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Ok,see if I can explain this............Have problems with swr with this radio..........the lower the DK the high the swr goes.
I've put some other radio's(94HP;95T;4600T;2970n2) in line with the antenna set and all had a 1.04 to a 1.07 on a LP100a meter,but hook the 6-175 back up and it would show 1.06 at 12 to 15watts and if I went below the 12watts the swr would start going up and at about 7watts it would show 4.60swr and went back to 15watts to 18watts and it would still be real high,but if I key 3 or 4 times real fast it would drop back down until you lowered the DK again......ok put a dummy load on and it would show 1.01swr from a 2watt to 25watt DK......go back to 2 different antenna's (wilson 5000 mag mount with LMR 240 coax and a 10-k mirror mounted and changed to different coax and tried different lenght and still the same........so I hooked the 94HP with a 6pill to the mag mount 5000 and showed 1.12 at 100watt DK and 1.28 at 1200PEP(on the lp100a),so I wouldn't thank there's any thing wrong with the antenna system.

So can there be somthing wrong with that antenna warning/shut-down system that Magnum has?

I'm just guessing as I have seen the same thing on my Magnum. The SWR will change with power output setting up/down. The difference between these two radios, is that the Magnum uses MOSFETS and the Galaxy uses bipolar transistors for output. I was told that the MOSFETS are a bit more sensitive to SWR changes. If that is true or not - I can't really say for sure.

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BASE: Kenwood TS-2000 w/Alpha Delta DX-CC, Diamond X50A, Sirio SY27-4, and IMAX 2000.
MOBILE: Galaxy DX99V w/RF Limited CR-577 mic, Sirio Z-180. Yaesu FT-8800R & Diamond NR-770.
"...2 WR-011..."

Are you doing this test with JUST the radio, SWR meter, and Antenna? It sounds like you're introducing a whole bunch of other variables like amps and stuff. I'm not familiar with that meter (although I know what it is), but does it require any type of calibration when you change the power level?