Sentry 5 prototype in the CQ WW CW contest

Chris Moulding

I mentioned a few days ago that I'd be running the Sentry 5 prototype in the CQ WW CW contest as a final test before starting the production PCB board design.

I've always wondered if it would be possible to make a SDR transceiver work full break-in on CW so that you can listen to the frequency in the gaps between the dots and dashes. I have an old TenTec Corsair 2 transceiver in the workshop that has full break-in and it's a simple elegant design that operates well.

The design of the Sentry 5 is such that on SSB, AM and FM it's a SDR transceiver with the transmit IQ baseband signal sent to an IQ modulator to generate the signal at the final frequency. For CW the local oscillator is shifted LF by 750 Hz on transmit and the IQ modulator is unbalanced by a FET with a timing circuit on the gate to generate the correct CW keyed waveform. When the radio is tuned so that the beat note is 750 Hz then the transmitter will shift to zero beat the incoming signal on transmit.

In the control circuitry there is a lot going on. There is a CW keyer with the speed controlled by a slider on the Sentinel SDR program. There is also a straight key option when one of the paddle inputs is shorted during start-up.

For the contest I decided to use the straight key option keyed by a USB interface using the DTR pin controlled by the N1MM+ contest logging program. For receive I used CW Skimmer controlling the Sentry 5 by CAT interface as a Elecraft K3. The CW Skimmer program has an IQ input for SDR receivers so it used the Sentry 5 receiver configured as a SoftRock-IF receiver in the program.

This allowed me to tune and change bands directly from CW Skimmer and decode the CW and transmit using the macros in N1MM+. I would enter the Assisted and QRP section of the contest as the power was set to 5W output.

So that's the technical set-up, how did it work?

On transmit I could hear between individual dots and dashes up to 18 wpm, beyond that it was between the character spaces. As most of the contest I was running at 28 wpm I could still here if another station was calling at the same time or whether the station I was calling was calling over me. So the break-in works, it just needs a bit more attenuation of the received signal during transmit to reduce the click and clunks but that's a minor tweak.

I don't know how people can operate in a CW contest without CW Skimmer. Some operators were operating above 40 wpm with many repeats and some morse was bad with reduced spacings between the elements. My morse is very rusty, I can copy the usual CQ, Test, AGN, and ? at any speed including my callsign as a happy little tune but I can only copy callsigns correctly the first time at 12 wpm so I did need assistance. I used headphones to hear the signals to get my timing right and also to try and improve my reading speed. I operated search and pounce as my QRP signal would be too small for sitting on a frequency calling CQ.

I thought I'd only make a few contacts but as usual I got carried away once I found that it worked. I ended up with 289 contacts using just 5W to the Terminated Inverted U antenna mainly on 40 and 20m.

I've attached a screen shot of the PC showing CW Skimmer, N1MM+ contest logging program and in the background the Reverse Beacon Network Aggregator that sent spots to the Reverse Beacon Network. This was just before I closed down. CU4DX was a gotaway at 41wpm!


Chris G4HYG

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