VHF modulating neighbors speakers?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by WM5TS, Oct 28, 2014.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Speakers alone cannot demodulate FM. That takes active circuitry.

    AM, on the other hand, can be demodulated by a bad connection!
  2. K4ISR

    K4ISR Ham Member QRZ Page

    Almost never is it a bare speaker not hooked up to anything. Once you add in the speaker input, small amp, power supply and so on, then it becomes very possible, and in my case happens every time I use them.
  3. VE3KUT

    VE3KUT Ham Member QRZ Page

    Agreed speakers won't demodulate FM. I've got a powered subwoofer that detests 2m fm though - Key up and a nasty hum comes out of it, but only when very near in proximity.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd try something else.:eek:
  5. KK4YWN

    KK4YWN Ham Member QRZ Page

    An amp can not demodulate FM. Do you have Cable? There may be strong leakage. It may not be in FM. QAM is more common on CATV systems.
  6. K4ISR

    K4ISR Ham Member QRZ Page

    Comes through at high 80s, around FM 89.1MHz I believe it was. I rarely use the speakers, but when I do, I turn the PC volume down some which forces me to turn up the volume on the speakers. when the volume knob is turned down, below 25% or so the radio station starts coming in, and the farther down the knob goes, the louder the radio station comes in up to approximately 30dB with the knob all the way down. So it must be something with the potentiometer knob and the circuit to the speakers that is picking it up.

    I am about ready to toss these old speakers and use a direct HDMI to the TV which resolves this issue.
  7. KA1MDA

    KA1MDA Ham Member QRZ Page

    So why do my computer speakers do such a great job picking up the local commercial FM broadcaster at 99.3? I've got a set of old Altec computer speakers (2 satellites and a subwoofer, all powered, with the amps in the subwoofer enclosure) and they've been picking up FM at 99.3 Mhz since I bought them over 15 years ago! Even with ferrites, dirty chokes, etc, it brings the FM station audio level down to a whisper, but still perfectly audible. FWIW, with a 12" whip, the received signal strength here is about +60 dBmV (13 mW) into 75 ohms! At least the transmitter is clean- the 2nd harmonic is all the way down to 0 dBmV!

    If the exciter were using phase modulation , couldn't there be some type of residual AM component to it? Or given enough signal strength, would it be possible for something in the amp to "brute force" slope detect the FM? Although it might be impossible in theory, it sure works in reaity on a daily basis here!

    Tom, KA1MDA
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Try tuning in that station using an AM receiver that will tune there (if you have one) and see what happens...
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    The AM receiver can slope detect the FM signal. Therefore, not really an indication of any AM component.

    Glen, K9STH
  10. KC2UGV

    KC2UGV Ham Member QRZ Page

    Slope detection? By chance, of course, but it is possible. I've been able to pull out an FM signal with a crystal rig, and with a receiver set on AM.
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