Hey all,
I owned my first Bird ( model 43 ) in 1968 when I started as a field
engineer in Mobil service. Today 2005 " The Bird " is still active
............with the same set of slugs. A termalin 50 Ohm 25 Watts is
still part of the team.
Robust for the field work, one can be sure. Accuracy ..... never saw one
better then 10% even 10% were rare birds in the Bird family. Our Birds
were controlled, not calibrated, every year by our metrology service as
part of contractual obligation. What was less knowen is that the % of
accuracy often approached 20% as a function during wide temperature
changes.
At least the Bird model 43 was, and I suppose still is, a marvellous
tool in the field to confirm our equipement behaviour was still in the
good direction.
Before I became in charge of our Mobil Radio service, I also leaded
military telecom bandwork,( for those who remember the ANGRC ) measuring
HF power for the military receptions where done with calorie meters as
Dr David Kirby mentioned before. How its done nowadays, I have no idea,
but I am still happy with my 37 years old BIRD. ( The older bird.....
that?s an other debate I am afraid.)
Jos on4kj
-----Message d'origine-----
De?: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] De
la part de David Kirkby
Envoyé?: samedi 19 mars 2005 16:58
À?: VK3HE Craig
Cc?: amps@contesting.com
Objet?: Re: [Amps] Bird Element Calibration?
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/
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