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GB3KD is incredibly simple, hardware-wise. It consists of a computer with sound card and Internet connection, connected to separate 2m receiver and transmitter units (Key KM150L) via a home-built interface unit. The receiver and transmitter share a single aerial via a 6 cavity duplexer and a circulator.
Audio from the receiver is fed via the interface to the computer's sound input, and audio from the computer's sound output is fed to the transmitter. The computer also receives status information and controls the radio's push-to-talk via its RS232 output and switching components in the interface. A single computer program, running in DOS, performs all timing, audio, tone decoding and logic functions, plus the Echolink functionality via an Ethernet connection to the Internet router.
Not shown are the essential fan to keep the transmitter cool, the PSU, the extra cavity filters which keep the signal clean and allow co-existence with the Packet Radio station, and the watchdog which guards against computer lockup.
From centre top to bottom this picture shows the following: loudspeaker, KM150 transmitter, KM150 receiver, watchdog, and modified switch-mode computer PSU which powers it all. To the right of the PSU is the audio interface unit (see below).
As you can see, it is completely unimpressive. The computer is not shown, because it's just a boring old Pentium 166!
These 3 pictures show the interface unit constructed by Richard G0EWH. Together with the computer, this forms the "heart" of the repeater, in place of a conventional logic unit.
The two black cubes in the center are the signal transformers which, together with the two opto-isolators near the rear panel, provide isolation between the computer and radio grounds.
There's nothing special about this interface, other than the fact that the transformers are correctly matched to the radios, to give a flat audio response.
This is the home-made 6 cavity duplexer, contructed of copper pipe and brass.
The aluminium boxes contain inductors and capacitors to make the notches asymmetrical, i.e. the baseline loss is lower on one side of the notch than on the other.
On the transmit cavities, this is arranged to give lower loss on the transmit frequency, i.e. the HF skirt of the notch, and vice versa for the receive cavities.
The Watson WX4 dual-band collinear is the 14 foot long white stick at the top of the mast. The camera resolution is too poor to show the aerial properly, but you may just be able to make out the 3 radials at its base.
The fuzz at the bottom of the picture is the treetops. A second set of guys is due to be added as soon as possible.
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