GPS voltage

[ir50]

Hi
Wondering if I could get some advice.

I want to share the car's GPS antenna with both the original receiver and an after market Android infotainment system. To do this I'm using a Fakra Y cable.

The problem I have is the Android unit locks up when connected in this way.

It's being suggested that the issue is that the original car receiver is supplying 5v to the antenna and this is being back fed to the Android unit. The Android unit only normally sends out 3v to the receiver.

Is there a way to stop the 5v travelling down the other leg of the Y cable and reaching the Android unit without affecting the GPS signal? Or maybe a way to drop the voltage down to 3v.

In my head I'm thinking a simple diode to only allow flow in one direction but my knowledge of electronics is just following instructions, not circuit design.

By the way the Android unit can run off it's own antenna but the signal is very weak. Plugging it directly into the car's antenna gives a great signal. Hence I'd like to share the antenna.

Appreciate I could probably buy an expensive splitter unit to do what I want buy wondering if there is a simple cheap option.

Hope this makes sense and thanks for any response.

 

[Diver300]

Just put a separate antenna for the Android receiver. It's far easier. A GPS antenna on top of the infotainment system, under the dashboard works fine.

If you want to use one aerial, you should really use an amplifier / splitter that does the job properly. Some cars have them from new.

You could put a capacitor in series with the wire from the Android unit. Anywhere from 100 pF - 100 nF would be fine. That will leave the GPS aerial powered from the original car's receiver and allow the GPS signal to get through to the Android unit, without letting the 5V go to the Android unit. However, with or without the capacitor, you are completely messing up the signal termination from antenna, so you will get less signal to the original receiver.

https://en.wikipedia.org/wiki/Electrical_termination says a bit about why it's important not to have splitters.

Also, if you get the signal leak out from your connectors, due to improper screening, you can get the antenna and the amplifier that is in it acting as a GPS jammer. That could be the "lock up" that you are seeing.

Also, the Android receiver may be looking at how much current the GPS antenna is consuming, and it may assume that if no current is taken, then the antenna isn't there, so it won't work. There are fixes for that but they are quite complicated.

I don't understand why so many GPS systems have remote antennas. It is the wrong way to use them. The signal from the antenna is at 1.575 GHz and has to use coax cable and special connectors, and can only connect to one receiver, or an amplifier or similar.

A GPS receiver weighs about 2 grams (https://www.u-blox.com/sites/default/files/NEO-M9N_DataSheet_(UBX-19014285).pdf) which is much less than the antenna, so it can be fitted where ever the aerial is. By far the best arrangement is to have the receiver within a few mm of the antenna, and then send out the data. The data is typically sent at 9600 baud, which is so close to DC that it can be sent in standard car wiring and can be split to multiple destinations.

 

[ir50]

Thanks so much for the detailed reply.

Like I mentioned, the separate antenna that came with the unit, no matter where positioned, gives a poor signal. The only exception was when I placed it externally on the car roof. Hence why I looked for an alternative.

The use of a capacitor is interesting and was the "easy answer" I was hoping for. I understand the signal loss issue but have nothing to lose by trying this.

Many thanks again.