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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. I read in sci.electronics.design that wrote (in
    20 turns does look too many for an 80-turn primary. If you have a
    standard 93 ohm input impedance in the radio, the load on the primary is
    93 x (80/20)^2 = 1488 ohms. If your 365 pF cap tunes the primary to 540
    kHz (I don't remember the lower bound of the AM band in US, but it's
    around that frequency), it has an impedance of:

    1/(2pi x 540.10^3 x 365. 10^-12) = 807 ohms,

    so your circuit Q is only 1488/807 = 1.84.

    Try 5 turns, would round the ground end of your coil. You have grounded
    the frame of the variable cap to the case of the radio, haven't you?
     
  2. Guest

    Try 5 turns, would round the ground end of your coil. You have grounded
    5 turns reduces the signal and noise level quite a bit. The volume
    level is much lower. 20 turns works better.

    I tried gounding the capacitor/inductor to the chassis with not much
    effect. I did notice a noise reduction if I connect the chassis to a
    electrical ground.

    I thought the impedance ratio of a transformer was the square of the
    turns ratio, so that 20 turns relative to 80 turns would be a ratio of
    16:1 ?

    -Bill
     
  3. I read in sci.electronics.design that wrote (in
    Yes. OK, designing by remote control with no input data to speak of was
    never very accurate. What you've shown is that the optimum is probably
    somewhere between 5 and 20, but it could be 25.
     
  4. Guest

    Yes. OK, designing by remote control with no input data to speak of was
    Yes, the secondary winding seems to be optimum around 25% of the
    primary. But I noticed the phase connections also makes a difference. I
    wound another air core coil using 1 inch PVC pipe, 6 inches length, and
    about 300 turns of #24 copper wire and 75 turns on the secondary for a
    1:16 impedance ratio. But it wouldn't tune very well until I reversed
    the secondary connections to the radio input and ground. Now it works
    fairly well. One side of the primary coil/capacitor is connected to the
    radio chassis.

    Any idea why the phase connections of the secondary output effects the
    operation so much?

    -Bill
     
  5. I read in sci.electronics.design that wrote (in
    It depends where on the primary coil you wound the secondary. The right
    place is at the end of the primary that is connected to the grounded
    frame of the tuning capacitor. Then the end of the secondary nearest the
    grounded end of the primary should also be grounded. Any other
    arrangement adds capacitances to the circuit which are not wanted and
    will upset the working in various ways, some difficult to explain.
     
  6. Guest

    The right place is at the end of the primary that is connected to the grounded
    Yes, that's the way it's connected. The secondary is close to one end,
    about 1 inch from the end.

    I'm using a couple lithium ion batteries from a laptop power pack for a
    7.5 volt supply, so the whole thing is self contained with no external
    wires. I disconnected the dial light to conserve power and it draws
    only about 80mA at low volume and works well down to 6 volts.

    The only problem I have now is a slight buzzing in the output on weak
    stations, which I suspect is digital noise from the front panel display
    circuit. If I hold the radio in my hand, (hands on chassis), the noise
    goes away and reception is good..

    I suppose there is no solution to the grounding problem other than
    connecting the chassis to some electrical ground?

    But, I was trying to avoid that and make the whole thing portable so it
    could be set on a table and moved around without any external wires.

    -Bill
     
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