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GPS aerial misbehaving

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Diver300

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I use some active GPS aerials. There's an amplifier inside them, supplied by 3 V that is put onto the aerial cable by the receivers. I've used these quite a lot.

I found that none of the receivers would work with one particular aerial, and nearby receivers wouldn't work when this aerial was powered up. It had basically become an oscillator and it was transmitting at GPS frequencies.

Has anyone got any ideas how it could fail in such a way that this oscillation happened?
 

MikeMl

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I wonder if the source impedance of the bias-tee inside your GPS receiver is badly mismatched with what that particular GPS active-antenna expects?

Do a Thev. equivalent measurement of the bias-tee inside the GPS receiver by measuring the open-circuit voltage and short-circuit current. You could make your own bias-tee to insert between the receiver and antenna if you use a dc blocking capacitor downstream from your added bias-tee. Play with the pull-up resistor in the new bias-tee, and I think you will find a value where the active antenna does not oscillate.
 

alec_t

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Diver300

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I happened to have a few of these GPS receivers:- https://shop.cyntech.co.uk/products/gps-gy-neo6mv2 and I was trying to use them with one of these external aerials:- http://www.roundsolutions.com/en/pr...e-glass-antenna-multi-band-900/1800/1900/umts (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.
 

Nigel Goodwin

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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.
It's a relatively common fault with aerial amplifiers, high gain wideband amplifiers (and even narrow band ones) are difficult to design and it takes VERY little to go wrong to make them oscillate. Prime causes would be a manufacturing defect, or a faulty decoupling capacitor.

We had a UHF masthead amplifier in the local area years ago, and it wiped out TV reception to over a 1000 yards away. It was found by the Postoffice Interference Service, who ran the service back then - nowadays it's run by OFCOM.
 

JimB

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Working blind with no knowledge of the insides of the amplifier, my first guess would be that the output coax braid is not well connected to the 0v line (groundplane) within the amplifier.
Second I would look for a screening can not connected to ground.
Third, bad decoupling of supply lines between gain stages.

All inspired guesses you understand.

JimB
 

Diver300

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Well thanks for that.

You've explained that it's quite possible for a small fault to make an amplified aerial oscillate, and also that it's happened before.

There's an aerial that I need to throw away.
 
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