along with the exact construction method. 
Thus it is apparent that in accordance with the present invention an 
apparatus that fully satisfies the objectives, aims and advantages is set 
forth above. While the invention has been described in conjunction with a 
specific embodiment, it is evident that many alternatives, modifications 
and variations will become apparent to those skilled in the art in light 
of the foregoing description. Accordingly, it is intended that the present 
invention embrace all such alternatives, modifications and variations as 
fall within the spirit and broad scope of the appended claims.