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The following list shows the most probable causes of high SWR in order
from the most common causes to the least common causes. If experiencing
SWR problems, check each of the following conditions in the order shown
until problem has been uncovered.
Shorted Stud Mount
Disconnect coaxial cable at antenna mount. Test continuity from mount to
antenna coupling nut. There should NOT be any continuity. If shorted, reposition
or replace insulators and retest.
Improper Type and Length of Coax
Single antenna installations require RG-58 type coax and dual antennas require
the use of RG-59 type coax cables. We recommend 18 foot long coax leads
from the radio to each antenna, especially on installations that are displaying
high SWR problems.
Shorted Coaxial Cable
Disconnect coax at radio end. Test for continuity between center pin and
threaded sleeve. There should NOT be any continuity. If shorted, repair
and retest.
No Chassis Ground at Antenna Mount
Test for continuity between the antenna mount and the vehicle's common ground.
There MUST be continuity. Repair and retest as required.
Open Coaxial Cable Center Lead
Disconnect coax at radio end. Test for continuity from center pin to antenna
base. There MUST be continuity. If necessary, repair and retest.
Open Coaxial Cable Ground Shield
Disconnect coax at radio end. Test for continuity from the antenna mount
to the coax connectors threaded sleeve at the radio end of the coax. There
MUST be continuity. If necessary, repair and retest.
Improper Installation Location
Transmit antennas need free space around them if expected to perform properly.
If more than 30% of the antennas overall length is parallel to the side
of the vehicle and within twelve inches of that surface, SWR problems are
probable, i.e. between truck cab and shell, corner of truck bed near cab,
low mount position on motorhome, etc. Relocate the antenna to a position
of performance (versus convenience or appearance) and retest.
Insufficient Ground Plane Available
Fiberglass, plastic and thin aluminum vehicle bodies lack the reflective
characteristics needed for proper antenna performance. On some occasions,
running a 12ga or heavier wire from the antenna mount to the vehicles chassis
ground will be sufficient. Otherwise, a no-ground-plane antenna system may
be required.
Low Quality Coax Cable
Delivering radio frequency to an antenna via poor quality coaxial cable
is the equivalent of watering your lawn with a hose full of holes. The bitterness
of low quality will linger long after the sweetness of low price is gone.
Use high quality coax only.
Antenna Tuned Without Tip
If the antenna is tuned without the tip then put on after tuning is completed,
the SWR will change when the tip is installed. All readings must be made
with the antenna tip in place.
Defective SWR Meter
Some are made bad and others have been known to go bad. Try to confirm readings
with a second meter.
Damaged Antenna
Aside from apparent damage that is visible from the antenna striking immovable
solid objects, you can test for any internal damage by checking continuity
from the antenna base to the tunable tip extender or end of wire. There
MUST be continuity. If there is no continuity, the antenna must be repaired
or replaced.
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