35
Maintenance
High VSWR at startup or
during operation
High VSWR (Voltage Standing Wave Ratio) is caused by any factor which
changes the impedance match between the transmitter and the antenna sys-
tem.
The following may cause high VSWR:
• Wrong antenna for the application and frequency. Make sure the
antenna is the correct frequency.
• Split bullet in the transmission line or in the baymount (see
Figure 1
on page 4). A split bullet is an inner conductor connector mis-
aligned such that one or more of its contact arms is stuck outside
the conductor instead of inside. (A missing bullet will cause infinite
VSWR.)
• Radiators out of sequence (especially on a center-fed, null-filled, or
half-wave-spaced system).
• Damaged feed strap(s) on a radiator. The feed strap is the brass
strip that extends back from the end seal. The length, angle, and
straightness of the feed strap are critical to the radiator's perfor-
mance.
• Components of other services that have entered the RF field (later
installations or broken components).
• Physical damage to the transmission line, feedline, or radiators.
This may be from ice, lightning, tower work, or any other source.
• Paint applied to the radiators, possibly during a recent tower paint-
ing.
• Failure of de-icers, causing excessive ice buildup on one or more
radiators.
• Domes missing from vertical arms. An overheating de-icer can melt
the solder from domes, spacers, and bushings at the ends of the
vertical arms.
Erratic VSWR during
impedance trimming
If VSWR readings during transformer adjustment as described in
Impedance
trimming
on page 30 do not respond reasonably consistently to transformer
adjustments, then either there is residual water in the transformer, or the
transformer is damaged.
Follow this sequence of actions:
a. Repeat the purging process as described in
Purge the system
on page 27.
b. Try again to trim impedance.
c. If VSWR is still erratic, Your transformer is probably damaged. Contact
Shively Labs.
Change in coverage
Changes in broadcast coverage may be caused by the same factors that pro-
duce VSWR changes. If coverage seems to have changed, look for VSWR
changes and use
High VSWR at startup or during operation
on page 35 for
troubleshooting.
It is important to recognize, however, that apparent changes in coverage may
be due to subjective factors or faults of the receiving equipment. Before doing
more than checking the VSWR, be sure that an actual coverage change has
occurred.
