500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page
4-2
15 January 2005
4.3.9
MISCELLANEOUS
(CABINET/
INTERFACE PANEL) CONTROLS AND
INDICATORS:
Figure MD-1 depicts the controls on
the transmitter cabinet and figure MD-29 depicts the
controls on the interface panel assembly. Table 4-10
is keyed to the reference numbers assigned to the
controls and explains their function.
PRESTART-UP CHECKS
4.4
Prior to applying input power to the
transmitter, observe the following:
(a) Verify the pre-installation requirements
described in paragraphs 3.9.1 thru 3.9.5 have
been completed.
(b) Verify the transmitter modules have been
properly installed in the cabinet and all panels
are installed and securely fastened.
(c) Verify interconnecting wiring has been
connected as shown in figures SD-1 and SD-2.
(d) Verify the transmitter's RF output connector J1
is terminated into a 50 ohm load.
-
An antenna that is interfaced by a tuned antenna
tuning unit for normal adjustment, testing and
operating procedures.
-
A 50 ohm, resistive, dummy load with a wattage
rating of twice the rated carrier power of the
transmitter for harmonic filter procedures and
for specified special adjustment.
(e) Verify the voltage of the input power source is
between 180 and 250 (230 nominal) V ac, at the
appropriate frequency and power rating for the
transmitter (1200 VA).
TURNING ON TRANSMITTER
4.5
Turn on transmitter as described in
paragraph 5.3.2 for initial startup and after repairs that
may have affected the adjustment accuracy, at other
times, set the switches to position tabulated for
Test
Setting
in table 4-1 initially and then to settings
tabulated for
Operating Setting
.
RESETTING TRANSMITTER
4.6
Transmitters that have been transferred to
the standby side or that have shutdown, may be reset
by momentarily switching the transmitter
off
and then
back to
on
. This action may be taken locally by using
the
RF
switch or remotely using remote on/off control.
Transmitters that have been reset will always go to
the selected main side of the subject transmitter
(original state).
MODULATION DEPTH WHEN USING A HIGH
'Q' ANTENNA:
4.7
A typical radiobeacon antenna is relatively
inefficient, since it is very short when compared with
the wavelength of the carrier frequency. The high
capacitive reactance of a typical antenna is tuned to
the carrier frequency, by an antenna tuning unit's
(ATU) loading coils, to produce a series resonant
circuit. The resulting net antenna system resistance is
then transformed to 50 ohms by a matching
transformer. When the antenna is very short
compared with the wavelength of the carrier
frequency, the resonant circuit has an extremely high
'Q'. Under these conditions, the antenna system may
present a 50-ohm load to the transmitter at the carrier
frequency but a different impedance at the sideband
frequencies. The mismatch at the sideband
frequencies will cause a standing wave on the feed
cable. Depending upon the length of the feed cable,
the sideband impedance of the antenna system will
appear to be more or less than 50 ohms. When the
transmitter is connected to a high 'Q' antenna system,
the difference between the carrier impedance and the
sideband impedance may cause RF stress current
limits to be exceeded, as described in paragraph 4.7.1,
or reflected power thresholds to be exceeded, as
described in paragraph 4.7.2. When RF stress current
limits are exceeded, instant remedial action must be
taken.
