[ARC5] Silly Inaccurate Wattmeters and a Tool to Fix Them.

[sbjohnston at aol.com]

I can confirm that a professional Bird 43 wattmeter is much, much more linear than any amateur meters I've tried.  +/- 5% of FS is their spec if I recall correctly.  But to get that you need various slugs to cover your frequency and power needs (can be expensive).  I have several 43s here with elements for HF, VHF, and UHF at my power levels.  

I don't think the linearity problems with the amateur meters is really a big deal in their normal operational use.  I check (and calibrate if needed) across the bands against one of my Bird 43s for the likely power levels where they are installed. 

For example, my operating desk wattmeter is a mystery "Macaw" brand unit that I rebuilt when its DC and calibration board started to get intermittent.  It reads quite close at the likely power levels from those transmitters:  100, 200, and 800-1000 watts.  I set it up so CW signals read the same on average and PEP, and on SSB and AM the PEP readings make general sense so that's good enough for the operating desk.  

But I would NOT want to use one of the amateur meters on the repair and homebrewing bench.  I have a Bird 43 and a dummy load set up there at all times.  I don't want unreliable metering to make me chase non-existent problems.  I added a peak-detecting board to this 43 and added a switch to choose AVE/PEAK.  The mod does not involve the RF or sampling, just the DC to the meter. 

I have only confirmed the performance of the Bird 43 meters against each other and by swapping the slugs around.  I have one Bird that I bought brand new so I am felt good when the old beater meters agreed with it.  I have not used other gear calibrated to external standards to confirm the Birds.
 
Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/http://af4k-crystals.com
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