[Amps] Bird Element Calibration?

[David Kirkby]

VK3HE Craig wrote:

>Anyone know of any companies  other than Bird who provide calibration and  
>service on Bird Elements.  I have heard  that Bird does not do such a great 
>job, and on many  occasions return slugs marked okay with large % errors 
>thats within their claimed accuracy and manufacturing  tolerances.
>So far based on the prices i have been quoted it would be cheaper to Ebay 
>the slugs and  buy new ones!
>
>Craig
>VK3HE
>

I used to work in the MOD as part of my sandwich degree. I checked the 
calibration of Bird elements and few were within the +/- 5% of FSD spec. 
Militrary groups would send the units in for cal, with a note saying 
+/-10% of FSD was acceptable - otherwise too many failed.

I bought a *new* element for 70cm for my personal use. Checked that and 
found it was out of spec. I took it back to Aspen Electronics (at the 
time the UK agent) and said it was out of spec, and said I was really 
only bothered about its accuracy at 432 MHz. The slug was bought back to 
me a few minutes later. I then took it into the MOD again, checked it 
and it was indeed within +/-5% of FSD at 432.

The lesson I learned from this (and was incidently told to me by a G3 
years earlier) is that the Bird meters are not precision devices, and 
are fine for tweaking a PA for maximum smoke, but are useless if you 
actually want to know the power output.

So I suggest selling your slugs on eBay and buying new ones would be a 
waste of your money.

Someone else measured the return loss of the throughline for me using a 
VNA.  I can't recall the figures, but it was very good. So the device 
does present a decent 50 Ohm impedance, but they are totallly useless 
for accurate measurements.

If you want to make accurate measurements, I suggest buying a low power 
meter (few mW), getting that calibrated and buying an attenuator (say 30 
dB) and getting that calibrated. Then you should be able to make 
measurements within a couple of percent or so - don't quote me on 
absolute numbers, but these will be far better than the Bird.

For even greater accuracy, use a water caloriemeter. You need to pay 
attention to detail, but that can be very accurate. It is how National 
Standards Labs measure high power. You could then use that to calibrate 
your Bird.

-- 
Dr. David Kirkby, 
G8WRB

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