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FM multipath/intermod?

Tim Osborne
Tim Osborne Member ✭✭
edited July 2021 in Amateur Radio Interests

I have one repeater in my area that seems to have a strange spectrum and terrible audio at my fixed location. There is no signal when the transmitter is not keyed, so it's not environmental interference. Is this due to some multipath issue from my metal roof? The repeater sounds fine when mobile on my handheld.

Next step is to hook my handheld to the fixed antenna to see if that sounds bad too (some miles away).


intermod.JPG


As an example, here's another repeater only 20 kHz away that is fine. Other repeaters are fine too, even when much higher or lower in amplitude.


normal.JPG


Answers

  • Tim Osborne
    Tim Osborne Member ✭✭

    Confirmed this happens with the handheld... but only with this repeater. Again it sounds great mobile, so all I can guess is multipath given the particular line-of-sight path.

  • halmassey
    halmassey Member ✭✭

    Neither multi-path nor intermodulation would look like that on a Spectrum analyzer...Multi-path would not be able to frequency differentiate like that at the peak of the signal. Nor would intermod unless it was mixing right in the transmitter. Are they both FM repeaters?

  • Tim Osborne
    Tim Osborne Member ✭✭

    Thanks for the reply.

    They are both FM repeaters.

    I also hear harmonics of the 103.5 Hz tone out to 1 kHz+ even with no audio modulation, but only on the Flex 6600 and not so much on my THD74... though it's likely the audio passband is narrower on the HT. Not implicating the 6600, just a data point.

    I am mystified as to why others don't seem to complain, but I will poke the repeater operator.

  • John KB4DU
    John KB4DU Member ✭✭✭✭

    Well, it looks to me like something in the audio chain is seriously over-driven. Maybe it sounds ok on radios with a narrow pass and, filtering out the distortion products.

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