Re-radiating anttenna problems

[MrT]

Just a word of warning about GPS re-radiating antennae. I use one with my TomTom unit as the aerial is placed on my roof and it gives me more satellites and a stronger signal than the TomTom unit alone on the dash. I also use a GPS hardware based accident blackspot detector.

The other day when driving, I lost all satellite visibility on both my TomTom and other GPS unit. It took me ages to work out what had gone wrong as normally both units will work without the antenna and I there was nothing else in common between the two. When the antenna had failed, it was now acting as a GPS jammer. On pulling the antenna wire out, both GPS units resumed to work as normal.

[Dave]

This can happen, and also depending on how far you position the wand stick (transmitter) from your GPS devices can also create the same sort of effect, which effectively is swamping the GPS Receivers with too much data and creating a reverb/echo scenario.

[MrT]

Yes I recently suffered from the reverb scenario. After deciding that the previous antenna was near the end of its useful life, I decided to buy a replacement.

There two basic types on the market, the small type with a lighter plug on one end, a T junction for the wiring and a 10cm rod about 7mm diameter on the inside end and a small antenna on the third wire. The transmitter needs to be upto 50cm from the GPS unit.

The other type has a transmitter about the size of a lighter plug and the antenna and power leads plug into this. This needs to be within 3m of the GPS unit.

I went for the latter as it is easier to pass the antenna plug through bodywork rather than cut the cables and re-solder the other hard connected type. However now having a 3m transmitting zone meant when I had the transmitter in the headliner where a sunroof motor would go, meant it transmitted to the antenna outside on the roof (through or around a solid metal roof). The meant that everytime the signal level rose, I would suddenly lose all satellites as it got into a feedback loop.

In the end I had to put the transmitter on the floor of the car behind the dashboard, where it works perfectly.

So don't buy the most powerful re-radiating antenna unless you need the longer range from transmitter to receiver, it does not improve the performance over the lower power units at short range.

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