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RF question - Plane radio aliasing?

  1. Apr 4, 2012 #1
    RF question -- Plane radio aliasing?

    I hope this is the right place to post it.

    My buddy and I were working on a jet earlier and turned on the HF communication system to perform an operational check out of the system. We tuned the radio to 2.5 MHz and picked up a local AM radio station. The station is 1250 KHz.

    Why are we able to receive this frequency when tuned to another exactly twice the frequency? We tried it on another jet and got the same thing.

    I searched google and couldn't find anyting.
    Any help would be awesome, thanks.
     
    Last edited: Apr 4, 2012
  2. jcsd
  3. Apr 4, 2012 #2
    Re: RF question

    Interesting!!! It is not twice the frequency, one is 2.5GHz, one is 1.25MHz, it is 2000 times different!!!
     
  4. Apr 4, 2012 #3

    berkeman

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    Re: RF question

    The HF band is nowhere near 2.5GHz. You're misreading something.
     
  5. Apr 4, 2012 #4
    Re: RF question

    lol

    My mistake, I meant to put 2.5MHz.
    The radio only lets me tune from 2MHz to 29.999MHz.
     
  6. Apr 4, 2012 #5

    berkeman

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    Re: RF question

    So when tuned to 2.5MHz, you picked up a radio broadcast on 1.25MHz? How far is that local broadcast antenna from the plane, and do you know what power level they transmit? You can get more info on the station by googling their call letters.
     
  7. Apr 4, 2012 #6
    Re: RF question

    Yes.
    wiki said 1kw day and night. It is roughly 17 miles from the jet.

    This site seems more credible though
    http://radio-locator.com/info/KZDC-AM
     
  8. Apr 4, 2012 #7

    berkeman

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    Re: RF question

    Hmm, even 25kW is not that big for being 17 miles away. If it were a mile away, I could see it maybe overloading the input stage of the HF radio and bleeding into the 2x frequency. Was there anything at 3x the 1.25MHz frequency? Are you able to try it at night when the transmit power appears to be lower?

    It could be a problem with the radio. Are there any other planes available to you with the same HF rig?
     
  9. Apr 4, 2012 #8

    berkeman

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    Re: RF question -- Plane radio aliasing?

    BTW, I noticed at the website that they have different locations for the daytime and nighttime transmitters. Are you sure that the daytime transmitter is still 17 miles away? There was something mentioned about new construction...
     
  10. Apr 4, 2012 #9
    Re: RF question

    I work from 7am to 4pm but I might be able to get another guy from grave shift to try it on a few planes. We had tried it on two different planes today about 100 meters from each other and got the same result. But yes all our planes are equipped with them. Tomorrow I will try multiple haha
     
  11. Apr 4, 2012 #10
    Re: RF question -- Plane radio aliasing?

    All three transmitters are roughly 16-18 miles following roads, maybe 12-15 line of sight.
     
  12. Apr 4, 2012 #11

    jim hardy

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    Re: RF question -- Plane radio aliasing?

    Hmmm

    Give a listen to that station on a good radio (like a car stereo) and listen for distortion. If it sounds scratchy and 'S' sounds are exaggerated and "SH"-y, it might be worth a phone call to that station's engineer. If he's clipping and making a harmonic he'll want to know pronto. tell him you're receiving him on twice his carrier.

    It could be your radio, though.
    A common IF frequency is 455khz so your local oscillator is probably 2955khz (2.5mhz + 455khz),
    which heterodynes with 1250khz to make 1705khz,
    which heterodynes again with 1250khz to make 455..
    in other words there's an "image" at half the frequency of interest and your radio doesn't have real good "image rejection"..

    i think they call these "Mixer Spurs"

    and they're discussed in simple terms around page 23 here.

    http://www.google.com/url?sa=t&rct=...xtnTDA&usg=AFQjCNFL4-aaqrjyCbg6dLpMxP3yIaGrqw


    Is the "HF communication system" a high quality one? If so it should have decent image rejection.

    Question to any genuine radio guy - is above plausible?

    old jim
     
  13. Apr 4, 2012 #12

    davenn

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    Re: RF question -- Plane radio aliasing?

    good thoughts Jim.

    this also could be just plain ol' intermod mixing

    Dave
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