Setting the SWR
All CB antennas are tunable. Antennas that have an easy, often
mechanical, tuning device are referred to as a "tunable antenna".
Accordingly, lots of people think the other antennas are not tunable
and so they make no attempt to have the antenna tuned. This is a
critical mistake.
Wire-wound antennas without mechanical tuners, that test electrically
long (higher SWR on ch 40), can be tuned by removing wire from the
top. Remove the antenna cap, remove one wrap of wire and cut it
off. Recheck the SWR again. As long as the SWR is higher on channel
40 continue to cut. Do not cut more than one wrap at a time. If
SWR testing indicates the antenna is electrically short (higher
SWR on ch 1), the coils at the top of the antenna can be separated
and spaced further apart or accessories that increase the physical
length of the antennas (springs, quick disconnects) can be added.
Solid fiberglass antennas (straight or helical wire impregnated
in fiberglass resins) that are electrically too long (higher SWR
on ch 40) can be cut shorter with a hacksaw or grinder. Cut only
1/4 of on inch at the most at one time. If the SWR test on one of
these antennas indicates that it is electrically short (higher SWR
on ch 1), the addition of a spring or quick disconnect (or both)
is the only way to correct for the short condition.
Base loaded antennas with wire whips have a set screw (or two)
just below the area that the whip is inserted into the loading coil.
By loosening up the set screw(s) you are able to slide the whip
up or down as required. If the SWR is higher on ch 40 slide the
whip down, If the SWR is higher on ch 1 raise the whip.
Setting
the antenna SWR with an amplifier can get involved. The antenna
SWR is calibrated at the back of the amplifier with the amplifier
in line and turned off. Once the antenna is properly tuned
the meter is removed and re-installed at the radio with the
amplifier in line. The antenna SWR level is then measured with
the amp off and then with the amplifier switched on. The SWR
level must be at an acceptable level at the radio under all conditions. The conditions are with the amp off on channel 40 and 1 and with the amp turned on, on all power levels of the amplifier
on channel 1 and 40.
Most shops don't check the SWR match at the radio with the
amplifier turned on. This is a critical mistake. With these steps added the SWR calibration can turn into a multi hour event at times. |
