In spite of the proliferation
of cellphones and near ubiquitous communications, there are still many applications
that require private 2-way communications systems. Emergency services like police,
fire, and ambulance; amateur radio, vehicular dispatch for utilities, delivery
and repair services; and anywhere that cellular service is not either available
or extremely reliable, cannot rely on cellphones for mission critical needs.
There are a lot of legacy 2-way radio system antennas and associated towers
still being used and many new installations in place. Word has it that use
of Citizen Band (CB) radio is on the rise amongst not just truck drivers but
everyday drivers and base station operators - largely for the anonymity factor. This article in
TV News gives a good overview of the issues of concern with 2-way radio antennas
Antenna Applications in Two-Way Radio Systems
An 8-stack. 48-element array operating in 160-mc. range
for a pipeline company in Arabia. All elements are in phase. Total gain is
about 17 db over single dipole.
By Tom J. McMullin - Communications Engineering Co.
Practical information on performance of directive antennas for mobile radio
bands 25-50 mc., 72-76 mc., 148-174 mc.
With the growth of two-way radio, greater use is being made of antennas which
will improve system performance and and reduce interference. Particularly this
true in the 25-50 mc. band where maximum performance is required in the face
of growing interference problems due to channel crowding and concentration of
In the past several years considerable improvements have been made in transmitting
and receiving equipment to help meet this problem. Since the antenna is a most
vital part of the two-way system, it too must keep pace with developments and
be used where applicable to the requirements.
In the mobile two-way radio field, improvement of system performance through
use of antennas is limited, for practical reasons, to the base and fixed stations.
This discussion will, therefore, be limited to antennas for base and fixed stations
in the commonly used bands of 25-50 mc., 72-76 mc., and 148-174 mc. with emphasis
on the 25-50 mc. band where most of the systems requiring maximum range operate.
Fig. 1 - Measured horizontal radiation pattern of bi-directional
Use of Directive Antennas
Directive antennas can be used to: 1) improve coverage where the area is
not circular in shape or where the base station is not in the center of the
area; (2) reduce interference between stations in the same area; and, (3) reduce
interference from noise sources which are directional.
Where can directive antennas be used to improve base station coverage? Most
engineers already are familiar with the use of directive antennas for fixed
station point-to-point application such as the remote control of a base station
by means of a radio control circuit. Many do not realize, however, that directive
antennas have a wide field of application for base station coverage.
Radiation patterns of directive antennas are misleading in terms of what
the range pattern will be. The usual pattern shown in values of relative power
or relative voltage makes the antenna look very directive, which in turn makes
the range look attractive in the major lobes and very poor in other directions.
The range of a system does not vary directly with the voltage or power of the
system. "Line-of-sight" distances can be covered with very low power, while
distance beyond the radio horizon require greatly increasing values of power.
This factor tends to smooth out the "nulls" and "lobes" of a directive antenna
pattern when the pattern is translated into terms of range.
Fig. 1 shows the conventional figure-8 pattern of a bi-directional antenna
plotted in terms of radiation in relative voltage (inside curve), and the coverage
pattern which this becomes (outside curve) when translated into terms of range
at antenna height of 200 feet, for a 50-watt mobile unit, 50 mc. talk-back to
the station with a signal of 1.5 microvolts at the 50-ohm input of the receiver.
Note how the figure-8 voltage pattern is changed to more nearly a rectangular
pattern in terms of actual operating range.
Fig. 2 - Range chart showing effect of base station
antenna gains and tower heights.
Range patterns for any given directive antenna or combinations of directive
antennas similarly can be plotted for given tower height using range charts
or range calculators which are available. Fig. 2 is a range chart showing
how the range varies with antenna gain respective to a vertical dipole (ground
plane or coaxial half-wave radiator) based upon average conditions at 50 mc.,
for a 50-watt mobile unit talking back to the station with a signal strength
input to the station receiver of 1.5. microvolts. On such charts or calculators,
decibels (db) is the most convenient form to express the gain since db gain
can be easily added or subtracted. If the antenna gain or loss is known at various
azimuths, the range can be quickly determined at those azimuths for any given
antenna height. Such charts also show the relative gain from using a directive
antenna compared to that of more tower height, and indicate where it is economically
advantageous to use antenna gain instead of increasing power or using a higher
In order to conveniently use available charts and calculators to plot antenna
range patterns, it is desirable that the antenna pattern in the horizontal plane
be plotted in terms of decibels gain (or loss) with respect to a dipole antenna
at the same elevation.
Fig. 3 - Power gain (in db) in the horizontal plane
of bi-directional and unidirectional arrays operating at 30-50 mc.
Figs. 3A, 3B, and 3C show typical patterns plotted in this manner from data
taken on field measurements for: a bi-directional antenna, a 3-element Yagi
directional antenna, and a 6-element directional antenna composed of two 3-element
Yagis fed in-phase. The 6-element directional antenna can be changed to a bi-directional
by reversing directions on one of the 3-element Yagi sections. Its gain in each
direction will be somewhat greater than that of the 2-element bi-directional
A side of tower mount antenna which gives an essentially
omnidirectional pattern. Half-wave folded dipoles fed in phase at fixed distance
mounts are attached to opposite legs or sides of tower. Vertical spacing is
from three-quarters to a full wavelength center to center. Frequency range
is 25-50 mc. A small 450-mc. Yagi antenna with a reflector and 3 directors
is also shown near the upper folded dipole.
