500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-12
15 January 2005
2.8.2
FWD/REFL POWER PROBE (A9A2):
Voltage transformer A2T1, current transformer A2T2
and their associated components provide voltage and
current arms of a forward/reflected power bridge,
which samples the RF output signal. Current
transformer A2T3 and a secondary winding of
transformer A2T1 provide voltage or current
waveform outputs (as selected by
RF MON
switch) for
monitoring purposes. The anti-phase voltages
developed across A2T2's secondary are summed with
the voltage from A2T1 to provide DC voltages at the
cathodes of A2CR1/CR2 which are proportional to
the forward/reflected power. A2R1 through A2R8
provide 50 ohm loading for their respective
transformers.
2.8.2.1 RF MON Switching:
RF MON
switch
A2S1 allows selection of either the voltage or the
current waveform to be monitored on RF monitor
BNC connector J1 on control/monitor A1's front
panel. The following provides the necessary
information for the setting of the
RF MON
switch.
2.8.2.2
The high capacitive reactance of the
antenna is tuned at the carrier frequency by the ATU
loading coils to produce a series resonant circuit. The
resulting net antenna resistance is transformed to 50
ohms in a matching transformer to provide the load
impedance required by the transmitter. When the
antenna is very short, compared with the carrier
frequency's wavelength, the series resonant circuit has
an extremely high Q. Under these conditions, a
perfect match may occur at the carrier frequency, but
the sidebands may be mismatched, causing a standing
wave on the feed cable at the sideband frequencies.
2.8.2.3
Depending upon the length of the feed
cable, the impedance at the sideband frequencies may
appear higher or lower than 50 ohms. If the sideband
impedance appears low, the current waveform should
be selected by
RF MON
switch to prevent excessive
current overloading. If the sideband impedance
appears high, the voltage waveform should be
selected by
RF MON
switch to prevent excessive
voltages occurring at the sideband frequency. The
correct setting of
RF MON
switch is made by choosing
the waveform which displays the greater modulation
depth.
2.8.2.4
The use of an inefficient antenna produces
a bandpass filter effect which reduces the modulation
depth of the radiated signal.
2.8.2.5
Under no circumstances should the
modulation level, as measured by the current or
voltage probe, be adjusted beyond 95 % in an effort to
offset the sideband attenuation which occurs in the
antenna. This will cause excessive dissipation and
distortion to occur and would create spurious
emissions which do not comply with national and/or
international specifications.
MONITOR PWB (A10)
(see figures SD-12 and
SD-13)
2.9
The monitor PWB monitors critical
parameters of the subject transmitter and produces
control signals which turn on display lamps when
these parameters are not met or a status condition of
the transmitter is to be known. The PWB also
produces and applies external alarm and status control
signals to a remote location for the user's information.
Test switches, located on the PWB, can be set to
enable calibration/repair on one side of the transmitter
while the other side is operating normally.
The monitor PWB provides adjustments for the
carrier and modulation thresholds, sets the changeover
time delay (seconds) and produces the unm15
V dc and -15 V dc for the subject transmitter. The
board also provides control signal outputs for the
forward/reflected power (remote and local) and
produces control signal outputs for the battery, SWR,
shutdown, standby and RF current alarm circuits.
2.9.1
TEST SWITCH CONTROL:
Under
normal operating conditions
TEST B
switch S1 and
TEST A
switch S2 are set to
NORMAL
.
When either is
set to
TEST
, the RF output from that side
A
or
B
will
be inhibited and it will be operating into an open
circuit. The switches are normally set to
TEST
when
performing maintenance procedures.
2.9.2
RELAY SWITCHING:
The relay
switching circuits contains remote on/off relay K1,
shutdown relay K2, standby relay K3 and A/B main
select relay K4. The following paragraphs describe
each relay switching circuit:
2.9.2.1
Remote On/Off Relay:
If the transmitter
is set to local control, the remote on/off relay will
have no influence. During normal operations, when
remote is selected, relay K1 will be de-energized
(transmitter turned on). When a remote ground
