[Elecraft] P3TXMON question?

[Bob McGraw - K4TAX]

This seems to have things really screwed up.   Yes, the Bird 43 is a 5% 
of F.S. accuracy instrument.   The Bird 43P is somewhat less accurate, 
although 5% F S with carrier, and more like 8% F S or so in peak mode. 
    In measuring a 100 watt radio with a 100 watt F S slug would seem to 
be most accurate.    Total accuracy decreases as the indicated value 
approaches the left side of the scale.   Now using a 250 watt slug to 
measure a 100 watt radio,  the earlier states that most accuracy is to 
the left of center, which by the way, is the compressed side of the 
scale.  This makes no sense as the scale markings are much more 
difficult to approximate.    In fact the most accurate area is to the 
right of center toward full scale is the expanded scale  which is where 
the highest accuracy actually occurs.   With a 100 watt radio and a 100 
watt F S  slug the error is +/- 5 watts at 100 watts.  Using a 250 watt 
element and measuring 100 watts near center scale the error is more like 
+/- 12.5 watts or 5% of the  F S value.

If one measures the same power source with identical elements and gets 
different results, they should refrain from buying ham fest elements for 
indeed the elements are not identical.  To wit, I purchased 3 new 100 
watt elements and 3 new 2500 watt elements. They all give identical 
results +/- the width of the pointer.  I don't buy ham fest Bird 
elements as I find they are junk and not worth the brass they contain.

And remember, Bird 43 instruments are calibrated for only a 50 resistive 
load.  Any other load being used leaves one with a "best guess" 
situation as to actual power.

73
Bob, K4TAX
K3S s/n 10,163

On 8/21/2015 11:48 PM, Wes (N7WS) wrote:
> So by your "logic" it would be best to measure one Watt using a 100W 
> element.
>
> On 8/21/2015 8:03 PM, Ken G Kopp wrote:
>> Bird accuracy is specified to be +/- 5% of meter full scale ... a 100W
>> element would be a poor choice to measure a 100W output level. Take a 
>> look
>> at any Bird's meter scale.  It's not linear and can be read with the 
>> most
>> accuracy to the left of center scale.  A 250W element is much better for
>> measuring 100W.
>>
>> Wanna chase your tail?  Measure the same power source with identical 
>> Bird
>> elements......
>>
>> Don's method is better, but all methods are obviously subject to the
>> accuracy of the final indicating instrument.
>>
>> FWIW .....
>>
>> 73l
>>
>>
>
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