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).




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.