When the range markers appear, operate the LO switch in the RAISE position and then in the LOWER
position until maximum marker intensity is obtained. Return the AFC-MANUAL switch to the AFC
position. The marker intensity should remain unchanged.
If a decrease in marker intensity occurs, the AFC circuit probably requires adjustment (page 211).
Note: There are two local oscillator frequencies that will cause the range markers to
appear at full brilliance; one is 30 MHz above the transmitter frequency, the other is 30
MHz below. The lower frequency is correct and is the only frequency at which the AFC
circuit will "lock in. " Proper action of the AFC circuit is indicated by the stability of the
range marker intensity and the AFC crystal current when the AFC- MANUAL switch is
placed in the AFC position. AFC crystal current may be measured on the test meter by
setting the TEST METER SELECTOR switch in the AFC XTAL CUR position. The
normal meter reading is 2. 5 (0. 5 ma).
If the range markers fade out entirely when the AFC-MANUAL switch is placed in AFC, the local
oscillator is probably tuned to the wrong frequency. To correct this, place the AFC-MANUAL switch to
MANUAL and operate the LO switch in LOWER position until the range markers reappear. Adjust the
local oscillator for maximum intensity of the range markers. Return the AFC-MANUAL switch to AFC.
No change in marker intensity should occur.
Turn the MARKERS switch to OFF.
a. General. In order to obtain accurate range data and an end result of accurate weapons locations, the radar
set AN/MPQ-4A must be range calibrated. Range calibration consists of alining the range circuits so that the radar will
read the correct range. Range markers are used to calibrate, and a fixed point may be used to check calibration. Range
calibration should be performed each time the set is emplaced and checked at least once a day during operation.
b. Procedure for Range Calibration. To prepare the radar for range calibration, insure that power is applied to
the radar, the transmitter is turned on, the local oscillator is tuned, the RANGE SHIFT switch is turned to the OFF position,
the DETENT switch is set to the OFF position, and ∆ RANGE is set in detent. (The latter allows the operator to set any
given range into the computer. )
Using the LOWER BEAM RANGE control, set the range at 2, 000 meters.