Johnson DM3474, Service Manual

Page: 36 / 64

CIRCUIT DESCRIPTION
4-12
Part No. 001-3474-002
C567, L566, and C570 form a discrete quarter- wave line. When CR561 is forward biased, this quarter-wave
line is effectively AC grounded on one end by C570. When a quarter-wave line is grounded on one end, the other
end presents a high impedance to the quarter-wave frequency. This blocks the transmit signal from the receiver.
C569 matches the antenna to 50 ohms in transmit and receive.
In the receive mode, no power is applied to L565, so all the diodes are “off”. The receive signal then has a
high-impedance path into the transmitter and a low-impedance path into the receiver because the quarter-wave line
is not grounded.