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Ferrite bar antenna for car radio?

Discussion in 'Electronic Design' started by [email protected], Sep 14, 2005.

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  1. Guest

    Any idea the best way to couple a ferrite loopstick antenna
    to a car radio input for AM reception?

    I experimeted using a 6 inch loopstick (230uH) and 365pF
    parallel tunning cap from antenna input to ground (chassis)
    with reasonable results, but I'm wondering what the best
    approach is.

    The tuning is very broad and I don't get much of a peak
    when adjusting the capacitor, but all the local stations
    come in.

    How should I wind the loopstick for best results?

    -Bill
     
  2. Pooh Bear

    Pooh Bear Guest

    You still listen to AM ! ?

    Awesome.

    Most poor ppl I know use wire coathangers to replace the car aerial
    when the original gets trashed by kids or whoever..


    Graham
     
  3. Dave

    Dave Guest


    IIRC the normal approach if you wish to make it tuneable is to have a
    large winding which is tuned with the cap, and a smaller winding with
    less turns, so there is an impedance transformation.

    I suspect very much depends on the device used as the front end -
    whether it is a high Z fet or a lower impedance device.

    It sounds like the Q is low if the tuning is broad, but do you really
    want the hassle of returning the capacitor every time?
     
  4. I read in sci.electronics.design that wrote (in
    You need to tap down the coil or wind a secondary winding on the rod.
    The car radio antenna input is a lowish impedance, not 50 ohms or
    anything like, but much lower than the resonant impedance of your tuned
    circuit. But if you tap down too far, you will need to retune the
    loopstick for each station.

    Experiment with taps or secondary windings. Alternatively, you could put
    a larger fixed cap in series with your 365 pF at the earthy end, and
    connect across it to the antenna socket. You might try a number of
    values from 1000 pF upwards.
     
  5. Robert Scott

    Robert Scott Guest

    Why bother with "best", unless you mean "easiest". There is no sense
    in trying to improve sensitivity beyond the point where you can hear
    the background RF noise. And on the AM band there is a lot of it.
    The radio itself is probably already doing as well as it can using
    selectivity to limit background noise. So if you connect the
    loopstick antenna any way at all that produces enough signal to hear
    the background hiss, then that is good enough, and no further overall
    improvement can be had by improved impedance matching or tuning. To
    check if your connection is adequate, just turn up the volume until
    you hear a good deal of hiss, while tuned to a frequency where there
    is no local station. Then disconnect the antenna completely. If the
    hiss drops to almost nothing, then you have as good an antenna as is
    possible. But if the hiss remains the same or drops only a little,
    then that hiss is coming from the radio itself, and a better antenna
    match could improve things.


    -Robert Scott
    Ypsilanti, Michigan
     
  6. Joerg

    Joerg Guest

    Hello Bill,
    Hard to guess from the distance. Most likely your ferrite antenna is
    fine but the impedance of the radio is a bit low. That loads down the LC
    circuit formed by your ferrite coil and capacitor, reducing its Q and
    thus making the tuning "broad".

    I am afraid you need a FET follower at the antenna, to provide a Hi-Z to
    the ferrite antenna and a lower Z to the cable that goes into the radio.

    Regards, Joerg
     
  7. Joerg

    Joerg Guest

    Hello Graham,
    I think Bill lives in the US. Out here AM is very much alive and
    kicking. While on the long drive to the airport I listen to it all the
    time. Same when doing some "real" hardware in the shop.

    Regards, Joerg
     
  8. Car radios used to be designed for a 93 ohm input impedance, and
    already has a tuned front end. Unless there is something to buffer the
    tuned antenna, you will detune the existing front end circuit. The whip
    "Antenna is really a RF probe fed to the top end of the tuned circuit.
    Good designs used a variable cap in the circuit to null out the cable
    capacitance's effects on the tuned circuit. You tuned the radio to a
    station near 1300 KHz and adjusted it for peak reception. I could scan
    the input circuit from one of the old Sams AR series car radio manuals
    and post it on ABSE if anyone wants to see it.
     
  9. Guest

    You still listen to AM ! ?
    I used to listen to FM music when I was a teenager, but nowadays, I
    just listen to right wing AM talk shows:

    Michael Savage
    Larry Elder
    Rush Limbaugh
    Shawn Hannidy
    Michael Medvits
    Dennis Prager
    Al Rantell
    Bill Handle
    etc.
     
  10. Ross Herbert

    Ross Herbert Guest

    Why?

    Doesn't your normal car antenna do this already? It does on my car
    radio.

    Most cars since around 1992 use diversity antenna's which work better
    for AM reception. If your car has a rear window heater it may also
    double as an AM antenna.

    I would also think that trying to use an add-on ferrite loopstick
    antenna would allow pick-up of all sorts of local interference coming
    from car ignitions etc. And it would also be directional thus making
    sensitivity dependant on orientation to the transmitter while driving.
     
  11. Pooh Bear

    Pooh Bear Guest

    It's so, so different over here.

    The AM band has been virtually abandoned.

