1296MHz or 23cm Beacon

A nearby Ham, and a good friend of mine, VK6KDX Jack is working on an EME station for 23cm. Locally our 23cm beacon has been off air for eons. For testing purposes Jack needed a distant signal source. Here then is the 23cm beacon, measuring 10cm x 5cm, under construction:

The beacon is now complete and tested. The ident is in CW every 60 seconds. The PLL chip is loaded at start-up by a STM8S1003F, or STM8S003F, micro running the fantastic STM8 forth with more details here. This micro also gates the amplifier chain to produce the morse ident.

I used parts I had in the junk box. The PLL was recovered from some TV modulator boards I have. The varicap came from another TV demodulator board that I had. The mmics were all recovered too. Except for the final mmic. I had planned to use the AH1 mmic shown in hte phot above which is a 100mW output device. However, both my recovered mmics were broken. So I used an AH101G instead which is a 500mW device. The standing current of 200mA from an 8V supply means 1.6watts is being dissipated which my board wasn't designed for (the AH1 is only 0.75W ). To solve this I used some copper tape to better connect thermally the two sides of the board and create a small fin. I also attached a heatsink to the opposite side of the board. So far, so good. The device hits 75 degrees Celsius and should survive the abuse it will receive.

I'll just have to buy some AH1's and either retrofit one, or build another beacon up. I have a lot of boards left over. Please contact me if you want one. I can furnish a partially populated and programmed board for you.

The code can be found here.

Regards
Richard VK6TT

40m Direct Conversion Receiver - Completed

Update: Also read this for fixing AGC for weak signals

It doesn't matter how good something appears on the test bench, it's the on-air baptism that counts. This weekend there was an ARRL contest. A proper baptism of fire. So imagine the smile when I turned the receiver on with a dipole connected. Wall to wall signals. North American stations, JA's, VK's.  They were all there. Strong, loud and clear.

This morning, Sunday, I listened to the local new broadcast on 40m. I could hear every station that checked in. And I could just cope with the strong signal of Chris VK6JI who runs the broadcast and call-backs. Chris's station is perhaps 5km away so it is a full scale signal on every radio I have. I hope to simultaneously  record the call backs one day, my homebrew receiver on one channel, my IC746pro on the other. I know there will be a difference, but not much.

Now that I have the bugs of this approach sorted out I'm using one of the commercial boards for an 80m version.

There is one thing that I learned from this version: how a voltage inverter can cause trouble. I used an ICL7662 to create a negative rail for the op amps.  What I failed to catch until I tested the unit was the ripple from the ICL7662 was large enough in amplitude to upset the squelch circuit. That was my fault for not bypassing the V+/2 rail the squelch used. One capacitor and problem solved.

Now all of my design goals have been met.

• lots of volume from a speaker since headphones don't work if you're moving about the workshop while you listen to a net,
• Passed - I can easily listen to a net while moving around my very large workshop.
• no audio instability, 
• Passed. Micro-phonics solved by using tantalum's instead of MLCC's in certain parts of the circuit.
• no hum, 
• Passed.
• good AGC so you are not jockeying the volume control, 
• Passed
• good audio filtering
• Passed
• a squelch. 
• Passed.

That's 6 from 6. I'll finish the 80m version soon but it's time to focus on updating the synthesiser and transmitter to accompany this receiver. Then I'll have a working homebrew 40m transceiver again.

Complete schematics except for the squelch. I'm happy to pass on details of the squelch circuit privately. But as far as I know it is copied from a Codan radio so I'm mindful of copyright.

Regards
Richard VK6TT