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G4ELJ England flag England

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Ham Member Lookups: 20817


Licensed since 1978.

Active on 17m through to 6m with a homebrew DSP based transceiver, Picastar, running 10W output.

Brilliant design by G3XJP. High IP3 (>+30dBm) H-mode mixer front end with narrow, amateur band only,
high IP3 bandpass filters, narrow xtal filter at 10.695MHz downconverting to 15kHz, where the DSP takes
over and does the rest.

Picastar works on 160m through to 6m but I only operate on 17m, 15m, 12m, 10m and 6m.




The Tx side feeds a homebrew tetrode linear using a Q1-P/42 (GU43B) giving me 400W on all bands,
including 6m. I had been using a Heathkit SB1000 (single 3-500) and a LinearAmp 6m 3CX800 but
could never get to 400W out. I know, what is 3-6dB. Nothing on HF but on 6m it matters as we use 400W
at the antenna so allowing to changeover relays, LPF's, antenna switching relays and coax losses, a bit
extra is needed. It all helps on 6m and I spend a lot of time trying to work DX on that band.

I wanted a linear that was quiet, high gain and covered HF and 6m. The latter was very important! I initially
tried a GU-81M pentode but a bit surprisingly couldn't get the gain I needed plus it was a little finicky on
12m and 10m. Never got it going on 6m. That was the Plan B linear as I had the GU-43B valves and base
well before that design. The advantage of the GU-81M was no fan needed and it should have more gain
which I was slightly worried about with the GU-43B, given the 'low ' output power of the Picastar.
I didn't want to have to run another linear between tx and main linear.

AnywayI reverted back to plan A using a GM3SEK Tetrode control board, a couple of Antek toroidal
transformers for Anode and screen supplies plus other toroidal transformers for heater, grid and general
12V supply. I couldn't find single transformer to cover all voltagesneeded over here in the UK.
I carried the Antek transformers back from a couple business trips to the US after having them shipped to
my hotel. I have 3400V on the plate and 350V on the screen. Fan is a shame but the drive requirement
is superb.

As it turned out, with the ceramic Q-1P/42 NOS valve I got off Ebay, I need just 6W in for 400W out and
even less drive on 6m. There is a Russian variable vacuum capacitor on the plate Pi network side using
a Chinese turns-counter and vacuum relays for Tx/Rx. Some fan noise but I intend to thermistor control
the fan next, plus there is a noise gate on the DSP tx on my Picastar . I'm also thinking about using a small
Mean Well switch mode PSU for the heater supply, as mains voltage here seems to fluctuate more than
expected. It is difficult to keep heater voltage correct. That is assuming the switch mode is quiet enough.
We will see.

The linear - finished just before New Year 2014 - still needs nicer labels etc.


Antenna for HF is a homebrew 4ele cubical quad below (based on Gemquad spreaders)
with the boom normally at 15m high.


Antenna for 6m is a homebrew 6 over 6 (DK7ZB 1 wavelength 50ohm design) with the
lower beam just above the quad boom. Spacing is 11ft.

Picture below before I took out an old ramshackle shed, bottom of the garden on the right.
Shack then was a newer garden shed but still an uninsulated garden shed at the botton left
of the garden. Very cold in winter and hot in summer!


Now I have a double glazed, fully insulated garden room for the shack where the
old ramshackle shed was. Much nicer!


Sorry but I don't QSL.


1637490 Last modified: 2015-01-25 17:08:13, 4904 bytes

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