I happened to have a few of these GPS receivers:- **broken link removed** and I was trying to use them with one of these external aerials:- **broken link removed** (with a suitable coax adaptor).
I had done this before, with no problems. Recently I only had one external aerials, and I couldn't get any of the receivers to work with it. The receivers were fine with the local aerials that came with the receivers. That was when I found that the external aerial didn't work, and would upset any other ones that were running.
This morning I got hold of another external aerial, which is fine.
I could measure the resistance of the source, but that would be at DC. I think that the receiver has an inductor in series with the DC supply, so its resistance may well be far less than 50 Ω. The aerials take around 20 mA, and are supplied at 3 V, so 50 Ω would give a voltage drop of 1 V, which is too much. I have no way of measuring impedance at 1.57542 GHz and 1.2276 GHz which I would have thought is what matters. Also, I had the same problem with several receivers.
What seems to have happened is that one aerial has changed in some way and it oscillates, transmitting enough to upset nearby receivers. I just wondered how that could have happened. I also wanted to know if there was a better way to spot it, as at first it just seemed that the receiver wasn't working. I don't have spectrum analyser that goes to those frequencies, and I think that the signals are incredibly small as well.