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Weird signal when receiving dual channels

Discussion in 'Becoming an Amateur Radio Operator/Upgrading Privi' started by KF7ACQ, Aug 29, 2009.

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  1. KF7ACQ

    KF7ACQ Ham Member QRZ Page

    I'm a new ham and finally picked up a radio the other day--a Yaesu VX-7R. The first night I was fiddling around with the dual channel reception and picked up some repeater traffic when I tuned in this specific combination.

    Main: 142.975
    Sub: 430.000

    The ht screen indicated the signal was coming through on the sub band, even though there does not appear to be a repeater on 430 in our area. The really weird thing is I lost the signal when I changed the freq on either the Main channel or the Sub channel. I'm sure what I was hearing was from a repeater, though I don't know exactly which repeater it was. (It's popular with night shift truckers in the Seattle area.)

    I figure it might be partially due to 430 being the third harmonic of 143 but I'm at a loss to explain why this was happening. This might be hard to explain without knowing what frequency the repeater is transmitting on but what exactly was going on here? Why would changing the frequency on either channel block reception? Why was I picking up signals at all on a frequency that wasn't transmitting them?
    Last edited: Aug 29, 2009
  2. K0CMH

    K0CMH Ham Member QRZ Page

    IRLP and Echolink often link repeaters into reflectors. This is done by connecting the repeaters through the internet. You may want to do a search for IRLP and Echolink to see if there are any "linked" repeaters in your area.

    If not, then possibly one of two things could be happening: 1.) what you figured, a third harmonic signal from a repeater outside operating limits, or 2.) a defective HT.

    Maybe others will have some more possibilities.
  3. KC7YRA

    KC7YRA Ham Member Volunteer Moderator QRZ Page

    I like the 3rd harmonic idea as well. Perhaps beating with some other "thing" that is transmitting.

    What we usually get around here is problems with the NOAA signal beating with railroad of all things. This plays funny tricks on radios.

  4. KF7ACQ

    KF7ACQ Ham Member QRZ Page

    I didn't find anything in the frequencies of interest, but to be honest I'm not sure what I'm looking for.

    That's an interesting point I hadn't considered. Wouldn't the repeater signal and "thing" have to be mixed at the source rather than just added together over the air in order to produce a frequency shift?

    I repo'd the test last night and found the original repeater signal on 147.080. I also found the following in the vx-7 user's manual.

    Here are the frequencies I know:
    Fm = Main band freq = 142.975
    Fs = Sub band freq = 430.000
    Fr = Repeater freq = 147.080
    Fmi = Main band IF = 47.25
    Fsi = Sub band IF = 46.35

    The fact that the signal only comes through when both Main and Sub are correctly set suggests I might be getting an image frequency. However, solving the equations doesn't give me an integer for n so maybe it's not an image frequency?

    Fm / (Fs + Fsi) = .300
    Fm / (Fs - Fsi) = .372
    Fs / (Fm + Fmi) = 2.26
    Fs / (Fm - Fmi) = 4.49

    I played around with the math for a bit and did discover an equation that does return an integer:

    (Fr + Fs)/(Fm - Fmi) = 6.02

    If I'm reading it right this equation essentially says that if the repeater signal were mixed with a signal of the same frequency as my sub band, I would have a situation where I could expect to hear an image. However, I don't know enough about radio receivers to know if that's done or if it can even happen inside the radio. Any radio designers around? ;)
  5. KD0FBK

    KD0FBK Ham Member QRZ Page

    What did you actually hear? Anything but noise? I have a VX-7 also and have noticed that certain frequency combos on mine cause a weak noise signal.
  6. KF7ACQ

    KF7ACQ Ham Member QRZ Page

    I hear the phone communications on the repeater. I set up a few hypermemories so I can switch back and forth easily.

    H2: Base=142.975; Sub=430.000
    H3: Base=430.000; Sub=142.975
    H4: Single Channel=147.080

    H4 is the normal repeater channel. I typically get a very good signal from it. When the repeater is transmitting I can switch over the H2 and continue listening to the discussion. The signal isn't quite as strong but it is still very easy to listen to. If I switch to H3 I get nothing.
  7. KX0Z

    KX0Z Ham Member QRZ Page

    I think it states right in the manual that you may get exactly what your describing.
  8. KF7ACQ

    KF7ACQ Ham Member QRZ Page

    I assume you're referring to the 'Dual Receive' notice about image frequencies? (That would be the same one I copied in post #4.) I considered that but the math doesn't work out. Assuming the listed values for the intermediate frequencies are correct 147.080 doesn't have an image on 430. Am I using those equations wrong?

    I do appreciate all the ideas. I'm still completely baffled as to why this is happening though.
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