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FM antenna curiosity

Discussion in 'Electronic Design' started by amdx, Mar 19, 2010.

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  1. amdx

    amdx Guest

    The receiver is an undercounter mounting Sony AM/FM, radio with CD.
    Nothing great, but Sony ususally does a fair job.
  2. amdx

    amdx Guest

    I have an FM radio inside an aluminum boat. The radio worked ok with the
    AC cord as the antenna but I got interference when I used my laptop.
    I found the circuit that ran from the power transformer to the antenna input
    on the
    FM IC. I installed a connector that is used on car radios and wired the
    center pin
    to the foil that went to the FM IC (capacitor on pcb isolated) and the
    shield side to
    dc ground near the IC. I then plugged in a telescoping car antenna and it
    great on the bench .
    So I installed the antenna on the outside of the boat and the radio inside,
    now one station I listen to is weak, but if I unplug the antenna and let the
    center pin touch ground of the connector on the radio it comes in great.
    Just curious why it is working this way.
    BTW, the mod did cure the computer hash.
  3. Baron

    Baron Guest

    amdx Inscribed thus:
    Sounds as if the antenna is fed with co-ax and has too much capacitance
    across the input. Put a trimmer capacitor in series with the centre
    pin and see if that improves things. Try 2-20 or 5-50pf.
  4. amdx

    amdx Guest

    Richard, after I read your letter I did a little better checking and found
    my signal is not weak it is to strong, I'm getting interference from other
    Also I get stations on the wrong frequency.
    I went out and collapsed the antenna to minimum about 1/3 of what it was
    my problem station is perfect and the other station I listen to is still
    good. The local
    NPR station isn't good though. But I can download the podcast of Science
    Friday :)
    Thanks, Mike
  5. Joerg

    Joerg Guest

    The usual, lousy FM tuner. They don't make'em like they used to. It's
    the same with television sets, one large signal and they fall off the

    If you have a radio with a signal strength meter you could notch out a
    strong station. But that only works it it's just one and far away from
    the NPR frequency. The only other options are to get a better radio, a
    directional antenna, or just live with it and use the podcast.
  6. Joerg

    Joerg Guest

    One solution: Try to get an old Becker car radio. And I mean old, at
    least 40 year, the first transistorized ones that could still be
    switched to 6V. They used to be standard issue in Mercedes Benzes.

  7. amdx

    amdx Guest

    Well the boat is a pontoon boat (maybe $6,000) that I run my business
    from, my wife or I are on the boat 70 hrs a week.
    The radio was fine when I used the original AC cord antenna, except when I
    was on my laptop, it caused hash
    in the audio. That's why I isolated the antenna from the AC, that did
    eliminate the computer hash.
    I have put together a car radio and wall wart that I use with a pillow
    speaker at night. Maybe later I'll put
    together another one for the boat.
  8. Phil Allison

    Phil Allison Guest

    "Richard Clark"

    ** Hearing the same FM station at more than one spot is still possible even
    with a 10.7 MHz IF frequency - if the signal is very strong. The reason
    is harmonics of the incoming carrier generated in the RF stage interacting
    with harmonics of the local oscillator in the mixer.


    A 100MHz FM carrier generates a harmonic at 200MHz in the receiver.

    When the local oscillator is adjusted to 94.65 MHz, its second harmonic is
    189.3 MHz.

    The difference frequency is then 10.7 MHz - so goes through to the FM

    In this situation, the FM deviation is doubled so the recovered audio will
    be distorted on loud passages.

    ..... Phil
  9. Phil Allison

    Phil Allison Guest

    "Richard Clark"
    "Phil Allison"
    ** Not by any method you alluded to - fuckwit.

    It has already been said.

    ** But not in any detail - fuckwit.

    No surprise a radio ham cunthead like YOU deleted all the facts.

    Pure embarrassment to a know nothing turds like radio hams.

    ..... Phil
  10. amdx

    amdx Guest

    This morning I got on the boat and the signal that was improved to good by
    shortening the antenna is now bad. 94.5 has interference from 101.1.
    Oh well!
  11. Joerg

    Joerg Guest

    You really need a better quality radio and with radios and a lot of
    other stuff older = better :)

    I mean, considering what the boat must have cost ...
  12. Joerg

    Joerg Guest

    I have learned not to trust any radio that's newer than 30 years,
    whether name-brand or not. And that's from experience. Unless it is from
    companies like Icom.
  13. Joerg

    Joerg Guest

    Best of all, it can be bolted down. That's really important on a boat.
    However, many newer car radios (newer as in "last 20-30 years") don't
    have very good tuners. Best to get one from the era of Ge-transistors,
    those radios were usually good.

    Better yet, get an Icom, Yeasu or whatever comms receiver. Most have a
    WFM setting. Ok, no stereo sound but one can easily listen to NOAA
    radio, ship-to-shore channels and so on.
  14. Phil Allison

    Phil Allison Guest

    "Dave" <>

    all antennas are dipoles, you just can't always see the other half.
    and Luxembourg has nothing to do with it, your silly frequency
    doubling notions should be packaged up in art's box and never see the
    light of day.

    ** ROTFL !!

    Dave should be writing scripts for Mickey Mouse cartoons.

    Cos he has the IQ of Daffy Duck.

    .... Phil
  15. The Luxemburg effect is usually contributed to the suspected Radio
    Luxemburg intermodulation products caused by the _ionosphere_

    Similar intermodulation effects can be obtained by the nonlinearities
    caused by rusty bolts in a transmitter tower.
  16. Joerg

    Joerg Guest

    Well, that's what I suggested above, notch = trap :)

    But it's tough and can be impossible if there are useful weaker stations
    near the one you want to muffle. In Europe they had a pager service
    right at the lower end of the FM band. Whichever committee signed off on
    that one should be dunked into a moat for gross incompetence, until they
    either learn or quit their career. Anyhow, the inevitable happened, and
    despite being a school kid I predicted that: A barrage of complaints by
    FM listeners. In Germany they pay a radio tax so that makes them sort of
    constituents with rights. Long story short the governement had to
    furnish rather expensive notch filters to anyone who complained.
  17. It's probably a severe impedance mismatch between your new antenna and
    the old pickup coil. You might need to create a little step-up
    transformer between the board and jack. I looked at a few FM chip specs
    and they leave it up to the designer to figure out the right input
    transformer. You'll probably need trial and error to figure it out.
  18. Jeff

    Jeff Guest

    Perhaps you should read up on what the Luxembourg Effect actually was!!

    It had nothing to do with antennas!!

    It was Cross Modulation in the ionosphere between Radio Luxenbourg and
    other radio stations, where by the modulation of Radio Luxenbourg was
    heard superimposed onto the second station.

  19. Joerg

    Joerg Guest

    I just hope someone was read the riot act for making that frequency
    allocation. I mean, that allocation was really borderline daft ;-)
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