Mobile antenna issue

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Otter
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Mobile antenna issue

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A friend bought a Thunderpole Atom (reduced size modulator) a while back and couldn't get the SWR down with it on his car. He handed it to me the other day to see what it was like on mine, and it's high on mine too.
I usually have a red devil and that has a minimum SWR of about 1.2. I get about the same with a Sirio 5000 so there's nothing wrong with coax etc.

DC-wise the resistance between the screw thread on the bottom and the whip is about 1/2 Ohm. I put it on an antenna analyser, and there is a very definite SWR minimum at about 27.4 MHz and the swr curve looks normal, but the minimum is at 2.1:1

Does anyone who knows about antennas have any idea what might be going on here? Thanks.
"To this day the words Stirling bridge conjour pride in every Scotsman's heart, while to an Englishman those same words conjour literally no feelings at all." - Cunk on Britain S1 E1
Gorgon Brown - a man with all the joie de vivre of a haunted cave in Poland. S1 E3

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