I've just tried making my own J pole antenna. While matching for lowest SWR on the SWR meter, I noticed that my FTM400 didn't show 5 W forward power.

I hardly saw the reflected power needle move so I increased the power to 20 W just to have a quick peek. There it showed a forward power of about 12 W.

On my normal antenna I get 5 W for 5 W input and 20 W for 20 W input, but this has me stumped. 

The homebrew is a J pole for 146 MHz and SWR should be in the ballpark. At 20 W I get a SWR of about 1.3.