G3rzp@aol.com wrote:
> In a message dated 20/03/2005 17:23:33 GMT Standard Time,
> david.kirkby@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/
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