2008-08-10

30 Metre QRSS Beacon

This weekend I had decided would be "QRSS weekend", I've been fiddling with the idea for too long now, it was time for a strong effort to complete the project. As luck would have it, David VK6DI found my 80 metre beacon and added some extra inspiration to get my act together on a 30 metres beacon.

I ordered my 10.140 MHz xtals from DL6JAN. The arrived quickly, nicely packed - it was an easy process, I highly recommend him as a source for QRSS rocks.

The Transmitter

The oscillator is a conventional Colpitts, with an 8 volts, 3-terminal regulator for voltage stabilisation. The buffer is a J310 JFET source follower with a 10 k input impedance fed through a 5p6 cap, the output is filtered through a LPF and delivers about 900 uW into 50 ohms.

Xtal Oscillator, Buffer and LPF

This is followed by a 2N3904 feedback amplifier giving about 20 dB of gain. This then drives my old SM0VPO QRP RF amplifier as a power amp.

Test Lash-up with Amplifier

A small low-pass filter follows to clean up the output suitable for delivery to the antenna. About 29 dBm is delivered into 50 Ohms.

Output Low-Pass Filter

The Keyer

The beacon keyer is a Atmel ATtiny13V running very simple code. It offers two outputs, one for CW keying (currently unused), and the other for FSK. The beacon CW idents three times at 10 WPM between the QRSS6 FSKCW idents. A small trimmer in the oscillator circuit is diode switched by the beacon controller to pull the oscillator an adjustable amount.

The Antenna

The antenna took much of Saturday to setup and debug. It is a loaded inverted V. Two ~27 uH coils allow the ~2.5 metre arms to be resonant on 30 metres.

Antenna Loading Coil

Precise tuning is achieved by a Z-match at the feedpoint.

Z-match at Feedpoint

The antenna is still very much work in progress, but looks like it will work OK for now. Return loss can exceed 40 dB, but seems to drift with the temperature of the day (can be tuned back up again). I suspect the loading coils are drifting around a little as they change shape with temperature. I'll have to lacquer the turns in place, but I am cautious about increasing their distributed capacitance and causing SRF and Q-reduction problems.

Results

David's grabber has been enormously useful during this development work. From the initial lash-up on solderless breadboards into the newly constructed antenna I've been able to see my wiggly signal on his grabber. Here is a shot of my first 250 mW signal through the antenna bridge while I was still playing around with the Z-match. The relatively stable signal is also mine, which I then QSYed to prove to myself it was really my signal I was seeing, that's the wiggly bit, I was using the C-jig to tune the xtal oscillator by hand.

First-Light for the Bread-boarded TX.

That's less than 1/4 of a watt from 3300 km away!

Improvements through the day lead to good signal reception at David's end.

My call on VK6DI's Grabber

Stability is still a problem, as the TX isn't boxed the air conditioning in the lab is causing the oscillator to drift around. I have some PTC thermistors that should make good oven heaters for the final assembly, I figure a nice diecast aluminium box will do the trick, with its larger mass and thermal inertia providing improved stability. Load variation frequency stability is excellent BTW, the JFET buffer and extremely conservative gain distribution gives good reverse isolation.

I've been monitoring the signal locally using my FT-817 and baudline running a 16384 point FFT.

Baudline monitoring my drifty TX

This isn't really optimal for QRSS visualisation, but gives me a good idea of my shift width, centre frequency and stability. Also, I found using my new 30 metre antenna into the same setup I could easily see many of the signals David can, in particular a stable carrier near 14.140000 MHz that seems to come and go. I may have to build my own grabber RX...

All in all, the results are very encouraging. I achieved an enormous amount of work in only 2 days, from building the antenna and Z-match, through to the various TX modules and software for the beacon. There is still much to be done however, I need to box and stabilise the beacon, build a dedicated 1W output amplifier for it rather than using the boxed test amplifier, and fix up the antenna in a more permanent manner, in particular water proofing the Z-match and fixing the end-insulators for the dipole arms. I'll have to post the technical details too, circuit diagrams, etc. Next weekend... :)

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Updates

2009-03-06: Hellschreiber QRSS
I implement sequential multi-tone Hellschreiber modulation in the 30 metre QRSS beacon.
2009-02-24: 30 Metre QRSS Beacon Calibrated
I finally sit down and calibrate the front-panel controls of the QRSS beacon.
2008-08-16: 30 Metre QRSS Beacon Updates
I complete and box-up the QRSS beacon, and experiment with a vertical antenna.