Operation of Directional Coupler and Wattmeter

TM 11-5820-554-12

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

background noise.

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

and Wattmeter

Switches

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

procedure:

(fig. 10-1)

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.

distance communication.

CAUTION

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

indicator.

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

10-7