7: Antenna Tuner Board
7-2
PRC1099A-MS
power to 500 mW. Because the output impedance of the transmitter is
50 ohms (determined by the resistive pad), maximum current flows when
there is a conjugate match or a resistive load of 50 ohms. This system is
simple, reliable, and accurately determines minimum VSWR.
7.1.3
Tuning Resolution
The antenna tuner typically tunes for a VSWR of less than 1.5:1 but can reach
2:1 if the resistive component falls midway between the impedance taps. In a
manpack environment, the impedance match changes every time the operator
moves. The final amplifier is tolerant of mismatched loads.
The PRC1099A final amplifier is capable of a power output of 30 to 40W and
can deliver 20W into substantial mismatches. At VSWRs of 2:1 there is
typically less than 0.5 dB output degradation and, even at a VSWR of 3:1
there is only about 1 dB performance change.
7.1.4
Tuning Program
The tuning program resides in ROM and is controlled by the processor on the
Processor board. When an operator presses the front panel
Whip Tune
button,
the processor checks the channel frequency and determines which elements to
use. The transmitter is keyed in low power and the firmware steps through the
inductors in a special sequence until it detects a current peak. If the tuning
program does not detect a current peak, the antenna is inductive and the tuning
program combines capacitors with the inductors until it detects a tuning peak.
At this point the tuning program checks the antenna current. If the current is
low, it checks the alternate tap positions, and selects the tap with the optimum
current. The tuning program then rechecks the current. If the current is low,
the processor signals an incorrect tune.
You can operate the transceiver even when the processor indicates an incorrect
tune—the tuner remains at the setting that achieved the best match. The
processor keeps the last tune settings in memory and automatically selects it
each time the channel is changed. The processor updates tune information in
memory each time the
Whip Tune
button is pressed. The processor
automatically selects the antenna tuner when the whip or long wire adapter is
used. A rod on the end of the antenna mount or long wire adapter activates a
microswitch that disconnects the 50 ohm connector.
Figure 7-1 on page 7-3 provides a block diagram of the antenna tuner and
control circuitry. The following sections describe the components of the block
diagram.
