In article , says...
SWR meter for CB is not suitable for VHF. You can get a dummy load through
most ham suppliers like www.aes.com. Have you tried to disconnect the coax
at the radio and antenna and checked for a short between the shield and
center?
SNIP

Some antennas present DC continuity between center conductor and shield.
That is, if you measure the antenna with an ohmmeter it may look like a
short (at DC) and be fine at the design frequency.

Moral: If your antenna is of this construction, you must disconnect the
coax at the antenna and radio to check the coax for shorts.

If you can find a ham that has an "antenna analyzer", it will do a good
job of testing the antenna system. The SWR meter is the next best
choice.

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If the SWR is high for some reason the radio may transmit at 1 Watt but
shut down to protect the transmitter at high power.

The comment about checking the DC supply is good too. A bad feed could
be supplying enough current to work at one watt, but severly dropping
voltage at high power.

Woody