PRC1099A-MS
7-1
Chapter 7: Antenna Tuner Board
7.1
Circuit Description
The tuning circuits on the Antenna Tuner board must match the radio RF output
impedance to that of the standard AT-271A/U whip antenna as well as a variety
of long wire antennas over the 1.6 to 30 MHz range. In a manpack radio, this
poses special design problems because the antenna is very short in relation to a
resonant antenna over most of the frequency range. Additionally, the ground
system is extremely poor in the manpack configuration. Since very small
movements by the operator detunes the antenna, it is very difficult to maintain
an exact match to the antenna. The tuner must perform quickly, have low
current drain in tune mode, and have no standby current drain. It is also
desirable to keep the tune power as low as possible and to retain the tune data in
memory for the fixed channel frequencies.
7.1.1
Matching Network
The resonant frequency of the whip antenna is approximately 24 MHz. This
means that over most of the frequency range the antenna has a capacitive
reactance that can be cancelled by an equivalent series inductance. The tuner
uses a binary-sequenced series of inductors to cancel out the reactive
component of the antenna. If the antenna has inductive reactance, the tuner adds
capacitance so the antenna appears capacitive and cancels small residual
reactance by adding series inductance.
The internal capacitors match the whip antenna and most long wire antennas,
but on some frequencies the inductive reactance can be very high. The long
wire adapter contains a series capacitor to bring the reactance within the tuning
range of the network. After the reactance is cancelled, the tuner presents a
resistive load between 6 and 400 ohms to the transceiver. A broadband
impedance transformer selects the closest available matching impedance: 12.5,
50, or 200 ohms.
7.1.2
Tuning System
The PRC1099A uses very simple and reliable tuning system. Instead of a
VSWR detector, a current detector measures the current flowing to the antenna.
The transceiver operates in low power (5W); a 10 dB attenuator reduces the
