Monday, 11 August 2014

Wispy on test!

After finally discovering that I had simply not been driving her hard enough, I have the 10m Wispy beacon transmitter testing to air.

Several problems had to be solved to get this far. First, I found the laptops clock is woeful! I now have Dimension4 running on both the laptop and the desktop to keep the clocks happy. Since doing this, my own decoding of the beacon (via the main station) is almost solid. Second, the main antenna, during high winds this morning, failed. The wire snapped right at the feedpoint, necessitating some quick repair work to get the antenna back in service.

The picture shows my own decodes of the beacon. From this, and some other test information I now now that the Wispy Tx takes around 30mins to stabilize during warm-up, after which its drift rate is negligible, but the initial drift during warm-up is about 60Hz. I  also know that the Tx takes a tad below 160mA, even in standby. This is due to the amplifiers always being powered. I might, later, add a Tx relay so the amplifiers can be off and allowed to cool when not in use, but that will add the complication of needing either a PTT input or a tone detector circuit, like in the FT-857D Interface.

Prior to any of that, I'd like to play with the biasing and see if I can lower the quiescent current somewhat. I also want to play with the drive and the output filtering to further reduce the harmonics in the output. If I can reduce the spurii going into the amplifier, then I can ensure that as much of the power consumed as possible is being used to generate my wanted 28MHz DSB signal.

To this end, Ive added the little 28MHz bandpass filter to the output of the SBL-1 DBM on the 'TennaLady' mock up, as well as adding the necessary parts to convert the oscillator to 14MHz self-doubling to 28MHz. If I can kill the spurii prior to the amplification chain then so much the better.

Another little beastie im looking at is a 28MHz 1W Power Amplifier, using parallel small signal bipolar transistors. Roger G3XBM has used this approach in the Tx only version of Wispy, plus in his 'tenbox' AM set. With 4x 2N3904 NPN transistors, Roger gets 500mW DSB, 250mW SSB equivalent. So, Im going to try with 8x transistors! Hopefully giving me an equivalent of 500mW SSB.

The photo just shows two transistors fitted, which would give the same output as the single 2N3866 im currently using in Wispy.

Unfortunately, so far my Wispy beacon has not yet been spotted. Ive had her running most of the day, but the propagation on 10m at present is dire. I'll let her run over night, and into the morning. Depending on the wind, I might put the 10m J-pole up and run her to that tomorrow, see if having a resonant antenna and not having to go through an ATU and long feeders makes much difference. At 100mW I think every little helps. At some point however I want to work on the receive side, so if I make decent progress on TennaLady tomorrow, I might try her to air for a bit while I do the Rx chain on Wispy.

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