Common Causes of High SWR

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113rsn025

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Feb 12, 2008, 11:27:58 PM2/12/08
to 113 Division CB Group Stations
There are many things other than the antenna that can cause high SWR.
SWR meter readings take into consideration the antenna, the coaxial
cable, the mount, the mounting location, the vehicle and objects in
the antennas near-field environment. We broke our list of common high
SWR causes into six primary categories. The order of the categories
does not imply that one is more common than the other, however, the
list within each category is arranged based upon our experience and
shows the most common causes higher on the list than the lesser common
causes.

ANTENNA RELATED

* Antenna was not tuned in its final mounting location
* Antenna tip installed AFTER the antenna was tuned
* Testing or tuning with doors, hood or trunk open (altered ground
plane)
* Antenna not mounted vertically
* Incompatibility between antenna and vehicle for undetermined
reasons
* Insufficient tuning range due to antenna/installation
incompatibility
* Antenna tested/tuned inside garage, carport, etc.
* Antenna was tuned then moved to another mounting location or
vehicle>
* Ground plane and no-ground plane antennas used with wrong coax
cable type
* Antenna grounded to vehicle chassis ground circuit
* Internal antenna issue (verify base to tip continuity)
* Antenna not designed for use with CB radio

ANTENNA MOUNT

* Ungrounded mount (re: ground plane dependent antenna system
only)
* Relying on a chassis ground via the radio (must test with coax
off of radio)
* Anodized or powder coated mounts (insulated from chassis ground)
* Antenna stud mount improperly installed (shorted)
* Coax shield side not grounding to mount (re: ground plane
dependent antenna system only)
* Improper positioning of required insulators on antenna stud

COAXIAL CABLE

* Open (broken) center conductor or shield
* Center conductor to shield short (undesirable connection)
* Severe pinches, sharp bends, and/or cuts in outer jacket of coax
* Excess coax rolled into a coil for stowing.
* Low quality coax cable
* Antenna/Counterpoise anomolies and potential coax cable
resonance
* Wrong type of coax (Use ONLY 50-ohm for single antennas, 75-ohm
for duals)
* Using a standard antenna with a no-ground-plane cable assembly
or vice versa

MOUNTING LOCATION

* Antenna running parallel to reflective surface over too much of
its length (and too close)
* Antenna mount affixed to an ungrounded in-bed pickup toolbox
* Antenna mount affixed to insulated roof rack (anodized or powder
coated)
* Antenna mount affixed to an ungrounded spare tire swing arm
* Antenna mounted too close to other antennas (24" minimum
recommended)
* Less than 60% of a top loaded antenna in free air-space
* Dual antennas not having unobstructed line-of-sight between top
60% of length

GROUND PLANE (COUNTERPOISE)

* Vehicle is a composite material (fiberglass) - NGP system
required.
* Insufficient ground plane size (motorcycles, ATV's, etc)
* Running too long of a wire from mount to chassis to ground the
mount
* Thin sheet aluminum vehicle shell (insufficient metal mass)
* Antenna positioned in a manner that doesn't allow it to use
available metal counterpoise

SWR METER

* "To Transmitter" and "To Antenna" coax leads reversed
* Failure to recalibrate SWR meter after every channel change
* Calibrating meter when in SWR/Ref mode instead of Cal/Fwd mode
* Innaccurate, defective or uncalibrated SWR meter
* Unable to calibrate meter to set line (radio output power issue)
* Improper testing procedure
* Faulty coaxial jumper between radio and SWR meter

CB RADIO

* Defective/damaged transceiver (radio)
* Insufficient current to transmitter is limiting output
* High resistance 12v power connection
* Partially broken wire on either positive or negative 12v lead
* Loose connection on either positive of negative 12v lead
* Corroded power lead on either positive or negative 12v wire
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