d. Adjust TUNE control for zero beat in head-
ing interference to the audio and bearing accuracy
set. The FREQUENCY dial reading should be
of desired station. Whcn this occurs, use the loop
mode for aural null homing (para 2-6).
identical with that obtained in b above. Turn BFO-
OFF switch to OFF.
c. Mountain and Coastal Effects. (groundwaves
from low frequqncy radio stations may be reflected
ranges and tune to a radio station in each range to
by mountainous terrain. This causes the radio
make certain that band switching takes place.
waves to bend and the bearing to a station may
change. The same error may occur when crossing
f. Tune thc receiver to a radio station the bear-
a coastline. To reduce this error, increase the alti-
ing of which is known. Set function switch to ADF;
tude or tune direction finder set to a stronger radio
visually see that the bearing indicator pointer indi-
station.
cates the correct bearing to radio station and coin-
cides with the magnetic compass.
pointer may oscillate and point to areas of electri-
g. Using the maximum speed position of LOOP
cal interference. If operating on adf, use the loop
switch (last position right or left), rotate bearing
mode and aural null homing (para 2-6).
indicator pointer 90 right and then 90 left of adf
bearing obtained in f above. At both the right and
left 90 bearing points, reset LOOP switch to its
29. Preflight (Daily) Operational Check
center position. Bearing indicator pointer should
Perform the following preflight operational check
return to the original adf bearing indication at a
during engine warmup and just prior to flight.
rate of not less than 25 per second (use a stop-
Report any direction finder set malfunction or fail-
watch).
ures noted in flight or during the preflight check in
h. Set function switch to LOOP.
accordance with the requirements in TM 38-750.
i. Using the slow speed position of LOOP switch
a. Set control unit function switch to ANT. See
(first position right or left of center), rotate bearing
that tuning meter, panel, and FREQUENCY dial
indicator pointer 360 in each direction and listen
are illuminated. Allow 5 minutes for equipment
for an aural null in headset. There should be two
warmup.
bearing indicator null positions displaced 180 from
b. Set range switch to the frequency range of a
each other. Stop bearing indicator pointer on the
radio station. Adjust TUNE control for radio sta-
null that points away from the station.
tion and maximum tuning meter indication, Note
j. Set function switch to ADF. The bearing indi-
FREQUENCY dial reading under the hairline.
cator pointer should rotate 180 and indicate the
c. Set BFO-OFF switch to BFO.
same adf bearing as in f above.
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