Today the ARNSW Home Brew & Experimenter's group got together at Dural to show and talk about our "challenge" receivers. Here's some edited highlights:
Mark ran the broadcast transmitter exciter to provide a signal but it took us a while to realise that the 3.7MHz morse beacon transmitter was running and causing problems for some receivers.
Peter, VK2EMUs receiver is constructed from modules:
I've written up mine here and it's in the video above.
Alan, VK2ZAY, was away but supplied some video. He's written up his receiver here.
Missing is Stephen VK2BLQ who we believe built a receiver, hopefully I can get some video of his rig to add to the collected video.
Thanks to whoever suggested the challenge project. I think because we were all to receive the same broadcast signal it made for a level playing field. Thanks to everyone who participated and let me video them.
John VK2ASU's notes
The receiver is built on a 5 X 5cm circuit board, Manhattan style. The circuit has been drawn as per the layout on the board.
Signal from the antenna enters via a 1K linear pot which is used to attenuate very strong signals. The 2 X 1N914 diodes clip all strong signals including the broadcast band.
I used rewound 10.7 MHz IF coils (from a junked 40 MHz hands free phone) for the tuned circuits. Slug tuning of these saves on having trimmer capacitors.
The 10 pf light coupling capacitor between the tuned circuits kills broadcast band feedthrough and also virtually eliminates interaction when peaking the coils.
The 3.5 mHz ceramic resonator is still cheaply available from futurlec.com. The 100pf tuning capacitor has a 6:1 reduction drive. All small capacitors are ceramic NPO.
A 78L05 provides regulated 5 volts for the MC3371 and also keeps the oscillator very stable, such that tuning into the WIA broadcast (3.595) at switch on does not need adjustment for the duration of the broadcast. Tuning is from 3.540 to 3.625 however the use of a 160pf polyvaricon could be used and should provide coverage from 3.500 to 3.620. The wire from the tuning capacitor to the resonator is thick and short to minimize inductance.
Duplication of the receiver is easy and non critical. When the circuit board mill gets going it will be a snap.
Alternatives to the MC3371 are MC3361, MC3359 and MC3357. Using these, all the left hand pins connections are identical. The other pins are different but are left no connection anyway.
Interestingly the old TV antenna trick did improve reception:
YouTube Insight is interesting.. it seems that the video above is not very interesting to women: