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|  MIL-N-81857(AS)
3.4.22 S y s t e m D e l a y :
3 . 4 . 2 2 . 1 Normal TACAN System Delay - The equip-
ment shall be so calibrated in the T/R mode as to effect an
indication of zero 0.1 mile when the delay at the antenna ter-
minal between the 50% amplitude points on the respective leading
edges of the second RF interrogation pulse of a coded pair and
the corresponding second RF reply pulse of a coded pair is 50
microseconds.
3 . 4 . 2 2 . 2 Air-to-Air System Delay - The equipment
shall be so calibrated in the A/A mode as to effect an indication
of zero 0.1 mile when the delay at the antenna terminal between
the 50% amplitude points on the respective leading edges of the
second RF interrogation pulse of a coded, transmitted pair and
t h e corresponding received RF reply pulse is 50 microseconds.
The delay at the antenna terminal between the 50% amplitude
points on the respective leading edges of the second RF interro-
gation pulse of a coded, r e c e i v e d pair and the corresponding
t r a n s m i t t e d RF reply pulse shall be 50 0.5 microseconds when
the level of the received signal at the antenna terminal is -40 dbm.
3.4.23 Frequency Channels - The receiver-transmitter
shall provide 126 crystal controlled channels as specified in
M I L - S T D - 2 9 1 . T o insure reduced spurious radiations, 126 crys-
t a l s shall be employed, one per channel. The receiver-transmitter
shall be automatically electrically tuned to the desired channel by
means of the remote channel selector switch on the control unit.
3 . 4 . 2 4 I m p e d a n c e L e v e l s - The RF input and output shall
provide for a common connection to a 52 ohm coaxial line. The
receiver and transmitter shall not be damaged under any mis-
m a t c h e d conditions.
3 . 4 . 2 5 Antenna Switching - An antenna switching feature
shall be incorporated in the equipment for the purpose of selecting
one of two associated antennas. The circuit shall be so designed
as to accept the first antenna which provides satisfactory signal
l e v e l . T h e equipment shall provide an "either-or" voltage for
the control of the antenna switch. The antenna switch shall not
be a part of the equipment specified herein and shall be a type
S A - 5 2 1 / A in accordance with Specification MIL-S-25879 or an
a p p r o v e d equivalent. In the absence of a usable signal the antenna
. . . . cont'd.
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