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.
Antenna Applications 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
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
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 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 of Fig.3A.
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
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 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.
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.
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
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
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.
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?"
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.
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 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 erection.
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.
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.
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 December 24, 2013