The ANTENNA CHANGEOVER RELAY POWER SUPPLY

The simple unit shown here provides around 22 to 24V DC, easily enough to operate Transco or Dynatech type microwave 24 -28VDC coaxial relays. It consists of an NE555 oscillator which drives a complimentary pair of transistors(TIP31/TIP32)into a diode pump (2 x 1N4001). A small piece of veroboard was used to mount all the components except for the two transistors, which are securely bolted to the wall of the small diecast aluminium box which acts as a heatsink. One of the transistors is fitted with an insulating washer as its flange is above ground potential. I'm indebted to Dave Robinson, G4FRE (WW2R)for this nice little circuit. Once again, the circuit and full details can be found in the PDF document concerning this transverter project.

The PA Power Supply was also made on Veroboard and housed in an aluminium box with all input and output connections made via feedthrough capacitors. The supply provides nominal +10VDC, +12VDC, +5VDC and -12VDC. The latter is produced by a DC-to-DC voltage inverter IC, the NMS1212, shown here on the left side of the photo. This was the only IC I could find that would produce enough current for the -12V PA bias. The +10V supply is produced by an LT1083CP regulator which is fed from the main +12V line of the transverter. This chip is capable of producing much more than the 4 or 5amps required by the PA and so is well under-run in this application. Nevertheless it requires good heatsinking as can be seen in the photo above. The supply has a built in failsafe circuit to protect the PA in the event of the -12V bias supply failing. The main +12V input is switched to the LT1083 regulator by a relay. When the bias supply comes on, a transistor relay driver is operated by the -12V from the inverter. The small relay (contacts rated at 10A DC)can be seen in the lower right of the photo and this then switches the +12V input to the 10V regulator. Similarly the +5V supply, which enables the PA, is only activated when the same relay contacts are closed. The NMS1212 bias supply IC was chosen because it supplies -12V even when the portable battery supply drops from a fully charged state. Normal regulators need some 2 volts headroom and thus an input of almost 14 volts would be required, not easily come by under portable conditions. The circuit of the PA power supply is a modified version of one supplied by John, G3XDY, and can be recommended for other PAs where voltages other than 12V are required at several amps.

The Sequencer is a DB6NT SEQ3. It can handle a PA supply of 12 volts at 18 amps, well in excess of my system! I believe in rating components very conservatively. That way you avoid system failure later. The sequencer comes ready made for around 25 Euro (about $25US) and is hardly worth the bother of making it yourself. I mounted this one in a diecast aluminium box with all supplies in/out via 1000pF feedthroughs. The sequencer provides proper TX/RX/TX changeover protocol so that the sensitive receiver GaAsFETS are protected from the RF damage that would almost certainly occur if no sequencer were used. A good friend of mine tried out his 15 watt PA 3.4GHz transverter in a contest and blew his receiver "front end" almost immediately ... he was not using a sequencer!