Thus, it can be seen that "range patterns" can be plotted for a directive
antenna, a combination of directive antennas, or combination of directive antenna
with non-directional antenna for a given frequency range and given antenna height.
If the range scale is selected to match a common map scale, the directive antenna
or combination of antennas can be selected which will most nearly fit the coverage
area outlined on the map. Height of tower can be scaled down or up without materially
changing the shape of the pattern - just the size.
Stacking for Added Gain
In the antenna arrays just discussed most of the gain is obtained by changing
the horizontal pattern from a circular configuration. If more gain is desired,
and the tower height will warrant the installation, two or more antennas can
be vertically stacked. The horizontal pattern shape will remain essentially
the same and the added gain will be obtained by compressing the vertical pattern
closer to the horizon. The pattern can be broadened by fanning out the stacked
antennas with corresponding loss in gain in the maximum direction.
In the 150-mc. band, as many as eight 6-element antennas (CE286) can be vertically
stacked in a collinear array with a spacing between centers of around one wavelength,
fed in-phase, and will have a gain of approximately 17 db over a dipole at the
same height. This is the equivalent of increasing the power by over 50 times.
On station-to-station contact with directional arrays at each end, the net gain
becomes the multiple of the antenna gains at each end. In such a case, gains
of around 2500 times in power are practicable. This means lower tower height,
less transmitter power, and circuits which would not be possible with a standard
Directive antennas can also be used in conjunction with non-directive or
general coverage antennas to cover a corner or sector of the area not being
reached by the general coverage antenna. Maximum efficiency is obtained by bringing
down separate antenna leads to separate receivers and switching the transmitter
to the desired antenna, automatically muting the receiver not used. At a sacrifice
of around 3 db in each antenna, these antennas can be paralleled at the top
of the tower and fed in-phase by one cable. The directive antenna should be
placed approximately one-half to a full wavelength above or below the non-directive
antenna. The vertical separation between the antennas should be such that the
near elements are separated by around one-quarter wavelength and in most cases
it is desirable that they be fed in-phase.
Effect of Noise
Although an antenna is a bi-lateral device and works equally well on transmit
and receive, certain considerations must be given to ambient electrical noise
picked up by the antenna. If the noise is generally distributed throughout 360°,
then the signal-to-noise improvement will be comparable to the gain of the antenna.
If the noise comes principally from sectors lying outside of the maximum radiation
lobes, then the signal-to-noise improvement may be greater than the gain of
the antenna. Conversely, if the noise sources lie principally in the beam of
the antenna, there may be little or no improvement in signal-to-noise over a
non-directional antenna. In planning station installations wherein directional
antennas will be used, it is a wise practice to locate the station tower on
the side of the city or built-up industrial area where the antenna will not
have to shoot over the high noise levels. In checking for noise sources surrounding
the proposed tower location, it is also good practice to investigate at further
distances in the general direction the antenna will point than would be done
for a non-directional antenna. It must be remembered that the gain of the antenna
will effectively bring the noise source closer in the same manner that field
glasses will bring a visible object closer.
Reducing Signal Interference
It is apparent that many cases of co-channel interference in the same general
area can be reduced by practical application of directional antennas. The greatly
increased expansion of radio facilities and the limitation of useful frequencies
available point more and more to the future necessity of putting the signal
only where it is required. In certain instances, considerable alleviation of
skip interference be achieved, although in most cases this is difficult because
of conflicting demands of system coverage plus skip interference.
In addition to co-channel interference, the effects of interference from
transmitter noise, desensitization and intermodulation often can be successfully
countered by antenna directivity. In general, a reduction of 7 db is the equivalent
of moving the interfering source twice as far away and controlling antenna pattern
radiation often is a practical method of reducing a troublesome signal.
Side Mounted Antennas
A 6·element unidirectional array operating at 72-76 mc.
Consists of two 3-element Yagi antennas one-half wavelength apart and fed in
phase. Gain is 10 db over dipole. Matches a 50·ohm coaxial line.
At v.h.f. and u.h.f. frequencies commonly used in two-way communications,
the only purpose of a high tower is to elevate the antenna above the ground
level and extend the radio horizon. A high tower is generally one of the major
items of expense, as well as the major "headache" item in installing a system.
The increasing difficulties of obtaining suitable quiet locations at reasonable
cost around cities with fast growing industrial and residential areas, which
are sufficiently removed from airports and airways, have caused a number of
users to mount antennas on the sides of existing two-way or broadcast and TV
towers. Where the frequencies are compatible, this often is a satisfactory solution
and results in greatly reduced capital investment and flexibility if change
in location may be required. It also may be a good investment or return on investment
for the tower owner, and a number of tower contractors are erecting towers up
to 500 feet high for the express purpose of leasing space to two-way users.
