A very pleasant Ham Radio day. My friends Joel and Mark (VK5QI) visited my home to build Peter Parkers (VK3YE) “Porta 40″ DSB receiver (from the November 2012 issue of “Amateur Radio” magazine). Joel did the assembly work, with Mark and I helping test the receiver.
We started with the local oscillator, and checked it could be heard on a nearby SSB radio. We then built the RF Amp, mixer, and AF Amp. We tested the mixer with the use of a signal generator (an Arduino controlled DDS) and an oscilloscope and verified the mixer loss was just a few dB. An oscilliscope was used to verify each amplifier stage had gain. At the end of the day we connected the receiver to an outdoor antenna and were surprised to hear Ham radio signals on the 40m band! It felt very satisfying – I think we were all a bit surprised that it worked! I enjoyed working with Joel and Mark – we all brought different skills to the project and worked well as a team. I took care of catering – cooking a nice curry for lunch and keeping the coffee flowing.
First FreeDV Contact
For the last few days I have been trying to make some contacts on the HF bands using my FT817 5W radio and various antennas, including a long wire (with antenna tuner), a commercial end-fed trap diople, and a magnetic loop. The long wire and dipole are propped up at one end with a 7m “squid pole” type fibreglass fishing rod:
I could hear quite a few signals, but no one could hear me. I think the problem was low power and low antenna height. This WebSDR receiver located about 800km away has been very useful. It lets me visualise big chunks of the band for signals I could transmit CW and SSB to it to test various antennas, listening to the results through my laptop.
Mark brought along his 100W Icom HF SSB Radio to try a little more power. While connecting his radio to my antenna we noticed some activity on the 20m band that sounded like FreeDV. We connected his radio to my laptop and could visualise the BPSK sync part of the FreeDV signal, but it was too weak to decode. However we did learn that the band was open to VK2 (about 1500km away), so Mark IM-ed Brenton, VK2MEV, and we started a FreeDV contact, using about 10W of power at our end.
We experienced SNRs in the 5-10dB range with about 80% copy. Some speech was lost when the fading got really bad. The fading on the channel was changing all the time, fast to slow and back again. I think the other Hams running FreeDV just before us were Peter and friends, as described in Peter’s FreeDV blog post today.
Being a speech coding guy I was initially unhappy with the coded speech quality but after a while I seemed to adjust and it seemed just fine. The microphone EQ was useful in improving Brenton’s voice. FreeDV was easy to operate and the colourful GUIs make it interesting and fun.
The tx/rx switching was slower than Push To Talk (PTT) SSB, due to internal latencies and modem sync times, plus the VOX delay. It would be hard to do any real time break in, but didn’t affect our conversation. We switched to SSB to compare and were struck by the “hiss and crackle”, with fading on the analog audio also obvious.
Later in the day Mark and Brenton sent some test data frames over the same channel. I will use this test data to start optimising FreeDV for low SNR channels. Having known data means I can measure the bit error rate, and even extract patterns of bit errors that I can apply to off-line simulations of the system. I can then compare different demodulation and FEC schemes before returning with the best candidates for some real world testing.