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Car radio antenna has an inline .85mfd capacitor, why?

Discussion in 'Electronic Repair' started by Bill Freeman, Sep 29, 2007.

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  1. Bill Freeman

    Bill Freeman Guest

    I have an 87 Nissan van with an antenna wire that has a 0.85 mfd
    capacitor soldered between the center wire and the center pin of the
    antenna connector, on the end that plugs into the radio.

    This vehicle actually has two antenna wires that connect side by side
    at the radio. One antenna is printed onto the centerline of the
    windshield, and the other is a conventional whip antenna mounted at the
    drivers side of the roof. The roof antenna lead is the one with the
    capacitor.

    Anyone know what the purpose of an inline capacitor would be on a car
    radio?

    Another car antenna question, why do they use such a small diameter
    wire inside the coax? The wire inside this coax is about .010 inches
    in diameter. Is it this small to save on copper, or is there some
    technical advantage to a small diameter wire.
     
  2. Meat Plow

    Meat Plow Guest

    Reduces engine noise on AM.
    Some technical advantage, maybe tuning the antenna circuit for AM.
     
  3. Smitty Two

    Smitty Two Guest

    Although I think Arfa recently met an audiophile who could supply the OP
    with an $1800 coax, whose center conductor is made of 300 strands of
    52AWG gold wire, specially braided to optimize the FM band.
     
  4. Jerry G.

    Jerry G. Guest

     
  5. Jerry G.

    Jerry G. Guest

    On the windscreen, in the winter dry snow will be brushing against the
    windscreen, and in the summer during a rain storm the windscreen can
    act in a way they could build up some DC static type voltages. The
    cap will block this effect. The cap basicaly acts as a protection
    against any DC from going in to the radio's front end.


    Jerry G.
     
  6. isw

    isw Guest

    Car antennas are really, really short at AM wavelengths; for that reason
    they are sensitive almost entirely to the "E" component of the EM field
    only.

    Interestingly, if that kind of antenna is hooked to a truly *infinite*
    load, the voltage induced in it is entirely independent of its length
    (it depends only on the total voltage swing from the E field of the
    signal that it encounters). With a very-high-impedance load, it doesn't
    take very much shunt capacitance to really attenuate the signal. The
    lead-in from the antenna isn't properly a "coax" at all; it's just a
    shielded cable, and it's constructed in a way that minimizes the shunt
    capacitance as much as possible -- very large shield diameter, very
    small inner conductor, nearly all air dielectric; concentricity is of no
    concern since its impedance doesn't matter.

    If cost was not an object, an insulated gate FET directly at the base of
    the antenna would produce the best results.

    Isaac
     
  7. At the frequencies used for AM reception lead capacitance is critical.
    Older radios used to have a trimmer to get an exact match- newer ones do
    this automatically. So all aerials should have approx the same
    capacitance. If you have a longer than usual lead - for a rear mount etc -
    the cable capacitance goes up, so you add a series one to bring it back to
    that standard. Same as an extension lead will have.
     
  8. Maybe to block DC in case something comes in contact with the antenna.
    Probably an impedance matching issue. The coax characteristic
    impedance is proportional to the the log of the ratio of the outer and
    inner diameters, so if you made the inner conductor 5x the size the OD
    would have to increase by 5:1 to get the same Z.


    Best regards,
    Spehro Pefhany
     
  9. 0.85uF? It would have an impedance of about 1/3 ohm at 540kHz--
    essentially a short circuit.


    Best regards,
    Spehro Pefhany
     
  10. Probably that 0.85uF number is wrong.. perhaps 0.85nF (850pF), 1000:1
    less, which would fit that function.



    Best regards,
    Spehro Pefhany
     
  11. Bill Freeman

    Bill Freeman Guest

    Spehro,

    You are correct, it is 0.85nF, not 0.85mF. The symbol on my DMM screen
    is so small I misread it without my reading glasses.
    I was estimating by eyeball when I said the original core wire was
    ..010. It actually measures .0065. I replaced the .0065 core wire with
    an insulated wire that is .0185 (.036 O.D. of the insulation). Sounds
    like maybe I should put the original size wire back inside for optimum
    performance.

    Thank you and everyone else who answered my questions so promptly.

    Regards,
     
  12. PeterD

    PeterD Guest

    The diameter (gauge) of the wire is important...
     
  13. Guest

    ..85 nF makes sense.

    I've quite a few car radios that had two complete tuners in them, side
    by side with LEDs on the board. There were two antenna inputs and you
    could plainly see it switch if you changed to the other input. Having
    worked on said radio, I think they did a pretty good job at making it
    switch without a transient.

    The theory is obviously that the multipath is not occurring in two
    places at the same time. However, it did not seem as though the AM
    section was duplicated in that manner. Looking at these boards, as
    this was years ago, they seemed to be strictly FM.

    Following the reason, for now, along these lines, there would
    obviously have to be a tapoff for the AM section, and this would
    operate pretty much like a speaker crossover, just a cap and coil.
    Both inputs, although seperate for the FM front ends, would be summed
    for the AM section. Not hard to do, pretty much like a summing
    subwoofer crossover. Only the frequencies have been changed to protect
    the innocent (c'mon it's Sunday).

    So, if my summary of the architechture of the radios on which I worked
    is correct we can assume the strange capacitor does something to the
    phase of the signal, but not to FM frequencies, only to AM
    frequencies.

    This makes sense because the two antennas are of different size. So
    the capacitor makes the two antennae into a basic, unsophisticated,
    yet probably effective phased array.

    Perhaps someone from Russia will ring in with some ideas on the phase
    they actually chose. The value of that cap will determine the
    directional sensitivity no doubt. I wonder what pattern they chose,
    hmmm, for one it is unreasonable to think that someone would drive
    right past the transmitting tower every day, I guess a cardioid
    pattern would be good. Actually that holds water because if you are
    driving right past the transmitter, what if you want to listen to a
    different station ? So cartioid it is right ?

    I dunno it might be spelled cardiod or something. I don't feel like
    checking right now, but it is most commonly used to describe the
    pickup pattern for a microphone. But you had me all screwed up at
    first anyway, a 0.85uF cap would serve no purpose except to let a
    static charge build up and blow the front end. I don't think that was
    their goal.

    Oh, the thing about Russia, as far as antennas and things, they are
    more advanced than just about anybody. We might have better ICBMs, but
    they have better antennas.

    So that's what I think it is, the cap brings the two signals into
    phase. Then as far as matching the radio to it, they are treated as
    one unit.

    That's my theory and I'm sticking to it. Pick it apart and I'll
    reconsider.

    JURB

    PS, I love shit like this, WHY did they do this and WHY did they do
    that.

    JURB
     
  14. Boris Mohar

    Boris Mohar Guest

    Why not stick that cap inside the radio?
     
  15. Guest

    Probably because there are different sized antennas on different
    models. It might also help to match it to the coax itself.

    JURB
     
  16. Bill Freeman

    Bill Freeman Guest

    Thanks again to everyone who answered my question about why the .85pf
    capacitor was in the antenna lead.

    I put back the original size .0065" diameter center coax wire.
    Afterward, I could not tell any difference in reception between it and
    the .0185 wire I had also tried. Listening to stations is rather
    subjective though. I didn't have any instrumentation to actually
    measure signal strength. At any rate, it's now back to the way Nissan
    built it.

    The whole antenna repair project was unnecessary. In addition to what
    I learned from the replies I got here, I learned that if you look for
    continuity through a car antenna with an ohmmeter and there is none,
    doesn't necessarily mean the antenna is defective.

    If it ain't broke, don't fix it.
     
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