18 Jan 2013

Mixed results with WSPR-X and WSPR-15

The new version of WSPR works well with plenty of reports given and received on 472kHz with it over the last 12 hours using the 2 minute version (as before but now with a waterfall and better interface), but mixed results on the slower 15 minute slot WSPR-15 which can detect much weaker signals.

My problem is TX drift over a long 15 minute transmission. My small 10W transverter is in a tiny plastic box. When using a 2 minute transmission the PA barely gets warm so there is little heat transferred to the crystal oscillator. With the much longer TX burst the PA warms more and the crystal warms up several degrees, moving the oscillator frequency by some 10s of Hz. So, although I was able to copy signals in WSPR-15, I was not able to be detected by others overnight. There are also far fewer stations monitoring yet on WSPR-15.

The solution is to rebuild the transverter into a bigger (metal) box with (a) better heatsinking so the heat rise with a mismatch is lower, (b) a slightly more efficient PA - problem here is with weather changes the antenna match goes off optimum sometimes and the heat rises as it the PA becomes less class E, and (c) put the conversion frequency oscillator physically further away from the PA, currently it is less than 10cm.

If I fix these issues and am sure that I transmit a more stable signal in the narrower (25Hz wide) WSPR-15 slot then I have a better chance of succeeding with the more sensitive version.  With another 7-9dB S/N detection improvement, my signal could reach some of the more distant stations like TF3HZ.

5 comments:

Graham said...

Another option might be to put the crystal in a simple oven so that it maintains it's temperature at some point above ambient.

These ovens can be very simple or quite complex. The QRSS crowd often use crystal ovens to maintain quite good temperature and frequency stability.

The simpler designs use a half dozen or so parts. I will post a few links later on if you are interested.

cheers, Graham ve3gtc

Anonymous said...

You may not even need an oven.. you simply need to increase the thermal timeconstant of the crystal.. for example, wrapping some thermal insulation around it (expanded polystyrene sheet works well). You may also need to lengthen the lengths of the crystal leads to add to the thermal timeconstat... maybe worth a try? regards, G6AIG (in France, with NO HOPE of 470KHz !)

Roger G3XBM said...

Thanks for the suggestions. I'd be interested in a simple oven circuit please Graham although I am confident that I can fix this. For 136MHz with my 10MHz crystal there are some nice OCXOs available at low cost with excellent short-term stability.

Roger G3XBM said...

I meant 136kHz of course, not 136MHz.

Graham said...

Good evening Roger,

Some info on simple xtal temperature control:

http://pensacolasnapper.blogspot.ca/2011/03/temperature-controllers-for-qrss.html

http://pensacolasnapper.blogspot.ca/2011/03/temperature-controllers-for-qrss-part-2.html

http://pensacolasnapper.blogspot.ca/2011/03/temperature-controllers-for-qrss-part-3.html

Those should give you a start.

cheers, Graham ve3gtc