"Ranga's GPS" Radio Station

[vnx205]

On a recent trip home from Darwin, I was traveling in convoy with my brother.

I thought he might like to share my collection of old Beatles music stored on my SD card in my GPS.

With his radio tuned to the correct frequency, a quick test showed that it worked quite well in the driveway. He could pick up the FM signal from my GPS and listen to the same music as me.

On the open road, a sensible distance behind him, we found that the signal started OK, but then faded. At first we put it down to an increase in the distance between the vehicles. Later we found that the signal faded even when the distance remained constant. It was only a strong signal for half a minute or so.

It's possible that I was in breach of some obscure communications regulations in trying to use my GPS as a radio station, but I don't really plan to make a habit of broadcasting music from my vehicle. However I am curious to know why the signal was always strong for a short while and then always faded. The music in my vehicle stayed the same, so it seemed as if it was a function of the radio in the other vehicle, not the GPS.

Does anyone have any theories?

[slug_burner]

A pirate broadcaster???

Devices are sold to allow the connection of audio sources to the vehicle sound system, most plug into a cigarette lighter and take an earphone jack from the source and FM modulate the signal.

One likely reason for the fade in signal strength is frequency drift on the transmitter which given a strong signal will not be noticed so your local receiver does not suffer from the fading signal. I suspect that when you first tune into the station you tune it exactly to the radiated frequency then as it drifts if you do not retune the radio it appears to fade? This would only result from a very poor oscillator stability on the transmitter.

[hoadie72]

Is it related to an increase in engine revs or is it a function of time? If you sat stationary for 10mins engines off and engines on do you see the same symptom?

[vnx205]

There was no change in engine revs or distance while it faded.

My money is on slug_burner's suggestion about frequency drift or something similar. I have noticed on a couple of occasions when my selected frequency is close to a nearby station that the intrusion from the radio station comes and goes a bit. That seems to fit the idea of there being a bit of variation in the frequency of the transmission from the GPS.