The BITX20A Transceiver


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SWR Bridge
for the BITX20A

swr bridge The simple resistive bridge (click on image, left, for larger version) is used to give an indication of SWR when tuning up the antenna. In use the tune button is pressed and the rig switched to TX, the 900 Hz oscillator is fed to the mic amp at a level that produces around half a watt of SSB. The AO91 diode detects any voltage arising from the antenna being higher or lower than 50 ohms, this voltage is fed via the RL2c contacts to the bargraph. When the SWR is 1:1 there are no LEDs alight on the bargraph.

swr board The bridge is constructed using two 100 ohm resistors in parallel in each resistive arm, this ups the wattage to one watt in each arm. The resistors are placed close to the relay and are wired directly to the contacts. The screened feeds from the TX and to the antenna are also taken directly to the relay. As can be seen, the relay is actually two 2 pole C/O relays, one of which switches the SWR bridge in and out of the RF path whilst the other switches the input of the bargraph driver from SWR bridge ouput to the RX signal level on receive and also switches the 900 Hz tone into the mic amplifier. Hopefully, the use of two relays will prevent RF getting into the mic amplifier.

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