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. This
article in Radio & TV News gives a good overview of the issues
of concern with 2-way radio antennas and towers.
in Two-Way Radio Systems
Tom J. McMullin
Communications Engineering Co.
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.
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 activities.
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.
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.
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 tower.
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
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
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 type antenna.
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.
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
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.
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 height.
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 static.
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 band.
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.