Setting the SWR of Your Antenna
The antenna subsystem is the
most important part of your CB system. It's tuning can make you or break you.
When checking the SWR or the "Standing Wave Ratio", you will be hooking up a simple form of a
directional watt meter between your antenna and the CB radio. The meter measures Forward power
to the antenna system and then compares
it to the reverse or reflected power from the antenna system. Instead of showing you
power, the meter is calibrated to show the RATIO of coupling from radio to antenna
system. If there is all forward power and no reflected power the antenna
system has a one to one (1:1) ratio with the transmitter. By checking the SWR at
channels 1 and 40 on your CB, you will have a good idea if the antenna is too "long" or too
"short" for the center of the desired band. You will also be able to
evaluate your system performance and troubleshoot problems. For instance a SWR too
high (over 3:1) all over the band can be an indication of a bad part or junction, poor
ground or poor location. An SWR too low over the whole band (1:1 for all 40
channels) can indicate a low efficiency antenna or lossy component. Safe operation
of your CB should be with an SWR of 2:1 or less. Most people would prefer to be
1.5:1 or less. A good quality antenna and proper installation (location very
important) should easily result in an SWR of less than 1.5:1 and for most long
antennas it will be that low across all 40 channels (plus a few). Shorter
Antennas tend to be more narrow banded and may have as high as a 2:1 at the top
and bottom of the band. Here's one of the places you can evaluate your antenna.
We EXPECT a high quality "short" antenna to be narrow banded. If you
have an antenna that's say, three feet or less, and the SWR tunes great (less
than 1.5) across all the channels than maybe your antennas feedline coax
or the antenna coil is "lossy". In "Lossy" I mean that maybe
the design or construction material is inferior and is wasting some of your
energy as ground losses or increased heat dissipation. This causes both the
forward and reflected power to be partially absorbed, thus showing a lower SWR
on the meter.
Lets get Started in the actual tuning of your antenna:
THINGS YOU WILL NEED
- Knowledge of what not to do .... read ahead.
- Properly installed antenna system (mount, coax and antenna)
- Functional radio.
- SWR meter. (See "SWR Meter Hook-Up")
- Short piece of coaxial cable (jumper) with PL-259 connectors on both end.
SWR TESTING REMINDERS:
- Remember to check for continuity, shorts and opens in your coax and mount installation
first if the SWR is high everywhere (1-40).
- Take measurements in an open area with the vehicle's doors and hatches closed.
- All measurements should be taken with antenna tip on, unless you do not plan to use the
tip in normal use.
THE SET UP
If already connected, disconnect the coaxial cable from the radio.
Connect the coax cable that normally connects to the back of the radio to the SWR meter
connector marked "Antenna" or "Ant". Now, connect one end of the
jumper cable to the back of the radio and the other end to the SWR meter connection marked
"Transmitter" or "Xmit". Your SWR meter is now in series (in-line)
with your radio and antenna.
You should have your vehicle in an open area, with all doors closed. Turn your
radio on and tune to channel one . If your radio has side band operation, make sure you
are in AM mode before doing SWR tests. NO MODULATION !!!
The following assumes that your SWR meter has a standard set of switches, knobs and
meters. That is, there will be at least one switch with the marking Forward (FWD or CAL)
in one position and Reference (REF or SWR) in the other. There will also be a knob or
sliding controller marked "Set" or "Adjust". Most meters come with
full instructions. If the common configuration does not match your meter you will need to
rely on the meters manual for assistance.
With the radio on the channel one and the SWR meters switch in the Forward (FWD)
position, depress the transmit switch (key up) located on the microphone. While holding
the unit in this transmit mode, adjust the meter needle to the SET or CAL position
using the Set or Adjust knob on the meter. As soon as the needle is in alignment with the
corresponding mark on the meter face, flip the switch to the Reference (REF) position. The
meter is now showing your SWR on channel one. Note the value and quickly release the
microphone switch. Record this reading on your paper to the nearest 1/10th. i.e. 1.8, 2.3,
2.7, 1.4, etc.
Finally, place your radio on the highest number channel (40 on CB). Place the meter
switch in the Forward (FWD or CAL) position, depress the microphone switch and adjust the
meter to place the needle on the SET or CAL position of the meter face. Once in the SET
(Cal) position, place the meter switch in the Reference (REF or SWR) position and note the
reading. Release the microphone switch and write this value down to the nearest tenth of a
ADJUSTING "LONG" ANTENNAS
If the SWR on channel 40 is greater than that on channel 1, your antenna is considered
to be "LONG" and reduction of physical height and/or conductor length will
correct this situation. Depending upon antenna model:
II, Firefly Screw down the tunable tip
2) Firestik, Road
Pal, and cheap truck stop antennas Remove the tip, make short slits in the
plastic covering and unwind and clip off short pieces of wire.
Fiberglass remove cap and lower stinger (cut if on bottom)
Fiberglass Slide the tuning sleeve down (may also use the procedure in step
2 if slide doesn't have enough range)
5) Wilson 1000,
2000, 5000, 500, Lil' Wil , and K-40 base load remove the whip and Cut
1/4" off of the bottom.
ADJUSTING "SHORT" ANTENNAS
If SWR on channel 1 is greater than that on channel 40, your antenna is considered to
be "SHORT" and increasing the physical and/or electrical length of the antenna
is required to correct this situation.
1) Firestik II,
Firefly unscrew the tunable tip
2) Firestik, Road
Pal, and cheap truck stop antennas add length with a spring, quick disconnect or
Fiberglass remove cap and raise the stinger
Fiberglass Slide the tuning sleeve up
5) Wilson 1000,
2000, 5000, 500, Lil' Wil , and K-40 base load raise the whip.
NOTE: The shorter the antenna, the more sensitive it is to adjustments. For example,
removing two wire turns on a 4 foot antenna might move the SWR by 0.3; the same amount
removed from a 2 foot antenna may move the SWR by 1.0. Make smaller adjustments on shorter
Measurements and determination of short or long conditions are the same
as the single antenna procedure. However, when tuning co-phased antennas, if you adjust
one antenna, adjust the other in equal amounts to keep them in balance
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