RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 4-3
15 September 2003
Table 4-1 Preliminary Switch Settings
NOMENCLATURE
REF
INITIAL
CALIBRATION
OPERATING
USED IN TEXT
DES
SETTING
SETTING
SETTING
KEYING
S1
OFF
OFF
ON
MOD
S2
OFF
OFF ON
POWER
S3
OFF
ON
ON
CONTROL
S4
LOCAL
LOCAL
User Determined
MONITOR
S5
BYPASS
BYPASS
NORMAL
TEST
S6
B+ VOLTS
B+ VOLTS
OFF
4.7.1.2
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 series 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 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. When this occurs,
remedial action must be taken instantly.
MODULATION PERCENTAGE INDICATION
4.8
Current and voltage transformers in the
forward/reflected power probe, which is in the output
of the harmonic filter A7, provide samples that are
representative of the RF output current and voltage
waveforms. When the transmitter's RF output is
being applied to a precision 50-ohm, resistive dummy
load, and the load impedance is precisely 50 ohms at
the carrier and sideband frequencies, the waveforms
provided by both transformers will be identical.
When the transmitter's RF output is being applied to
an antenna system, the sideband impedance will be
higher or lower than 50 ohms. If the sideband
impedance is less than 50 ohms, the current
transformer's waveform is selected as the source for
the RF monitor sample. If the sideband impedance is
greater than 50 ohms, the voltage transformers
waveform is selected as the source for the RF
monitor sample. The RF monitor sample is applied
to
RF MONITOR
connector J1 for external monitoring
and to a modulation detector circuit. The output of
the modulation detector circuit is displayed on
TEST
meter M1 as a modulation percentage indication.
