Radio in my car bad due to antenna

  1. I searched around this forum and google but couldn't come up with anything definitive.

    Now, I know that the radio in your car needs an external antenna because your car acts as a Faraday cage, and most radio waves don't make it through. But I'm having some reception issues.

    I own a 1986 Porsche 944. Anyone familiar with the car will know that the antenna is embedded into the windshield in the form of a thin wire ring. That wire erupts from the bottom of the window and connects to a metal point under the hood which leads into the car to the back of the radio via a standard antenna plug. The FM reception is really bad, however.

    The car does not have an external antenna of any kind, nor a mount for one and I do not wish to do anything to the car itself to mount an antenna. My question is how can I make the reception better without modifying the car for an external antenna? Is there anything I can do that little bit of exposed wire between the glass and the firewall? Some drivers say their reception is fine, while others who drive the car said that it's always been a weak point of the car. I have an aftermarket head unit in the car.

    Is there anything I can do to help? I'm new to the EM physics realm so be easy :-p
  2. jcsd
  3. berkeman

    Staff: Mentor

    We did have a thread similar to this several years ago -- I'll try to search it up when I get a moment.

    It was similar in that the OP didn't want to put anything outside the car, which makes it tough. I think in the end there was a suggestion/question about maybe in the external fiberglass rear spoiler/wing -- I don't remember if it was a stadard spoiler or an aftermarket one. Does your car have any external fiberglass things that you could embed the external antenna in?
  4. I believe the car is metal mostly. The spoiler is rubber. The only area not metal on the outside of the car are the two valences that droop underneath the bumpers. They're plastic as far as I can tell. It's 23 years old, after all. The car does have a sunroof, but no glass.

    Here is a picture that's similar to mine. As you can see, there is little that's not metal. Oh and I do not have the rear wiper showon on this image. I have a big rubber plug that goes into the windshield, which I'm not keen on removing anytime soon. hehe.

  5. Have you tried cleaning the window, both inside and out? I've never tried it myself but a car salesman told me that it would make a noticeable difference. Also, clean around the area where the antenna comes out of the glass and connects to the coax.
  6. I've done what I can in terms of cleaning, removing debris, and making the wire connection secure.

    The question is, there's nothing I can do on the inside of the car to fix this? You're convinced that the antenna on the outside of the car needs to be better?
  7. berkeman

    Staff: Mentor

  8. Danger

    Danger 9,663
    Gold Member

    Where do you live? There are some areas in which FM just doesn't work properly. If you're in hilly territory, for instance, your lack of reception might have nothing to do with your car.
  9. Central PA, which is hills and valleys and more hills and valleys. Though, in all other cars, reception is fine for the local (in-town) stations. It's just my car that has trouble.

    And XM/serious isn't an option, sorry. Broke, unemployed college student here. I'd like to, but the antenna issue seems like it would still present itself.

    How much interference would there be if you mounted the antenna under the valences in the front or the back of the car, like, down by the fog lights? Is this plausible at all?

    Edit: Quote from the Porsche Forum. Looks like someone else trying what I'm interested in. he posts:
    Is there anything to this?
    Last edited: Aug 10, 2009
  10. Danger

    Danger 9,663
    Gold Member

    The FM wavelength depends upon what station you're trying to receive. Frequencies (wavelengths) are what you use that little knob on the dash to tune in.
    Bear in mind that FM is pretty much a line-of-sight medium. It won't bounce off of the ionosphere, as AM does. If there's something such as a hill between you and the transmitter, you're pretty much hooped.
  11. berkeman

    Staff: Mentor

    In the US, the FM band is around 100MHz, which has a wavelength of 3 meters, so a half-wavelength of 1.5 meters. Damn inches units -- who came up with these anyway?...

    I'd think that the low wing mounting positions would not work so well, especially in the front with the engine compartment exposed to the antenna...
  12. turin

    turin 2,326
    Homework Helper

    If it isn't too inconvenient, I would follow the coax all the way from the antenna to the tuner to check for damage. You can also ohm it out to quick check for any obvious shorts, but your problem, if it is in the coax, is probably intermittent/subtle, so requiring a careful visual inspection (or more expensive equipment).
  13. If I touch the head unit itself with my finger, the signal comes in perfectly. What would this signify?
  14. Danger

    Danger 9,663
    Gold Member

    That sounds as if the capacitance of your body is acting as an extra antenna. We used to have that sort of thing happen with the 'bunny ears' on our TV. Try putting some conductive substance in contact with the same area where your finger works. With our bunny ears, wrapping some aluminum foil around them improved the picture incredibly.
  15. Could be that something is not grounded.
  16. QuantumPion

    QuantumPion 883
    Science Advisor
    Gold Member

    Is there rear window tinted? My radio reception became much much worse after I had the window tinted.
  17. Indeed it is tinted. I got the car from someone who used to live in Atlanta, so it's tinted down by 70%, the legal limit there.

    How would this affect the radio signal??
  18. QuantumPion

    QuantumPion 883
    Science Advisor
    Gold Member

    Car window tints often have metal particles in them, to reflect sunlight and reduce fading. My guess is that this shields the antenna somehow.
  19. That's the reason I asked if the window was clean. Anything on the glass surface that is even slightly conductive will add capacitance which can change the antenna impedance so that it no longer matches the impedance of the transmission line. In simple terms, it shorts out the antenna.
  20. Removing the tint is not an easy job. It's not a spectacular application, probably done by a second-hand hobbyist, not a shop, as it's peeling in multiple places...but it can't just be torn off. Otherwise I would. Operating the car at night is a HAZARD. Can't see anything at all.

    Today, I'm going to go make sure the connection from the glass to the firewall of the car is clean and secure. There's not much behind the headunit that can be adjusted though. The antenna wire runs right up to that point under the windshield. I've done this before though, and it didn't fix anything up. I'll have to do some more research on the window tinting theory.

    Not that there's anything on the radio anyway :-p
  21. Here's a picture of the setup. This is directly underneath the windshield on the firewall.

    In the first picture, you can see the wire coming from the glass and into a connector. All "brown" in the photos are dirt, and not broken/exposed wire.

    The second picture is what it looks like when the plug is pulled.

    The third is how the plug goes into the firewall. As far as i can see, there are no frayings or exposed wire.

    The tinting exists on all windows though.


    Many responses I've seen online regard AM reception as interfered by window tint, but not FM.

    Also, something to note, the CD player will NOT play CDs when the car is running. When it's off, however, it plays them fine. It reports "disc read error" when nonfunctional. Friend of mine suspects engine noise. This related in any way?

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