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When I transmit, it there seems to be something mixing and reflecting my signal. For example, when I transmit on 14.300 MHz, I see signals above my transmitted signal at at 14.500, 14.700, 14.900, etc. and below my signal at 14.100, 13.900, 13.700, and so on. I tested on another frequency in twenty meters and saw the same pattern, with both tests radiating signals with frequencies "reflected" around 14.400 MHz.

This sounds like some issue with my signal getting unintentionally mixed with some other signal. The weirdest thing is that I still see at least the primary reflection---e.g. the 14.500 MHz signal when I transmit on 14.300 MHz---when I'm using a dummy load.

I even see noise signals being reflected around this 14.400 MHz point. What could possibly be doing this, and how can I track it down?

Some information about my system:

  • I am watching the spectrum using an RTL-SDR, with SDR# software
  • Rig is an Icom IC-718
  • Antenna is a dipole cut for 20 meters, run around the interior perimeter of my shack
  • Antenna is connected to the radio with 12ft of DXE400MAX coax
  • Transmission line has a common-mode choke wound into the receiver end of the coax
  • There is a 1:1 current balun is at the antenna side
  • The following points are all bonded together, however they are only bonded to the AC ground since I am in a third-floor apartment:
    • The transmission line shield at the antenna end
    • The transceiver (ICOM IC-728)
    • The power distribution bar (MFJ-1129)
    • The power strip this is all powered by (an MFJ-4230MV)
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    $\begingroup$ This type of spectrum pattern could be caused by RF mixing inside a nearby (or not so nearby) DC-DC converter (for LED lighting, USB charging, etc.) running at at frequency of 200 kHz. $\endgroup$ – hotpaw2 Aug 8 at 20:14
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    $\begingroup$ Do you still see this distortion if you go a half-mile away with a receiver? Or perhaps if you view your own transmission on a WebSDR? The distortion could be in your measurement setup. $\endgroup$ – Phil Frost - W8II Aug 8 at 21:27
  • $\begingroup$ @PhilFrost-W8II Unfortunately I'm unable to see my own transmission on a WebSDR, even the Utah one with me being in northern Colorado. I'm trying to address that as well. I'm seeing this effect using my own SDR on a different antenna in the same room. That said, this could definitely be an artifact of how I have my SDR set up. I wasn't able to hear some of these signals on a receiver which points to a problem in the SDR setup. I'll set up my receiver on one of these mirror frequencies and see if I'm actually transmitting that way or just receiving that way. $\endgroup$ – Rosa Aug 9 at 2:41
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    $\begingroup$ Hi! This sounds very much like a case where you produce regularly spaced spurs through overloading your receiver, not anywhere on the air. $\endgroup$ – Marcus Müller Aug 9 at 15:14
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    $\begingroup$ @Rosa Utah may be in the skip zone. voacap.com might help to find locations that are more likely to hear your transmission. $\endgroup$ – Phil Frost - W8II Aug 9 at 22:07
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This is caused by the RTL-SDR I was using to view my signal. Here's a quote from the guide at https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/:

Note that this feature makes use of direct sampling and so aliasing will occur. The RTL-SDR samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 - 14.4 MHz while tuning to 14.4 - 28.8 MHz and the other way around as well. To remove these images you need to use a low pass filter for 0 - 14.4 MHz, and a high pass filter for 14.4 - 28.8 MHz, or simply filter your band of interest.

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    $\begingroup$ Please feel free to accept your own answer, since you have apparently found the problem. $\endgroup$ – rclocher3 Aug 10 at 14:01

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