GB3ZZ is Bristol’s 23cm television repeater. It receives signals on 1249MHz and re-broadcasts on 1316MHz with 6MHz inter-carrier sound. Its receiver and transmitter both work in FM mode.
While idle, a testcard is transmitted, at 30 minute intervals this changes to a short sequence of text screens which describe the repeater and give tips on how to use it. When it sees a signal on its input frequency, the testcard or information page is switched off and the signal is relayed through the transmitter instead.
To optimise the signal reaching the receiver, different receive antennas can be selected by sending keypad tones on the audio channel of the transmission. Codes *00# through to *05# will select a directional antenna. *00# selects the antenna facing approximately north, the others select antennas at 60 degree clockwise intervals (going east first). Keying *06# through *09# or losing the received signal will reselect the omni-directional antenna.

The equipment rack at GB3ZZ holds monitors for incoming and outgoing signals, the receiver, transmitter, antenna switcher, control computer and a VCR to record and later replay signals.

Also on the repeater site is a VHF radio link, used to provide feedback from users while maintenance or installation work is being carried out.

A modified satellite receiver can be ‘patched’ into the repeater to allow pictures from GB3XG, its sister station to be relayed.