Tuesday, November 9, 2021

DC in to RF out with QDX on 40 meters

I made some measurements today to compare the RF out with a given DC input for my QDX operating on 40 meters. The results surprised me - I was expecting the QDX to drop out at around 8 volts or so.

T1 is wound (3:2) so that the QDX operates at 12 volts. I used a variable PS, Fluke 87 and a WM-2 QRP wattmeter to perform the test.

        DC in       RF out

  • 12.5V - 5 watts
  • 11.3V - 4 watts
  • 10.0V - 3 watts
  • 9.2V - 2.5 watts
  • 8.4V - 2.0 watts
  • 7.5V - 1.5 watts
  • 6.1V - 1.0 watt
  • 5.4V -  750 mW
  • 4.5V - 500 mW - Receive sensitivity is much decreased
  • 4.0V - 100 mW - No receive
  • 3.7V - 10 mW
  • 3.6V - WSJT disconnects

Surprisingly, the QDX stops receiving before it stops transmitting - and I was still being received and decoded at this level by other stations. And this is all the way down at 4 volts.

My main reason for doing this test was for WSPR. I have no need to use the full 5 watts on that mode nor do I wish to use external attenuators to decrease the output of the QDX (while also reducing rx sensitivity by the attenuated amount).

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2 comments:

PE4BAS, Bas said...

Interesting. At 6V it still gives 1W output and it receives. Very usable. 73, Bas

John AE5X said...

Unfortunately, a lot of experienced builders seem to be having problems with their QDX. I'm anxiously monitoring the postings to see how things get resolved.

73,
John