A
wattmeter is essential for station maintenance.
Whether
you're operating a Travelers Information Station (TIS), a Highway
Advisory Radio (HAR) or an Emergency Advisory AM station, a
wattmeter is a valuable tool for installing and maintaining your
radio station. A wattmeter can be used to tune the station's
antenna and to determine if the antenna and transmitter are
working properly (by allowing the measurement of the system's
forward and reflected power).
There
are a couple of ways to use it.
 Both
the SX.100 Wattmeter and the new SX.200 model now provided on all
new radio stations are passive units, placed between the
transmitter and the antenna. They may be inserted for test
purposes, or they may be "hard-wired" into the system
for convenience.
On the back
of the meter, find two UHF coaxial termination points. Route the
coax from the lightning surge arrestor and antenna to the
connector, labeled "ANT." From the connector labeled
"TX", route the coaxial cable to the transmitter.
Measurement
steps include . . .
With the
wattmeter now placed in series and the transmitter off, you can
measure the forward RF power. Here's how:
1. Set the
meter's FUNCTION switch to the POWER position.
2.
Set
the RANGE switch to the appropriate setting. (For the SX100, use
the 30W range; and for the SX200, use the 20W range).
 3.
Verify that the TX and ANT output connections are secure.
4.
Turn
on the transmitter and turn up the output power to 10 watts.
5.
Set
the POWER switch on the wattmeter to FWD (forward) and record the
reading. If you're using the TR6000 transmitter, the power-adjust
dial is labeled on the front of the unit. If you have the TR20
(Phase II) transmitter, open the transmitter lid and adjust
the blue power potentiometer, located vertically just above the
power-switch. (A properly tuned antenna should allow a
reading of 10 watts.)
6.
Record
the reflected power by turning the meter's POWER switch to REF.
This reading should be less than 1, and less than 1/10 the forward
power reading.
The
higher the ratio of forward to reflected power,
A
radio system with a high ratio operates more efficiently,
resulting
in longer transmitter life.
If your
readings show reflected power to be nearly the same as forward
power, turn the transmitter off. There is a problem with the
antenna system. Confirm that your coaxial, feedline and
groundplane connections are secure. If these connections are
secure, with a VOM meter, verify that coaxial lines have not
shorted or opened; and check the antenna for disconnects and
damage.
If the wattmeter indicates that there is very low forward power
and reflected power, there is internal transmitter difficulty.
Turn the transmitter off and place a dummy load on the
"ANT" connector of the wattmeter (in place of the
coaxial connection to the lightning arrestor and antenna.) Turn
the transmitter back on. If the meter continues to show low
forward and reflected power with the dummy load in place, service
the transmitter. (With a dummy load in place and the transmitter's
power at full, the reading should be 10 watts forward power and 0
watts reflected.)
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