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AO40 The Easy Way by VK3CJS

Page 1

Like me, you may have despaired of receiving AO40 on 2.4 ghz. I love to homebrew, but draw the line at messing with precision printed circuit boards and tiny surface mount components. If this description fits you then NIL DESPERANDUM ! Grab a big magnifying glass, hitch up your trousers and read on.....

  To the left is a Conifer terrestrial downcoverter which will cover the AO40 2401 mhz downlink band. This one came in new condition with it's open wire dish from the Eaglehawk (Bendigo) rubbish tip "shop".

The Conifer converts 2401.0 mhz pretty near exactly down to 450.0 mhz, but how to receive SSB on that frequency? Here's how to do it on the cheap..... Most scanners will cover 450 mhz. As far as I know most scanners use a first IF of 21.4 mhz, so we can use a scanner as an intermediate converter to bring our 450 mhz signals down to the 15 metre band. The scanner first IF can be tapped with a small (20pf) capacitor and a bit of coax to bring it out to your HF receiver. Handheld scanners are fine too, providing they have coax antenna sockets.

You can verify if your scanner has a first IF of 21.4 mhz by looking inside for the IF filter. It looks like an ordinary crystal but has 3 legs and will be marked 21. The tapping point should be BEFORE the crystal filter so there will be sufficient bandspread for the HF receiver tuning. Look for the IF transformer that feeds the crystal filter and try tapping around there. In my case there was an unused low impedance link on the first IF transformer.... how's that for good luck!

I also disabled the second local oscillator in my scanner by cutting the PC track to one leg of the 20.945 xtal. No point in having unnecessary birdies. Keep it simple!

Now you have a general coverage VHF/UHF SSB receiver to play with. Verify that all is well by dialling in to a beacon or some local activity with the scanner connected to an antenna while tuning around 21.4 with the HF receiver. IMPORTANT.... The scanner LO is ABOVE the desired frequency, so signals will be inverted. Set the HF rx to LSB to resolve USB (the convention on AO40). Also, to tune HIGHER in frequency using the HF receiver, you must tune DOWN from 21.4 .... Hmmmm.... confusing! You'll get used to it.

  Here is my bare bones AO40 receiving setup. With the scanner readout set to 450.150 the Middle Beacon appears around 21.250 (centre of 15m band) depending on doppler shift, which can be around +/- 30 khz at 2.4 ghz. The dish is roughly pointed at the satellite.... don't worry, you'll hear it.

The downconverter needs 15 to 18 volts injected into the co-ax downlead. An old TV preamp injector box will do, otherwise a nice clear diagram can be found Make sure there is at least 14 volts at the dish end, otherwise the LO will not fire up properly.

Once we've achieved results, let's consider some improvements....