EMRFD Message Archive 277

Message Date From Subject
277 2006-12-30 00:36:00 Rick 505 kHz experiment
This evening I comfortably copied wd2xsh/20 on 505 kHz CW on a 6m dipole. Distance is
127 miles, and he's running 18 watts. Pretty cool. There are a number of 500 kHz
experimental stations on the air scattered around the US. Do a search on wd2xsh/20 for lots
of information on the web.

The receiver is a Racal 6790/gm. Then I swapped the same antenna over to the TS570 and
heard nothing. The receiver in the Kenwood transceiver tunes to the right frequency, but it
isn't very sensitive there. Clearly a 500 kHz preamp is needed for the TS570. Time to dig
into EMRFD.

Best Regards,

Rick kk7b
280 2006-12-30 16:21:27 Rick Re: 505 kHz experiment
Good comments. One reason 500 kHz is such a interesting band is that there are so many
options for technology. One possibility is a bandpass filter from 500-520 kHz and a little
signal conditioning followed by a fast DSP engine. Another is something as simple as a
microR2, with the VFO and LNA inductors wound on ferrite cores instead of powdered

Incidently, VFOs with ferrite cores can be quite stable at 500 kHz. Many 455 kHz BFOs
were built that way, and in those receivers it's seldom the BFO that limits stability.

The reason most general coverage receivers switch in a 20 dB pad when tuning below 1.8
MHz is that AM broadcast stations can be brutally strong, and few mixers will handle those
signal levels. Even order distortion of AM broadcast signals in the mixer of an
upconverting mixer will trash the ham bands. The Racal 6790 is one of the few receivers
with a wide open front end from true DC through 30 MHz, straight into a very strong
passive FET mixer.

For 500 kHz listening with a more conventional receiver, I'd suggest a tuned preamp
preceeded by a good low-pass filter at about 550 kHz. If I were going to build one
tonight, I'd just take the microR2 LNA design and multiply all the element values by 14 (7
MHz divided by 0.5 MHz) with a coax connector on the input and output so I could
measure it on the bench.

For low frequencies, the old URM-25 signal generator is hard to beat. They are cheap,
available, and designed to last as long as the aircraft carriers they were used on.

Best Regards,

Rick KK7B
281 2006-12-30 21:58:17 S. Cash Olsen Re: 505 kHz experiment

Rick and group,
Just before Christmas I ordered and have assembled the Jackson Harbor LF up converter kit. It fired off with no problems, I chose the 30M optional crystal. The kit also comes with a 75M xtal.
I am using it as a frontend to my Icom 706. I built it into a altoids tin. And I'm looking for the best sound card based software for decoding of QRSS modes. I'd appreciate some suggestions that will help get the most from this effort. I'd like to work both the 137 and 500kHz signals.
One local issue at my QTH is the proximity to a loran station (very high power 100kHz burst) just 15 miles to the south. I'm considering a notch filter, for this beast. Any suggestions?
Cash Olsen
Kits to build Scotty's Spectrum Analyzer http://www.zianet.com/erg
Scotty's Spectrum Analyzer website http://www.cpu-net.com/host/wsprowls
Sam Wetterlin's website http://www.wetterlin.com/sam/
Yahoo Builders Group http://groups.yahoo.com/group/spectrumanalyzer/
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282 2006-12-31 04:03:31 Luiz Amaral Re: 505 kHz experiment
It may be very interesting to analyse such signals using a SDR.

Luiz - PY1LL

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