> Lou, also take a look at the LP-100A at www.telepostinc.com. It is reputed
> to be better than either of the other two.
> I will at that unit to the mix.....anymore ideas? Lou
Buy whichever gives you the look, shape, size etc. that you want.
Don't get carried away with the specification claims - you're
unlikely to get the extreme accuracy in practice, and you don't
need it for a ham station anyhow.
Once you're away from 1.0:1 VSWR, measuring power with a
throughline system is fraught with error, mostly due to the
directivity of the directional coupler (how well it isolates
forward and reverse power pickup).
In the LP100, the construction of the coupler looks unlikely to
maintain high directivity at the top end of hf and above - the
paperwork says the 6m 'raw' readings might be up to 10% out before
calibration. If the coupling has drifted that far off, the
directivity will probably have gone as well.
Reading the leaflet for the V/UHF coupler heads for the Power
Master had my BS alarm going. There's a section about how lead
length affects VSWR readings, and how this is caused by harmonics.
While harmonics can cause the effect, it's usually down to poor
coupler directivity - sure enough, further down the page are the
specs. giving values of low to mid 20s dB. Something that poor
will tell you if your antenna has broken, but all the accuracy
claims become nonsense.
A freshly calibrated HP power meter guarantees a few% accuracy in
a pure 50 ohm system. At best it's optimistic to imagine you can
get anything like that once you factor in directional couplers,
real world VSWR etc. The good thing is it doesn't matter much.
Just don't forget that the third digit on any power display won't
be accurate, and the fourth is for amusement only.
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