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Tuesday 9 March 2010

SDR Rx with a Valve Tx in a contest how?

I demo'd SDR at CDARC rally at w/e and mentioned using an SDR Rx with an old TX and was posed this question by email just now:

> Hello
> Dominic,
> I'm sure that you had thousands of discussions with people at the Wood Green Rally on Sunday;
> however, you were telling me that you were planning to use a SoftRock SDR (possibly)
> receiver with a valve transmitter for contests, etc.
> How are you planning to get the tx on the same frequency? Just by frequency meter?

This was my response:

Hi ,

If using a valve TX then usually you know the TX frequency in advance. Especially (mine is) if xtal controlled, so you cannot usually tell the other station the freq so hope they are playing nicely and they move their RX to it. If VFO based you will have to match your TX to their RX, unless you are the run station.

What I have managed so far is this:

My xtal controlled 6V6 ( 1st 2 images) paired with a Softrock SDR 80 RX, on TX I flick a switch that switches out the RX signal, clunky, very clunky! I must create a better interface. but this is how I managed so far.

I listen for the remote station using SDR RX that is frequency agile, and TX on the 6V6. I am the run station so I do not move the TX frequency and to some degree don't really care what it it as long as stable, in band etc... As per most xtal based qso the frequencies are well known and established 3.560, 3.558 or 3.579 are three that many know and use. G3VTT recently sent me a whole bag full of FT243 80 xtals so I have the ability to cover 3.500 to 3.600 almost every 15kHz or so. Xtal based TX are limited and only on fixed frequency so if you have DX or contest operators that don't work with you then you will have trouble.

However, if my TX were a homebrew frequency agile (VFO) then I'd use a frequency counter to confirm my TX freq. If I were using a commercial valve TX then I can usually read the TX frequency off a dial or whatever.

Most contests are simplex so Rx freq = Tx freq. Some using valve TX as sure you know will allow Rx VFO to help cope with the Tx limitations of fixed frequency operations.

The method all depends on if you are 'run' or 'search and pounce'. If I were HB0/M1KTA or even 3B8/M1KTA again I don't think it would matter what the Tx freq was hihi..

Does this help?

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