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My old (2001) Seville had a cell phone with antenna mounted on the rear glass. The coax connects inside the glass, with signal passing through the glass to the whip mounting plate.

I'm wondering if I can use the old cell antenna mount (through glass) and just replace the whip to make a dual band 70cm/2m antenna. I discovered that the old cell radios in this car were on 800-850MHz, which would make a full wavelength antenna 0.35m long. Which is approx 1/2 wavelength for 440MHz, and <1/4 wave for 140Mhz

Has anyone done this before? The base inside the glass is more than a piece of metal I suspect, perhaps a loading coil. Any advice is appreciated.

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These types of antennas are typically capacitively coupled. They will not work on metal tinted glass or glass with near by wires in it.

They have to be calibrated for glass thickness and dielectric constant. If it was meant to be used generically rather than on a specific vehicle, there may be a set screw or something to adjust the calibration. Likely the external antenna and mount need to be matched to the inside piece, but you might be able to calibrate for that.

You would need an antenna analyzer for the frequency in question (or a VNA) to do this right.

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  • $\begingroup$ Since the antenna came with the car (cell option from GM), I assume the glass etc. should work. The portion inside the glass is a 1cm deep black square, which I assume has a loading coil - but I'm not sure. Being new to this, I wondered are such coils / base / capacitance across glass tuned to a particular frequency making it impossible to use on another frequency? Or can I just lengthen/trim the whip to tune it? I will look into VNA $\endgroup$
    – TSG
    Commented Sep 3, 2022 at 13:24
  • $\begingroup$ Glass mount antennas are almost always capacitively coupled, instead of a loading coil. The gap and the antenna and base are all tuned together for a specific frequency range, changing the frequency will require redesigning and returning the whole thing, so no, you probably can't use it on a different frequency just by trimming the whip (unless it's really close). $\endgroup$
    – user10489
    Commented Sep 3, 2022 at 15:18
  • $\begingroup$ Also, the whip is probably hardened spring steel, so you will break whatever tool you try to trim it with. $\endgroup$
    – user10489
    Commented Sep 3, 2022 at 15:19
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    $\begingroup$ It might, it might not. That's why you test with an antenna analyzer. But modifying it beyond adjusting the tuning is not likely to work, assuming it has a tuning adjustment at all. $\endgroup$
    – user10489
    Commented Sep 3, 2022 at 20:47
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    $\begingroup$ @user10489, Re, "hardened spring steel," Good point, but once somebody knows what they're dealing with (Thanks for the warning!) then it's just a matter of choosing a suitable tool. $\endgroup$
    – Solomon Slow
    Commented Sep 6, 2022 at 16:19

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