With modern equipment of good design, stations as close as 500 kc. in frequency
can be located on the same tower without interference. At some locations five
or more separate systems are located on the same tower with separate antenna
systems and are operating without interference.
The conventional coaxial or ground plane antenna mounted on a support arm
out from the side of the tower is not well suited to such application. Unless
properly balanced around the tower, a number of such installations tend to overload
the tower and endanger its safety during high wind or heavy icing. Of greater
consideration, however, is the strong possibility of upsetting the antenna radiation
pattern due to the capacitance and shielding effect offered by the tower structure
itself. Quite often an otherwise good antenna will show decided directional
effects that are quite unpredictable.
To overcome this problem and to simplify installation, antennas have been
developed consisting of folded dipoles mounted close into the tower, vertically
stacked on opposite sides and fed in phase. The antennas are provided with integral
mountings to keep them at a fixed distance from the tower structure. The capacitance
of the tower is taken into consideration in the adjustment and matching of the
antenna. When these dipoles are mounted on opposite sides of the tower and staggered
vertically so that the spacing from center to center is around three-quarter
wavelength (near ends separated by at least several feet), the effective horizontal
radiation pattern will be very nearly circular. The natural question is, "How
does such an antenna perform compared to a ground plane or coaxial half-wave
antenna mounted on top of the tower?"
Such an antenna is practical only where the tower cross section dimension
is less than one-quarter wavelength of the operating frequency. Otherwise, radiation
from the dipoles on opposite sides would produce considerable distortion from
a circular pattern and we would be back where we started with a standard antenna.
Table 1 - Gains in typical installations.
At 150 mc., applications for use of folded dipoles for side-of-tower mounting
are limited to relatively small towers - if an omni-directional pattern is desired.
If the tower faces exceed approximately 10 inches, out-of-phase components will
partially cancel the effective gain. For larger towers this means that only
the top of the tower can be utilized - using a pipe extension. If an omni-directional
pattern is not required, and coverage primarily is over a 180-degree sector,
the half-wave elements can be stacked on the same tower leg, with resultant
gain depending on the number of elements employed.
In the 25-50 mc. band, tower faces of between two to four feet can be used
and this makes available most guyed towers. Table 1 lists the gains to be realized
on typical installations compared to a half-wave dipole at the same effective
If one 180-degree sector is to be favored. then the half-wave folded dipole
elements can be stacked in-line on the same tower leg. This will result in up
to 3 db additional gain and a corresponding loss over the back 180-degree sector.
The side-mount antenna is easy to mount and readily accessible for inspection.
It does not clutter up the tower appearance and has minimum effect upon the
impedance of an AM broadcast tower. Furthermore, experience has shown that the
side-mount antenna is less susceptible to lightning damage and to precipitation
A 3-element Yagi unidirectional array composed of coax radiator,
director, and reflector. Gain is 6 db over dipole. operates at 33-50 mc. with
50 ohm input impedance. Stainless steel whips are used to reduce vibration
fatigue for oil well drilling rig and barge installations.
Design of Directive Antennas
It is said that there is nothing new under the sun and basically this is
true in the design of antennas. The principles of antenna radiation and how
to direct and shape that radiation have been known for many years. Basic electrical
designs have been developed, all of which perform well under proper applications.
The application is important because we know that best performance always can
be obtained when the design covers a specific purpose. An antenna which must
cover a wide band of frequencies cannot be designed to have as much gain or
as a low standing wave ratio as one which must cover only a relatively narrow
Above all, the electrical design must be selected which will best complement
the mechanical design. There is little point in reaching for high gain performance
if it will be short lived by certain early failure due to mechanical limitations
inherent in the design. In the most simple terms, this means that a good antenna
should work well and live long.
High gain antennas and arrays can be as reliable as simple half-wave antennas
if proper attention is given to the mechanical design. A good mechanical design
should have the following characteristics:
1. A high strength-to-weight ratio. Materials and construction should be
used which will withstand the force and stress of maximum wind velocities and
ice loading to be expected. Light weight means less tower loading and easier
2. Materials which are resistant to the effects of weathering; corrosion,
and electrolysis. This applies to the antenna elements, insulation, hardware
and mountings. To avoid electrolytic corrosion, metals of widely varying chemical
potentials should not be used in direct contact.
3. Insulation should be used only where necessary and then not between high
voltage points. It is also desirable to avoid the use of insulating materials
at points of high mechanical stress.
4. Plugging of tubular elements to lessen vibration fatigue and fastenings
which will not loosen with wind vibration. The small but constant force of wind
vibration can be just as damaging as a hurricane if precautions are not taken.
Directive and special purpose antennas are now commercially available which
have the mechanical and electrical reliability of the simpler dipole antennas
commonly used. This makes it possible for system planning engineers to use the
techniques of the broadcast service where the radiation is directed and patterned
to where it counts-and pays. In the mobile service it pays in terms of improved
coverage, shorter towers, and reduced interference to adjacent stations.
With a better understanding of their application, it is believed that such
antennas will be increasingly used to help meet today's and tomorrow's requirements
in two-way radio communications.
Posted February 12, 2021(original 12/24/2013