output is accomplished by turning all PA Modules OFF through
modulator action. This occurs in less than 20 milliseconds and
may not be noticed by listeners, or will be noticed as a slight
“click” or “pop.” If a VSWR fault cannot be cleared by turning
the PA Modules OFF for a short period several times, the
transmitter will reduce power.
6.27.2.1.1
First Stage VSWR Protection
a. SYMPTOM: VSWR LED flashes RED, carrier level
pauses at half-power and then returns to normal power; a
slight “Pop or Click” is heard on the air.
b. ACTION: This is the first step in the VSWR protection.
The VSWR detectors act in less than a millisecond to
detect a VSWR fault and turn off the transmitter RF output
for approximately 20 milliseconds or less. The VSWR
status indicator flashes RED for approximately one-half
second, then returns to GREEN. If the VSWR condition
is no longer present, no additional action will occur. The
VSWR LED will not stay RED since this is not a serious
type of VSWR condition. These types of VSWR actions
can occur indefinitely, as long as they do not occur for a
consistent period of longer than one second.
6.27.2.1.2
Second Stage VSWR Protection
a. SYMPTOM: The VSWR LED latches RED, the LOWER
button illuminates and the PA Current and Power meters
read low. Within 10 to 30 seconds, the LOWER indicator
extinguishes and the power output and PA Current stay at
a lower than normal power level.
b. ACTION: The VSWR sensor has detected a serious
VSWR problem. The first stage of VSWR protection was
attempted, but the fault existed for more than 1 second of
continuous recycling. A LOWER command was given to
the transmitter to fold back the power to a level at which
the transmitter could still operate. The power level will
remain there until given a RAISE command. If the fault
still exists the transmitter will again LOWER the power
and disregard the RAISE command. In this case, the
source of the VSWR fault must be repaired before full
power operation can continue.
6.27.2.2
Antenna VSWR Fault
6.27.2.2.1
Possible Causes of VSWR Overloads
VSWR overloads during stormy weather may occur normally,
and may be no cause for concern. Proper installation of static
drain and static discharge equipment in the antenna system can
minimize, but not eliminate, this problem.
Causes of VSWR overloads may be listed in three categories.
They will first be listed, then will be discussed in more detail in
the following paragraphs.
a. ARCING in the impedance matching network, phasor,
switching equipment, transmission line, tuning equip-
ment, or at the tower ball gaps. Once an arc occurs,
transmitter output power would sustain the arc. When the
transmitter power output is removed, the arc will go out
(unless there is some other voltage source to keep it
going).
b. TRANSIENTS, or other signal pickup, fed back into the
transmitter output from the antenna system.
c. COMPONENT FAILURES causing a change in load
impedance at the transmitter output connector.
6.27.2.2.2
Possible Causes of Arcing
Common causes of arcing include:
a. Defective vacuum capacitors. VSWR overloads will prob-
ably occur at a certain power level or under modulation.
b. Static discharge or discharge due to lightning, across ball
gaps, guy wire insulators, or possibly across components
already operating close to their voltage ratings. Static
charge buildup can occur on towers that do not have
provision made for static discharge, such as static drain
chokes. Charge buildup can also occur on insulated guy
wire segments. Static charge buildup can occur before,
during or after rain, snow, or even blowing dust or sand.
c. Dirt build up or moisture (including condensation) on
insulating surfaces, causing the voltage breakdown rating
to be reduced. VSWR overloads will probably occur on
modulation peaks.
d. Condensation inside a transmission line may cause re-
duced breakdown voltage of the line. This can occur if
pressurized gas filled lines lose pressure or if the dehy-
drator in the line pressurization unit fails. VSWR over-
loads will probably occur on modulation peaks.
e. In new systems, insufficient voltage rating of compo-
nents, such as capacitors or insulators, or spark gaps that
are set too close.
6.27.2.2.3
Possible Causes of Transient Signals
TRANSIENT signal pickup may occur during thunderstorms,
even from distant lightning strokes in some cases. Lightning
strikes may induce currents in towers, causing currents on the
transmission lines that can reach the phase detectors and give a
VSWR overload indication.
Other station signals can also induce voltages and currents in
antenna systems that are large enough to be detected by the
phase detector and cause VSWR overloads. The solution in such
cases may be a trap or filter in the antenna impedance matching
network or phasor.
6.27.2.2.4
Load Impedance Changes
The Reflected Power reading and DETECTOR NULL (An-
tenna) reading on the front panel multimeter are the best indi-
cations of the antenna operating impedance, once the system is
initially tuned into the antenna. An impedance change in the
transmitter load will change the DETECTOR NULL (Antenna)
indication and, to a lesser extent, the reflected power. The load
impedance should be checked with proper impedance measur-
ing equipment and corrected if possible.
“Dummy loads” should also be treated with caution. Dummy
load resistance or impedance may change with time, and
dummy load resistance or impedance may also change as the
load heats up when power is applied. If reflected power changes
after power has been applied to the load, this is probably the
cause.
Section VI - Troubleshooting
Rev. R: 11-11-96
888-2297-002
6-15
WARNING: Disconnect primary power prior to servicing.
