Shortwave broadcast aliasing into 40m band

  • 2
  • Problem
  • Updated 3 years ago
  • Solved
  • (Edited)

Setup: Flex 6500, SDR ver1.3.0

I have noticed aliasing in the 40m band when there is a high-strength shortwave broadcast near. The issue is dependent upon the spectrum bandwidth I have selected. Curiously the problem occurs when the bandwidth is small. The first picture below is when I am zoomed-out on the band - no aliasing and the received audio is clean. Notice the strong out of band shortwave broadcasts. The second picture shows the aliasing problem when I have decreased the bandwidth by clicking on the +zoom button a couple of times. It becomes hard to discern the desired signals from the aliased shortwave signal and the aliased signal is very noticeable, err annoying, in the baseband audio. 

Photo of SteveM


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Posted 3 years ago

  • 2
Photo of Tim - W4TME

Tim - W4TME, Customer Experience Manager

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Official Response
The images issue took a bit of troubleshooting to characterize and ultimately resolve.

In direct sampling radios with no analog mixers, IM mixing (images) is not technically possible, unless there is some unexpected component failure that would act as a rectifier. In SteveM's case, a faulty lightning arrestor was "re-purposed"  as a mixer, demodulating foreign shortwave broadcast stations. These were real signals and the radio received them like it is designed to.  Disconnecting the faulty lightning arrestor removed the source of the extraneous signals and resolved issue.