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Re: [Amps] digital wattmeter

Subject: Re: [Amps] digital wattmeter
From: Steve Thompson <>
Date: Thu, 25 Sep 2008 08:05:10 +0100
List-post: <">>
Lou wrote:
> Lou, also take a look at the LP-100A at  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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