|How does a CB radio
antenna work and why
does it need to be tuned?
A Citizen Band or "CB" radio as it is widely known, is a radio
transceiver for two way communication, transmitting and receiving, over a CB radio antenna.
The job of the antenna on a CB radio system is two-fold. First it captures
inbound radio-frequency signals which are then converted into electrical signals
by the CB's receiver. Second, it converts outbound electrical signals
coming from the transmitter and converts them into radio-frequency signals that
are then broadcasted onto the air waves, hopefully to another receiving CB
radio. This is the transmitting function of a CB radio. It is during
the transmitting function of a CB radio that proper tuning of the antenna
This is due to the fact that an antenna is radiating
radio-frequency signals of a specific wavelength. A properly tuned antenna
will have a length that will accurately match the radio frequencies wavelength
it is transmitting.
Determining the proper length of the antenna can be done using
Wavelength feet = 984 / frequency megahertz
Citizens' Band (CB) frequencies begin at 25.01 MHz. A full
wavelength antenna would be slightly longer than 39.34 feet tall. For
mobile purposes, the height of a full wavelength antenna would be impractical.
So it is common practice to use antennas that are a fraction of the full
wavelength. 1/8, 1/4, 5/8, and 1/2 are all commonly used wavelengths for
antennas. For mobile CB antennas the quarter wave antenna is just under 10
feet long and take to form of the common whip antenna.
The dilemma is that there are 40 channels on today's citizen
band transceivers. Each channel is on a different frequency, thus
requiring a different antenna length to exactly match each frequency.
However, a separate antenna for each frequency is simply not practical so a
compromise must be made and antenna manufacturers design antenna lengths from
the middle of the CB's 40 channel frequency range.
However with any compromise, there are tradeoffs and you have to
adjust the Standing Wave Ratio or SWR of the antenna
and wire until the SWR is acceptable. The antenna and antenna feed line
have what is called characteristic impedance, or a measure of resistance to the
current flow through the wire and out the antenna.
When a transmitter is connected to an antenna by a feed line,
the impedance of the antenna and feed line must match exactly for maximum energy
transfer from the feed line to the antenna to be possible.
In the best possible scenario, there would be a perfect match
between antenna and the feed line where there would be a 100% conversion of
electrical energy (sent from the CB to the antenna) into radio wave energy and
radiated out into the atmosphere. When perfectly matched, the SWR
measures 1:1 and the antenna is maximizing it's potential output.
In the less than ideal scenario, a portion of the electrical
energy (from the CB) is not converted into radio wave energy and instead is
reflected back down the feed line and into the CB radio. This energy
reflected back from the antenna creates standing waves of electrical energy in
the antenna wire and in turn can cause damage to the radio circuitry.
When tuning the SWR of an antenna, an SWR meter is attached
between the CB radio and the antenna feed line. SWR meters vary and some
allow the meter to generate a signal on different channels while other SWR
meters use the CB by keying up the microphone to generate a signal. As a
signal is generated, the SWR meter is observed.
Ideally its best to tune your antenna for the channels that you
use most if you typically only use one or a few channels. Otherwise use a
broader test across the full range of 40 channels. If the SWR never
goes above 1.5:1, then your antenna is well tuned. If the SWR does go
above 1.5:1, then test the SWR on the lower channels such as 1 through 5 and
compare those SWR reading with the upper channels such as 35 through 40.
If the SWR is greater on the lower channels then your antenna is too short and
needs to be lengthened using the antennas height adjustment. Make small
adjustments and retest. If the SWR is greater on the higher channels, then
gradually shorten the antenna and retest with each small adjustment. When
adjusting in either direction, if the SWR begins to test greater, reverse the
adjustment to find the "sweet spot".
Keep in mind that the electrical ground of the antenna, the
shape of the vehicle including open doors, hoods and hatches as well as the
location of the antenna on the vehicle, all play a part in proper adjustment.
Test with the door and hatches closed and experiment with different mounting
locations of the antenna.
Last, be sure to read and understand the instructions of the SWR
meter as well as the antenna itself. In the end, with good SWR, you'll be
transmitting to your fullest potential and ensure longevity to your radio.