Summer Ham Project

Back in the early 80’s I used to mess around with Ham Radio. That’s what people interested in electronics did, especially before PCs were common. Similar culture, lots of hacking and open source hardware. Good to see it coming full circle now with open source software people starting to mess with hardware through projects like the Arduino. I also notice a lot of FOSS people getting Ham licenses which is cool.

Summer always makes me think of Ham Radio, as thats when our big High School/University holiday break was (and still is) so we had time for projects instead of study. Nothing like clambering over a corrugated iron roof and soldering when it’s 40 degrees C. So after a break of about 20 years I have been messing around with a small home brew Ham radio project with Kim Hawtin, a local Ham and Linux guy.

We have been building a Pixie 2. This remarkably simple circuit is a fully functional 80m band (3.5MHz) CW (morse code) transceiver.

Kim and I had fun working through the various bugs, over a couple of afternoons. It’s funny for me coming back to a hobby I did as a teenager. I actually understand some of the electronics now, rather than just assembling circuits and hoping they work. Actually that doesn’t sound too much different to the way I build IP-PBXs!

Here is a picture of my Pixie 2, check out the 3-dimensional construction style:

The Pixie transmits 200mW into a 50 ohm load and I have heard a few SSB signals on the receiver. Unfortunately as it’s crystal locked I can’t tune the SSB signals clearly. I am a little scared about trying to make a Morse Code contact as I am 20 years out of practice with Morse.

I have also built a simple inverted-V 80m dipole antenna, suspended from my Wifi antenna mast. My dipole is just two lengths of wire (each 20m long), fed in the center by coaxial cable. Getting quite an “Antenna Farm” in the back yard now:

I tuned the dipole using a very effective impedance bridge that Kim built:

Here is the bridge circuit. The bridge is basically two voltage dividers, with the transformer and LED measuring the difference between the middle of the two dividers. If all resistances in the bridge (including the antenna) are 50 ohms then the voltage across the transformer and LED is zero. I made one of the resistors variable, then while transmitting adjusted the resistor for minimum voltage across the transformer. Then I measured the variable resistor with a DC multimeter to get the antenna impedance.

I measured my antenna impedance as 30 ohms at first, so I shortened one side of the dipole by about 20cm and it measured bang on 50 ohms. I must admit still find it a little amazing (and hard to understand) that an antenna can have an impedance of say 50 ohms. The antenna theory is beyond me. But it was interesting to measure and see the effects of adjusting the antenna.

Electrical noise from all the computers made the receiver hard to test in my home office, so later that night I tried it outside. However most of what I could hear was AM radio – a nearby AM radio station was strong enough to “break through” my receiver. A little Googling suggested that a “balun” would reduce AM break through on the Pixie. The reason is something to do with feeding the dipole antenna from coax. Coax is unbalanced, which means the outer conductor remains at a fixed potential while the inner conductor moves around with the signal V. However dipole antennas are balanced, both side of the antenna move in opposite directions of 0.5V each. If you feed a dipole with coax you can get problems like the coax shield picking up and radiating signal. I must admit this is all a bit fuzzy to me, need to do some more reading.

Anyway the solution is a little transformer called a balun (balanced to unbalanced) that is installed where the coax connects to the antenna. I made one up from a AM radio ferrite rod which I think is OK at 3.5MHz:

It actually worked – the AM radio break through appears to be gone and I managed to here the “Donald Duck” sound of a couple of Ham SSB stations on the receiver.

Not quite sure about my next step. I might use an AVR microcontroller to key the transmitter and turn it into a beacon for testing 80m propagation. The current frequency is right in the middle of the SSB part of the Ham band so I might need to re-crystal for the lower CW part of the 80m band.

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