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Date: Fri, 13 Jul 2007 22:52:05 +0100
From: John GM4SLV <gm4slv@sighthound.demon.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Message-ID: <20070713225205.2cec3a03@lurcher.twatt.local>
Organization: The Gammy Bird
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Subject: LF: 500KHz signals in Shetland
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Status: O

Hi all, 

A brief update from the far north.

I've now got a working TX and I'm waiting for my NOV to arrive.

I built a version of the recently published design by Jim M0BMU,
although not using his ceramic oscillator.

My version uses an VFO that I'd previously built for a phasing RX. I
moved it from 40m to 8MHz, and divided by 16 to get 500kHz. It has a
huff/puff stabilizer, and I'd already gone to great lengths to get the
VFO reasonably stable without the huff/puff. With the stabilizer in use
I've found I can transmit (into load) for > 10 hours of constant
carrier and the output frequency stays constant, to the accuracy I can
measure.

I left it on overnight recently, into load, and fed my RX audio into a
PC running SpecLab. In "QRSS-30" mode, the waterfall line stayed
on a constant frequency throughout, and was still rock steady the next
morning! Keying the TX has no effect on the frequency either, no
chirp, no jumping huff/puff "lock points". It should certainly be good
enough for QRSS-10 or slower.

The TX produces 16W, and my 2 stage 1/2 wave LPF has the 2nd & 3rd
harmonics at 64dB below the carrier. It produce a constant carrier
indefinitely with no overheating (65% efficiency). I chose not to key
the dc feed to the PA via a FET and current limiting circuit as Jim's
design - to get just a bit more voltage to the FETS! I altered the
Drain transformer ratios to get a bit more than the 11w initially
available. I'm toying with the idea of upping the volts a bit to get
more power in hand in case I can't get enough RF current into the
antenna.

I've just refurbished my poorly AR7030 receiver, replacing all the front
end LPF/HPF filter components and the 1st mixer IC to rectify the
damage it suffered a few months ago. Sensitivity is better, as is
linearity, whether the sensitivity is necessary on MF is a moot point,
since external noise seems to be the limiting factor.

I have heard a few stations this week, not as many as I'd hoped, but
not had much time to listen/watch.

I've seen QRSS-3 from G3XIZ, heard Finbar GI4DPE's CW beacon in daylight
at RST229 and this evening, again in daylight, heard G3UNT calling CQ on
502.1kHz. He was also only 229, it took some time to be sure of the
callsign, but it was a nice bright trace on the waterfall and QRSS-3
would have been easily possible.

I've been getting lots of advice from Alan G3NYK and next step is to
prepare some form of variometer tuning to replace the L/variable C that
I'm tuning the antenna with for receive.

I've been experimenting with an impedance bridge to estimate ground
loss, and the results are encouraging so I should be able to get the
thing resonant and matched ready to radiate as soon as the NOV arrives.

Best 73 from the far north. At least it's starting to get a bit darker
up here now as autumn approaches!

John

-- 
G-GRP-Club 2377, QRP-ARCI 12384, SKCC 3214
Member : RSGB, ARRL
Shetland Islands (EU-012) IP90GG 
Lerwick Radio Club : http://www.gm3zet.org