[Amps] Bird Element Calibration?

[David Kirkby]

G3rzp at aol.com wrote:

> In a message dated 20/03/2005 17:23:33 GMT Standard Time, 
> david.kirkby at onetel.net writes:
>
>     I don't know what 'Type Approving' equipment is,
>
> Equipment that has to be 'Type Approved' before being placed on the 
> market.

Of course, what it is marketed for would determine how accurate you need 
that instrument to be, and I would guess the tracability of the lab 
approving it.  Surely an expensive Aligent VNA would need to be approved 
by a better lab than the SWR meter sold in amater radio outlets. This is 
outside my area of knowledge, so I am guessing here.

> Conducted measurements to 5% by calorimetric methods do rely on the 
> mismatch errors being very small at the operating frequency.

Agreed - I know mismatch errors are going to be the most significant, 
but with a decent load, I reckon 5% could be achieved by amateurs using 
a Calorimeter  - which is half or a third the error they are likely to 
get using a Bird 43. I can't think of a more accurate way, that is 
affordable.

And if the original poster is keen to get his elements accurate (which 
he clearly is, to consider selling old ones on eBay and buying new 
ones), I think such a method would be his best option to calibrate the 
elements. Perhaps you can. The method we used was attenuators and power 
meters, but the mismatch errors and attenuation of the attenuator were 
known accurately - not something that an amateur can do.

I suspect Bird use Calorimeters for internal use, but would not use them 
to calibrate each and every element they sell, as the method is too time 
consuming.

Another big problem for amateur power measurement would be line voltage 
variations. Since any such measurement by Calorimetric methods will take 
10's of minutes at least, there needs to be a way of keeping the RF and 
DC power constant.

You could argue that the mismatch makes no odds at all for a valve amp, 
as you tune the amp into whatever load impedance the dummy load 
presents. But of course, that only gives you a power measurement that is 
accurate into that load. It does not give you a power measurement that 
is accurate into an antenna unless you make *exactly* the same as the load.

At the end of the day, you need to ask yourself what accuracy do you 
need. I think for RF power measurements, the answer is not very much for 
amateur radio use. You tend to tweak for maximum, keeping an eye not to 
exceed the current ratings on the tubes. Exceeding the anode dissipation 
by 10-15% will not be a problem if the temperature is kept down, so 
knowing the exact efficiency is not such a big deal. I suspect using the 
temperature rise across the tubes would be reasonable to work out anode 
dissipation. How that would compare to the accuracy of a Bird I don't 
know, but suspect even that would be better than the Bird.

You could argue you would need to be a bit more fussy about that with 
semiconductor amps.


-- 
Dr. David Kirkby, 
G8WRB

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