I'm installing an Android tablet as a carputer in my '97 Hunk-o'-Junk Crapwagon, and saw a reddit post detailing a carputer install where OP linked his Jaguar's antenna to a USB GPS adapter. OP also mentioned that adapters for such connections are available for cheap on eBay, but doesn't seem to be responding to further questioning.

I understand that antennas are a complicated subject that I would do well to bone up on, but this would at best amount to a nice touch to an otherwise low-priority kind of project. Would finding a way to connect my car's AM/FM radio antenna to the GPS module inside a tablet appreciably improve the reception?

Obviously, it would be far from optimal, but would it be anywhere near worth the trouble? Even if it boosted the signal, would it translate into a noticeable benefit for the speed and accuracy of location services? I'm betting the tablet would work okay unassisted anyway, but the idea of utilizing a pre-existing connection appeals to the scavenger in me.

  • \$\begingroup\$ I don't think you will get any improvement. It will get worse. GPS and FM radio have completely different frequencies. \$\endgroup\$ – Botnic May 27 '16 at 6:42
  • \$\begingroup\$ I think the guy was talking about the existing GPS antenna, not the FM antenna. \$\endgroup\$ – HandyHowie May 27 '16 at 7:45
  • \$\begingroup\$ I think it is worth the shot. Is not a complicated project anyway. I still remember how much TV reception would improve when anyone touches it, and I don't think our body is a matched UHF/VHF antenna. The main advantage of your AM/FM external antenna is that one, that it is external, outside the Faraday club of your car. \$\endgroup\$ – Claudio Avi Chami May 27 '16 at 8:23
  • \$\begingroup\$ Y'all are so strange for commenting instead of answering the question. Doesn't anyone want credit for accepted/most-helpful answers? fwiw I'm with you, claudio, but the question now is whether I want to crack the case open to make a modification of such uncertain benefit. \$\endgroup\$ – Ryan Lue May 27 '16 at 8:29

No, it will not help. GPS uses a much higher frequency than FM radio and trying to receive that with an FM antenna will not work well at all. Also, GPS signals are circularly polarized while FM signals are linearly polarized, so not only will you be operating way outside of the antenna bandwidth you will also have a polarization mismatch. If you want to use an external GPS antenna, I recommend either a patch or a helical antenna with an integrated LNA our complete GPS receiver to avoid cable losses.

  • \$\begingroup\$ Okay, very cool, these are technical details I was curious to know. I guess the next question is, would an external GPS antenna make a big difference? My phone can determine my location accurately enough to navigate on the roads, so what applications would there be for external GPS antennae? Would my signal last longer before cutting out in a long tunnel, or what? \$\endgroup\$ – Ryan Lue May 27 '16 at 8:47
  • \$\begingroup\$ I would expect that you would see some discernable improvement in the performance of the GPS is you used a powered (amplified) external GPS antenna mounted in an appropriate place (like on the roof or somewhere with a good 360 degree "view" of the sky). Certainly, powered GPS antennas are inexpensive enough to try it with some reasonable expectation of improvement. \$\endgroup\$ – Richard Crowley May 27 '16 at 12:30
  • \$\begingroup\$ You may see improvements in time to first fix and time to reacquire the signal after a disruption (say, when leaving a tunnel). You may also see improved accuracy due to a better sky view. However, a good antenna will not help you when you have 1000 tons of rock above your head. Some devices actually have inertial measurement components (accelerometer + gyro) that will keep track of position (coarsely) for a little while after losing a GPS signal, though. \$\endgroup\$ – alex.forencich May 31 '16 at 19:38

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