[Amps] Bird Element Calibration?

[David Kirkby]

G3rzp at aol.com wrote:

> For real fun, how do you measure the power (with what accuracy?) into 
> a load such as marine antenna at 2MHz?

No idea.

> The dummy load is typically 10 ohms in series with 250pF: a DC 
> calorimetric method might do to calibrate the resistive part, but the 
> RF and DC resistances are probably slightly different.


I have no direct experience of using Calorimetric methods, but I know 
they were (25 years ago, and probably still are), the most accurate 
method for measuring high powers. Whether that is by DC substitution, or 
computed directly from the specific heat capacity, temperature rise etc  
I do not know.

To what level of uncertainty I might have known 25 years ago, but if I 
did it has long since been erased from my brain cells. And I'm sure the 
state of the art has progressed in that time.

But I still maintain three things - none of which I can prove to anyone 
today.

1) For amateur use Calorimentic measurements would be the most accurate 
without spending a small fortune on test equipment and calibration costs.

2) For amateur use, we don't need much accuracy.

3) 25 years ago Bird 43's  rarely meet the +/- 5% of FSD spec.

>  I have a very hard time with people who insist on quoting numbers to 
> meaningless digits - like 2 decimal places of dBs,

I had a student today give me the first draft of his undergrad report. 
He computed the noise factor (i.e. not in dB) of a spectrum analyser 
using resolution bandwidths of 30 and 300Hz. I think the results 
differed by about 20%, but he quoted the numbers to 3 decimal places, 
then took an average.

He had read the noise floor from the spectrum analyser in dB (resolution 
0.01 dB) converted to noise factor (now of course his calculator 
probably gives him 10 or more digits). 

-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/