I decided to see what my ham transmissions look like on an SDR and was kind of shocked to see how much leaks out. The waterfall display is split between two handhelds during transmit. There are a couple large energy spikes at +/- 800kHz to the 431.5MHz transmitting frequency.

Given that both radios spurious emissions are lined up and similar I'm guessing it is common and due to some sort of harmonic effect, possibly from implementing similar architecture.

Is there any negative to the less dispersed transmission of the bottom device/waterfall? The top half of the waterfall display is from a handheld transceiver that is much higher in quality than the one captured on the bottom half. Interestingly the SDR picks up the better handhelds audio much better even though voice communication between handheld devices is fine.

  • 1
    $\begingroup$ Welcome to SE! Is there any chance the difference is related to overload of the SDR? For example, is there any indication that the ADC is being saturated? Also, are the radios putting out the same power, into similar antennas and coupling equally to the SDR? $\endgroup$
    – Brian K1LI
    Mar 30, 2019 at 22:47
  • $\begingroup$ @BrianK1LI I was just using the 'low' settings on the handhelds about 3ft from the antenna. I just tried both about 30ft away and got the exact same waveforms. $\endgroup$
    – RobC
    Mar 30, 2019 at 22:55
  • $\begingroup$ In that image, what software is that? $\endgroup$ Mar 31, 2019 at 18:21
  • 1
    $\begingroup$ @MikeWaters GQRX, I've also been playing with OP25 which shows the actual QAM constellations which is pretty cool. $\endgroup$
    – RobC
    Apr 2, 2019 at 22:09
  • $\begingroup$ Thank you, I've edited your comment with a link to GQRX. :-) I want to see if it works with my Softrock RX Ensemble II in Linux Mint MATE 18.3. $\endgroup$ Apr 2, 2019 at 22:28

1 Answer 1


This is very likely to be be due to overload of the receiver. A quick, rough way to tell the difference:

  1. Tune your SDR receiver so the waterfall center frequency is not the same as the transmitter's frequency.
  2. Transmit.
  3. Check whether the spurs you see are symmetric about the transmitter's frequency or the receiver's frequency. This tells you which side generated them.

Also, if transmitting causes the entire waterfall to rise up then you definitely have an overload problem.

How to eliminate overload: First, make sure your receiver's RF gain is set to the minimum level, and AGC (if any) is off. Then try these things:

  1. Replace the transmitter's antenna with a dummy load.
  2. Replace the receiver's antenna with a dummy load or terminator, or if neither is available, just leave the antenna port unconnected.
  3. Get more distance.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .