the vtvm between the PHONE
used to compute vswr. In some cases, it may be
PATCH OUT jack (J4 on the Collins
convenient to multiply the forward and reflected
312B -5 ) and ground. Slowly adjust
power readings by a factor of 10. The method of
the VOX BAL control for a null as
reading swr from the chart remains the same. For
indicated by the vtvm.
example, 50 watts forward and 5 watts reflected
i. Hang up the telephone, turn off the phone
result in a value of swr slightly less than 2 to 1.
patch, and reset all transceiver controls for normal
Power levels of 500 watts forward and 50 watts
vox ssb operation.
reflected produce the same value of swr. If the
j. The phone patch is now ready for use. The
reflected power is greater than the forward power
normal procedure is to set the PHONE PATCH
indication, the coupler unit may have been
switch to ST'N MUTE and call the third party on the
improperly installed. Check to make sure that the
telephone. Switch the phone patch on, and adjust
coupler has not been installed backwards in the
the AF GAIN on the transceiver to provide normal
transmission line. Also make sure that the d.c.
telephone line level. If the received signal is strong,
meter leads have not been reversed.
the RF GAIN may be decreased to reduce
e. If forward and reflected power indications
are equal, or reflected power is nearly equal to
k. Depending upon telephone line characteristics
forward power, the swr on the line is extremely high.
and voice volume of the incoming telephone signal, it
This usually indicates a shorted, open, or very
may be necessary to make slight adjustments of the
poorly terminated line. The coupler will respond to
MIC GAIN and VOX GAIN. In case of extremely
harmonic and other spurious output as well as RF
weak signals from the telephone line, manually
output at the desired frequency. If the transmitter
switch the phone patch back and forth from REC
output contains a high level of spurious emission, a
ONLY to XMIT ONLY.
high swr indication may be obtained even if the
transmission line is terminated properly.
10-7. Operation of Directional Coupler
10-8. Operation of Station Control
The transmitter output power that can be handled
safely is relative to the swr on the transmission line.
Table 10-2 lists operating conditions for the various
If the swr is extremely high, as when the line is open
settings of the FUNCTION and PHONE PATCH
or shorted, it is possible to obtain a forward power
switches in the Collins 312B-4 and 312B-5. Refer
indication up to 2 kilowatts with very little power
to figure 10-1 for switch identification.
output from the transmitter. To avoid damage to the
10-9. Operation of Vfo in Collins 312B-5
instrument, carefully observe the following operating
a. Set the wattmeter switch to the FORWARD
The switch positions of the VFO switch designate
200 position. (If a linear amplifier is being used, set
the two oscillators as number 1 and number 2.
the switch to the FORWARD 2,000 position.)
Number 1 is the vfo in the Collins KWM 2/2A.
b. Couple RF into the transmission line using
Number 2 is the vfo in the Collins 312B-5. The vfo
cw emission. The meter should show a forward
switching arrangement allows instant selection of
power indication. Note the reading.
either of two preset frequencies within one band (one
c. Switch to the appropriate REFLECTED
for net frequency and one for the frequency),
scale. Unless the antenna or load is perfectly
scanning for a clear channel because of severe
matched to the transmission line (52 ohms,
interference, and checking the local transmitter
resistive), the meter will indicate reflected power.
frequency for interference while making long
Note the reading.
a. With the VFO selector switch in the REC 1-
Where doubt exists in regard to expected
XMIT 2 position, the receiver frequency is
power output from a transmitter, always
controlled by the dial settings of the Collins KWM-
select the highest wattmeter scale first and,
2/ 2A, and the transmitter frequency is controlled by
if necessary, switch to the lower scale.
the dial setting of the Collins 312B-5. This allows
This action will prevent damage to the
the transmit circuits to be set within one portion of
the band and the receive circuits within another. For
d. The forward and reflected power readings, in
example, with the Collins 312B-5 dial
conjunction with the chart of figure 10 6, may be