Have you ever used a walkie-talkie and
felt there was something wrong with the particular radio you were
using, or with the system, because the range was reduced? Have you ever
wondered why some walkie-talkies seem to have much farther range than
other portable radios on the same system?
Case in point: A new VHF radio system
recently installed for a public-safety agency worked sometimes, and
sometimes not. Specifically, it had poor range with some of the
walkie-talkies but great range with others. Why did some of the units
work with a range of better than 20 miles, while others exhibited a
range of less than 2 miles?
The range of a radio system is influenced by the
aggregate of several factors, including the following: transmitter
power; receiver sensitivity; transmission line losses; antenna gain,
efficiency and height; line of sight; obstructions; terrain; noise
floor; and RF interference. But the items dealing with the antenna have
the greatest effect on the range.
Because the fixed part of the system, often
referred to as the system infrastructure, is constant for all users,
the system's base station and mobile and portable radios have the same
operating parameters. The main difference is that the external
environment in which a portable handset operates can change simply by
the user turning around. Reception also can be affected by where the
radio is worn or where the user's hand is placed in relation to the
Even though antenna efficiency has the greatest
influence on both the transmitting and receiving paths, it is rarely
tested. While it is easy to measure a radio's transmitter and receiver
parameters, it is quite difficult to measure the antenna parameters.
When testing antennas
for walkie-talkies, especially for VHF radios, it is very important
that the antenna always be at least 1 to 2 inches away from the body if
the radio is worn on the belt, as close proximity of the body to the
entire length of the antenna seriously detunes the antenna. In fact, it
is not uncommon for the range to shrink to 1% of normal should the
antenna come in contact with a person's body. Most of the radios on the
market today have the antenna mounted to the case in a manner that
creates suitable distance from the body, or they have leather or canvas
cases that provide adequate separation. Those of us who have “extra
handles” at our midsections must use the external cases or pouches, even
on the radios that have the distances built into them.
The best way to determine how well an
antenna performs standing in free space compared with sitting just above
a belt is to use one of the antenna analyzers — such as Anritsu's Site Master or Bird Technologies'
Site Analyzer — with a phase-stable cable, along with the proper
adapter to match the end of the phase-stable cable to the antenna.
An antenna analyzer will give a graphical
representation of the antenna performance, which is better than just
looking at the reflected power on a specific frequency using a
Under normal circumstances, a standing wave ratio (SWR) of under
1.5:1, which is equivalent to -14 dB return loss, indicates a good match
at the operating frequency. A SWR of 2.0:1, which is equivalent to a
-9.0 dB return loss, is the worst case that would be tolerated. At 2.0:1
SWR, the antenna is operating only at 90% efficiency, which is the bare
Field tests show that many antennas operate only within a 5 MHz
window where the SWR is less than 2.0:1, and the antenna manufacturers
normally label such limits on the antenna or by the model designation.
In further testing, when an antenna that
is tuned correctly and has a good SWR on the normal operating frequency
is placed directly next to a body — as would occur when a walkie-talkie
is mounted on a person's belt and the antenna is touching the body — the
SWR goes from acceptable to terrible. In the case of the public-safety
agency installation mentioned earlier, the antenna was detuned to the
point that it was operating at less than 1% efficiency.
Using an antenna analyzer, it was shown
that just moving the antenna away from the body by at least 1 inch
caused the antenna to behave as designed and the SWR returned to the
acceptable range. Some manufacturers, such as Motorola and Kenwood,
have addressed this problem with antennas that are mounted so that they
are moved away from the body even when there is no holster case and
only the belt clip is used.
Radios manufactured by other companies that were
outfitted with the recommended holster cases — resulting in the antennas
moving away from the body at the recommended distance of 2 inches or
more — worked as designed. (See Figure 1 on page 34.)
Should someone want to leave the radio on
the belt and use a lapel microphone, then the antenna must be away from
the body by more than 1 inch — preferably by 2 inches — for the radio
to work properly in the VHF band. As the frequency goes up, the distance
between the body and the antenna can be reduced. Although VHF antennas
need the most separation distance, it can be reduced to less than an
inch for UHF and higher frequencies and the antenna will continue to
The ultimate way to determine whether a radio and antenna
combination is working properly is to test the field intensity to see
whether it matches the predicted value for the power at a controlled
Field intensity is a quantitative measurement that indicates how
much signal is radiated from a radio transmitter. One of the laws of
physics that prevails here is that as you increase the distance between
the transmitter and the receiver, the field intensity drops by the
square of the distance. In simple terms, if you double the distance, you
quarter the field intensity. If you triple the distance, the field
intensity will drop by a factor of 1/9 the original value. If you
quadruple the distance, the signal will be 1/16 the original value. A
second factor is the free space attenuation of radio signals, which is
expressed in Equation 1 above.
This formula works for line-of-sight
signals. Any obstructions such as trees, buildings, vehicles or people
will attenuate the signal to some extent. In this case, the body could
block the antenna signal path to and from the base station site. Also,
if line-of-sight conditions do not exist due to terrain obstructions,
there will be severe losses between the transmitter and the receiver.
Finally, it should be noted that the curvature of the earth prevents a
signal from being line-of-sight over very great distances.
When a walkie-talkie radio is used
without a lapel microphone, the radio is brought up to the face of the
user, and the antenna is usually above the user's head so that it does
not matter which direction the user is from the base station or repeater
with which they are communicating. When a lapel microphone is used with
a portable radio, the handset is directional in such a way that when
the body of the user blocks the radio and antenna from the other user or
repeater, the signal will be attenuated to some extent.
This condition also is present with VHF
radio systems, but not to the extent that most users need to worry about
it. On UHF and higher frequencies, such as 700 MHz, 800 MHz and 900 MHz
systems, the manufacturers of the radio systems put the antennas on the
lapel microphones to lessen these directional effects. It was through
field intensity tests that the manufacturers determined lapel microphone
antennas were needed for these higher frequencies. Many of the UHF and
higher frequency systems use multiple receivers and a voting system so
that users in the field do not need to worry about which direction they
are facing when using their radios.
A walkie-talkie still is the preferred
mode of communications for many companies and agencies. Over the years,
such radios have become very small, reliable and user-friendly. The one
component that has not changed across the decades of use is the
antennas. Due to the laws of physics, the antennas must be a certain
length that corresponds to the frequency being used. When the antennas
are placed so that they touch the body of the user, they will be detuned
and no longer perform properly. Just an inch or two away from the body
will ensure the antenna works as designed for VHF radios, and less for
the higher frequencies. When your radio system fails to work as designed
or expected, do not forget to look at the antennas of the
walkie-talkies as the problem.