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simple choke for power supply - how ?

Discussion in 'Electronic Basics' started by Skeleton Man, Dec 31, 2004.

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  1. Skeleton Man

    Skeleton Man Guest

    Hi All,

    I'm having interference issues on weak stations with a car radio from two-way
    radio equipment and I figured a choke on the 12V power supply would be a way to
    address this.. I was figuring on using a Pi or "T" filter (LC/LL), and I know
    how to assemble such a thing.. _but_ I have no idea how to determine component
    values.. I don't know the frequency of the interference and have no way to
    measure it.. (ie. expensive equipment like CRO) so I need a choke with a fairly
    wide band..

    While I'm at it I'll kill two birds with one stone.. I want to boost the signal
    strength of the weak stations too.. the car has an on glass antenna (flat coil
    shaped strip on back windsheild) with an existing antenna booster built in
    (without which the signal strength of even local stations is very poor) , but
    this is not sufficient.. I would like to clearly pick up stations from
    interstate (300+ KM) away without interference from power lines, etc..

    If someone can help me with both or either of the above it would be greatly
    appreciated.. (I have minimal electronics theory behind me, half of which I
    never actually used and since forgot)

  2. Andrew Holme

    Andrew Holme Guest

    Have you established that the interference is entering via the power supply?
    It could also be picked up on the speaker wires, the antenna or the booster.
    Does it afffect cassette tapes and radio reception? AM and FM? Is it still
    there if you unplug the antenna?

    What sort of two-way radio interference is it? 27 MHz CB? Taxi? Hams?
    License exempt? Most are VHF or UHF.

    You could make an LC low-pass filter with a low cut-off frequency. This
    would pass DC power and audio frequencies, but attenuate RF frequencies.
    Alternatively, you could make a "balun" by winding the speaker and/or power
    cables through a toroid - like the lumps often seen in power leads of
    consumer products. You will find plenty of info on choke baluns in radio
    amateur literature. Hams often deal with interference to radio and
    television caused by their transmitters.

    LC filter components can be obtained from standard tables. Choose your
    filter type e.g. Butterworth, input and output impedance, and cut-off
    frequency. The impedance choice is a bit tricky for power and speaker
    leads! Try the lowest impedance that yields practical component values.

    The choke baluns might be easier ...
  3. Art

    Art Guest

    FYI: The RF interference from two-way equipment is coming in via the
    antenna, normally, being received with the other information you are trying
    to listen to.
    Increasing the antenna ability to receive information may only acerbate the
    What you are trying to do with a Car Radio may be better attempted with a
    commercially available communications receiver and a good antennae array.
    Then you will probably still have the interference via the two=way radio
    equipment depending on your proximity to the offending transmitter, etc.
    Not a simple quandary to rectify/ Happy 2005, Have a Safe One!!
  4. Skeleton Man

    Skeleton Man Guest

    Have you established that the interference is entering via the power supply?
    Only occurs when the radio is on (not tape/cd).. and largely weak AM stations..
    (don't notice it on FM or stronger AM)
    Guess that means it comes in via the antenna then and not the power supply..
    it's worse with the booster disconnected (power antenna wire disconnected..)
    Taxi.. intergrated computer (lcd display shows job information) and 2-way
    radio.. I'm not sure if it's VHF or UHF..
    Wind the power/speaker leads around a ferrite toroid you mean ?

  5. Andrew Holme

    Andrew Holme Guest

    So, the booster is not at fault. Do you have another AM radio? Can you
    hear the interference on that? Do you live near a taxi transmitter? It may
    be the taxi office is broadcasting spurious signals in the AM band. If
    that's the case, you can't filter them out! You'll have to raise the issue
    with them, or take it to your national radio licensing authority.
    Yes, as close to the radio as possible. This could still make a difference.
  6. Andrew Holme

    Andrew Holme Guest

    Spurious products can even be generated by rusty metalwork forming a
    rectifier which acts as a mixer. It happens! It could be a nearby fence,
    or even your car bodywork.
  7. Skeleton Man

    Skeleton Man Guest

    Thanks for the help, but I'm not sure I was clear enough in my previous posts..
    the two-way equipment and the car radio are _both_ inside the taxi.. the owner
    listens to a mainland AM station most of the time instead of anything local..

    We don't live near transmitters, and the interference is pretty constant
    regardless of where the car is and whether it's moving or stationary.. the car
    is also brand new so rust, etc is not an issue..

  8. Andrew Holme

    Andrew Holme Guest

    OK, I see now.

    So, does the taxi equipment cause interference all the time, or only when it
    is transmitting?

    In your original post, you said you also wanted to improve reception. Have
    you tried an external mag-mount vertical on the roof?
  9. Skeleton Man

    Skeleton Man Guest

    OK, I see now.
    From the minute it's turned on to the minute you switch it off it interferes
    (doesn't matter if it's transmitting or not)
    You mean just a roof mounted antenna ? I have tried just plugigng one in and
    holding it, the reception is better than the on-glass antenna, however would
    like to boost the glass one.. drilling holes or glueing is not an option.. and
    it can't leave a mark.. there is the ski-bar I suppose (roof rack bar that the
    taxi highlight is attached to).. it would have to have some kind of external
    plug so the bar could still be removed tho.. I'm not sure what cutting and
    shutting coax leads does to gain.. (I don't have a crimper)

  10. Andrew Holme

    Andrew Holme Guest

    The power lead could be radiating; you might one or more supression
    techniques (LC filter / screening / toroidal balun / re-routing /
    shortening) on that.

    The interference is almost certainly in the AM band - you can verify this
    using a small handlheld transistor radio inside the cab. This makes it very
    hard to seperate wanted from un-wanted signals. Supression may be the only
    way to go.
  11. Skeleton Man

    Skeleton Man Guest

    From the minute it's turned on to the minute you switch it off it
    So put the LC filter on the 2-way power supply and the radio, or just the 2-way

    How do I calculate input/output impedance for the filter ? (so I can find a
    table listing component values)

  12. Andrew Holme

    Andrew Holme Guest

    It's trial and error, I'm afraid. Try the easier things first.
    Actually, it's not that critical. We're not matching a 50 ohm signal. I
    might try a PI-network with (say) 100n capacitors to ground and (say) 100uH
    in series. The capacitor leads need to be short.
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