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Interesting find on UHF today

Discussion in 'Interesting DX Contacts' started by nkeck72, Nov 18, 2017.

  1. nkeck72

    nkeck72 New Member

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    Around noon today (November 18), I was attempting to DX with an amplified antenna of mine and my trusty PRO-97 scanner (from Radio Shack) in Maryville, TN. After spending some time on 2m and 70cm for a while, I figured I may as well just set the thing up to scan everything with the amplifier cranked up and see what I could get. Eventually, the scanner stopped at 853.55 MHz, with some really noisy audio. What got my attention was that, unlike other things I had picked up in this band, there was no trunking or scrambling going on, it was clear audio. I played around with the antenna gain levels and the attenuation on the scanner for some time until I could get a relatively clean signal, which was still pretty crap. Through all the noise, I could pick up "Brought to you by ...[unintelligible]... and viewers like you."



    So, I was picking up a TV audio carrier, possibly PBS. This was odd for a couple of reasons. After scourging the internet for band plans and other such info, the nearest TV audio carrier was on 853.75 MHz, too far from 853.55 to actually get heard on that frequency. The second reason is that this frequency is smack in the middle of the Public Safety Band, which AFAIK is prohibited for any other use except public safety. I have posted with this the audio I could capture split into two parts, the first part being what I could hear in FM mode, the other in AM. (Yes, I know TV carriers are FM, but I wanted to make sure just in case I wasn't picking up a TV transmission.) By the time I made this recording, the reception had decayed significantly and almost all the audio is unintelligible, but it is there. I'm uploading in FLAC format simply so that I don't lose any more readability than I already have. For those who can't use FLAC, I'll also upload a WAV in the best quality I can get.

    I never did figure out where the transmission was coming from, but it couldn't have been our local PBS station because it transmits on a far lower frequency, albeit still UHF after accounting for the transfer of VHF TV stations to the lower end of the UHF band.

    Edit: It appears the forum's file upload feature is broken, so here are Drive links to the files.
    WAV: https://drive.google.com/open?id=1DN2ixV4_hdF4Rn8FlcsSnvx6pf7TCKAx
    FLAC: https://drive.google.com/open?id=1e7DXh4zfiT2YdM9fdoU-nTTqful2VeZe
     
    #1 nkeck72, Nov 18, 2017
    Last edited: Nov 18, 2017

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Congratulations. You have just discovered what intermod is. That signal you heard was not really broadcasting on that frequency. Either the scanner front end or the amp was being creamed by a STRONG off-frequency signal and causing that signal to appear elsewhere in the RF spectrum as a product of mixing inside the receiver or two separate frequencies both of which are strong were mixing inside your receiver and producing a third frequency which you heard.

    https://www.midians.com/html/intermod-explained.php
     
  3. nkeck72

    nkeck72 New Member

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    Interesting, I had thoughts that something similar might have been happening with all the noise on the signal, and especially since it cleared up so much when I put the scanner in AM mode. Is there a way to prevent this from occurring in the future with my amp? There have been occasions where I had to crank that thing all the way up in order to hear some DX conversations and I don't want other signals coming in from nowhere and ruining my reception.
     
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    not unless you had a really tight (narrow band) filter that was tunable and was able to track the frequency that you want to listen too. way back in the day when everyone carried a pager it was not uncommon to hear paging signals on some of the newer wide band 2m VHF radios for the same reason. Eventually some of them had a tracking front end filter which really cleaned things up a lot. the problem is that the receiver (or amp) front end transistor cannot handle such a strong signal and causes all sorts of problems. educing the signal is the only way to solve the issue.
     

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