30 Sep 2010

137kHz receive challenges

Several people have emailed me about receiving signals on 136/137kHz. Now I'm certainly not greatly experienced in this, but a few things I've picked up from others are worth passing on:
  • Many HF rigs are deaf on 136kHz, for example the FT817, although this is less deaf if you switch IPO to "on" on 136kHz. A small preamp may help as long as this is selective and doesn't increase overload and intermod from strong out-of-band stations. 
  • The receiving antenna may have to be directional e.g. a small loop/preamp to allow local noise sources to be nulled. 
  • DCF39 on 138.830kHz sending FSK data with a burst every 10 seconds is a good indicator of receiver performance: it should be a good strong signal in the UK. 
  • Ultimately it is S/N rather than noise figure that matters on this band.
  • Be prepared to use PC software such as Spectran or Argo to look for weak QRSS mode signals.
Here, I'm using the deaf FT817 (no preamp) with my 80sq m transmitting loop facing W-E at the moment and this is able to receive G8IMR's 20mW ERP WSPR beacon most of the time (188km) despite not being in the main direction of the loop. I've also copied G3XIZ, G3XDV, G3YXM and G3KEV on the band, the last being on CW. So, even a poor receiver can produce results, but it is really worth optimising the set-up (including here at G3XBM) to get more decent results. This is on the agenda.

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