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 TR6100 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.) |
