Tuning
Your CB Radio Antenna by Andrew
Youderian
The most important and often overlooked step
in installing a mobile CB is tuning the antenna. While a
properly tuned antenna will result in strong radio performance,
a badly tuned or untuned antenna can result in poor performance
and can even damage your radio.
Before you start fiddling with the antenna,
it's important to understand the basic theory behind tuning
as well as the term "Standing Wave Ratio" (SWR).
What Is Antenna Tuning?
Tuning is the process of adjusting an antenna
so that the radio's output power is turned into radio signals
as efficiently as possible. In more tangible terms, it involves
incrementally increasing or decreasing the length of the
antenna. The goal of antenna tuning is to match the antenna's
physical length with that of the radio frequency being used,
so as to ensure optimal performance and transmission efficiency.
An antenna's efficiency is measured by the
Standing Wave Ratio, or SWR, which represents how much of
the radio's power is being turned into radio signals by
the antenna. An SWR reading of 1.0 indicates that 100 percent
of the radio's power is being transmitted, while a ratio
of 3.0 means that only a third of the power is being transmitted
by the antenna.
The goal of antenna tuning is to achieve as
low an SWR reading as possible. While low SWR readings result
in the best performance, high SWR readings have more worrying
implications than poor transmission range. In fact, operating
a radio at SWR levels above 3.0 can cause damage to the
radio because all that excess output power is reflected
back at the radio as heat, which can result in permanent
damage to the unit.
CB Antenna Tuning Instructions
Enough theory - let's tune that antenna! To
tune a CB antenna, you'll need to have correctly
installed both your CB radio and your CB
antenna. You'll also need an SWR meter and a coaxial
jumper cable; both can be purchased for around $20 from
most CB shops. Now for the step-by-step instructions.
1. Find an open area. Drive your vehicle
to an open parking lot or field; make sure there are no
trees, buildings or other obstructions within 15 meters.
To ensure accurate SWR results, make sure everyone is in
the vehicle and that the doors are closed when you take
the readings.
2. Hook up the SWR meter. Disconnect
the cable going to the antenna from your radio and attach
it to the connection marked "ANT" or "Antenna"
on the meter. Next take the jumper cable and attach one
end to the radio and the other end to the connection marked
"TX" or "Radio" on the meter. The SWR
meter is now installed in-line between the radio and antenna.
3. Take SWR readings. You're now ready
to take SWR readings. You'll be taking readings using both
Channel 1 and Channel 40, with the goal of reaching the
same SWR level - below 2.0 - on both.
Depending on your results, you'll have to do
one of the following.
Channel 1 SWR is greater than Channel 40
SWR. If the reading on Channel 1 is greater than the
reading on Channel 40, your antenna is too short and you'll
need to lengthen it. Most fiberglass antennas have a tuning
screw at the tip of the antenna that you can loosen or tighten
to adjust the length of your antenna. Nearly all other antennas
allow you to slide a steel whip in and out of the base to
adjust the antenna's length. In either case, make very small
adjustments and then recheck the SWR readings on both channels.
If your antenna has a weather cap that covers the tuning
screw, make sure it's back in place before taking your SWR
readings.
Channel 40 SWR is greater than Channel 1
SWR. If the reading on Channel 1 is less than the reading
on Channel 40, your antenna is too long and needs to be
shortened. Use the appropriate tuning feature, discussed
above, to make the adjustment. (In the unlikely - and unfortunate
- event that your antenna isn't easily tunable, you'll need
to physically shorten the antenna by cutting it.) Make very
small adjustments and then re-check readings on both channels.
If your antenna has a weather cap that covers the tuning
screw, make sure it's in back place before taking your SWR
readings.
Tuning Goals
The goals of antenna tuning are (1) to achieve
SWR readings of 2.0 or less on both Channel 1 and Channel
40 and (2) to match SWR levels on the two channels as closely
as possible.
Continue to lengthen or shorten your antenna
based on your SWR readings until you've achieved one or both
of the stated goals. When you're done, remove the SWR meter
and the jumper cable from your installation and reattach the
antenna directly to the radio using the original coaxial cable.
Several problems can keep you from reaching the
tuning goals. A full discussion of these problems is beyond
the scope of this article, but Right
Channel Radios CB Radio Resources Library has several
troubleshooting articles that can help you solve common tuning
problems.
Andrew
Youderian is the founder and manager of