    The FM band has whole MHz worth of bandwidth hi-jacked by the BBC so as to
    provide nationwide coverage but it strikes me as over-kill. Each BBC FM
    service uses about 2 MHz. I can often pick up one of their stations on 3
    different frequencies. E.g. I can get Radio 4 on 93, 93.2 and 93.5 MHz.

    Stupid regulations intended to ensure breadth of programming simply mean
    that most commercial stations are virtual clones of each other with bland
    look-alike programming. In a similar vein 'narrow-casting' is discouraged,
    the only 2 stations offering a unique and specialised programming style
    being Jazz FM and Classic FM. There's no 'rock channel' at all ! Oh - and
    BBC's Radio 3 already offers a full classical music service anyway.

    No shortage of phone-ins to let the local retards spout off about rubbish
    though !

    BBC's Radio 1 which is their 'contemporary music' channel plays the kind
    of stuff that you'd expect to hear at the kids' dance raves mainly ( and
    almost exclusively - to the extent that a spokesman for that channel once
    stated that they weren't intereted in an audience aged over 25 ? ) whilst
    their Radio 2 offers MOR for oldies that's 80s weighted - Yuk.

    No wonder that pirating is popular in London.

    A really screwed up radio service here.

    Graham
     
  12. Joerg

    Joerg Guest

    Hello Graham,
    Sad. Maybe it can be revived with some local stations.

    FM is bland here as well. IMHO that is because of conglomeration, large
    companies owning a whole slew of stations and "adjusting" the music to
    mainstream, the same songs over and over. But the local ones is where
    you can occasionally find the real blue grass or old style country. AM
    is a bit better, lots of ethnic broadcasts but don't count on
    understanding the languages.

    AM talk radio is popular here, mostly conservative. Some of it has
    pretty good quality content so I listen to that once in a while.
    Surprisingly most of the contractors seem to listen to it as well when
    on a remote job. Also, the driving distances are huge out here. When you
    are going clear across a place like Nevada AM is the only thing that
    really works in many areas. Actually, when I drove in north eastern
    Scotland in the 80's it was similar. Not many FM stations but this was
    in an old Ford Cortina with a weak radio.

    Regards, Joerg
     
  13. Guest

    The radio is used indoors, not in the car.
    Yes, I want it to be directional so I can null out
    strong stations adjacient to weak ones I want to hear.

    -Bill
     
  14. Mark

    Mark Guest

    try a tap on the coil to create a lower z output to feed to the
    radio...

    or a small C coupling to the radio from the top of the coil.

    A "real" whip anteanna on a car looks like a capacitor at AM
    frequencies.

    Mark
     
  15. You forgot Hugh Hewitt.

    The first 6 are nationally syndicated. I don't think Al or Bill are so
    you must be in the LA area, Right ? (I fixed your spelling)
    GG
     
  16. Jim Thompson

    Jim Thompson Guest

    While Bill Handle (Handle on the Law) is on KFYI, I would hardly class
    him as right wing... just seems to be really savvy about law.

    Actually Handle is the only one on the list that I listen to.

    Mornings, I listen to Barry Young (here in Phoenix), then I switch to
    Air America just for groans and to keep the "canal" cleaned out ;-)

    ...Jim Thompson
     
  17. Mike

    Mike Guest

    I bought this new microwave oven that trashes every AM radio in the
    house while it's running, So I built a Shielded loopstick antenna for
    my old RCA Radiola. It is untuned, Faraday shielded, and I used a
    mosfet amplifier with the drain capacitor coupled to the external ant
    input to the set. No impedance matching to speak of, but it works
    really well! No noise from the microwave and it picks up more stations
    than I care to listen to.

    Mike
     
  18. Guest

    You forgot Hugh Hewitt.
    Yes, I'm in the LA area, I can walk to Disneyland.
    I have a year round pass.

    I forgot a couple others in addition to Hewitt.

    Kevin James late at night on KABC.
    Also, Doug McIntyre and Bill O'Reilly
    in the morning on KABC.

    -Bill
     
  19. Didn't know he had a radio show.
    Hannity - like the TV show, not the radio version
    Medved - not my style.
    Didn't know he had a show either.
    G. Gordon Liddy
    And don't forget Phil Hendrie. ;-)

    I've become hooked on Bill Bennet's "Morning in America". XM dropped
    the show 9/1 so I had do buy a subscription (via the internet) and an
    MP3 player with an FM modulator to get my daily fix. :-(
     
  20. Guest

    Yes, a separate short winding on the loopstick improves operation. The
    main winding is about 80 turns, and I added another winding of about 20
    turns feeding the antenna input. I get a sharp peak on weak stations,
    and it picks up stations I can't get in the car, so it seems to be
    working better than the normal car radio setup.

    There are 8 tunable coils inside the radio which is a Ford modle from
    1980s. I tried tuning a couple coils near the antenna input, hoping to
    improve it further, but didn't have much effect.

    I might try adding or subtracting a few turns from the secondary
    winding to see if I can improve it further.

    -Bill
     
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