Re: Power meter for 23 cms over 300 watts

Alan G0XBV

Agree about the comparison with amateur fare in this respect, Geoff.  I once saw an outrageous amount of power indicated (~1100W) from an otherwise very well amateur-designed/built 50MHz power fet amplifier, which showed to be ~480W around the point of output compression on a Bird 43.  The particular meter/insert combo was probably a somewhat stingey combination, but the (amateur) brand name meter was *very* optimistic. I probably fall into the 'over-rated' camp, in the sense that I have an over-rated opinion of them.  A combination of not knowing/not having anything better, well certainly at UHF and above.  Since John, 'XDY very kindly joined me to this group recently, I have quickly had to come to terms with the true meaning of 'microwaves' (thank you, John, and for the Amphenol relay data too!) and clearly this is an ongoing process :> Happy to learn though!  A very timely thread from my point of view  - I was only enquiring of Sam, G4DDK, about E inserts and their suitability/his usage for 1296 just the other day.  I'm sure you won't be surprised to hear that 'Indication only' was his reply too.

Al G0XBV   (with apologies for quoting a DC band example :>)

-----Original Message-----
From: Geoff Blake geoff@... [ukmicrowaves]
To: ukmicrowaves
Sent: Thu, 26 Feb 2015 12:42
Subject: Re: [ukmicrowaves] Power meter for 23 cms over 300 watts

Hello David, Group,

David, I totally agree with what you say about the Bird 43 (~2% FSD) and the slugs (~5% spec., ~10% FSD in practice), but they are better than much on the amateur market. I have in front of me the manual for a Diamond SX200 "Precision SWR - Power Meter" which quotes various accuracies between 5% and 20% FSD. From my own quantified measurements, those figures are wildly optimistic.

Sadly, so are yours for the HP/Agilent power sensor/meter kit. The HP436A, for example has an instrumentation accuracy of ~1%, the sensors vary but are typically no better than about 3.5% when you take mismatch uncertainty, noise etc. into account. John Hurl of UKAS produced a brilliant document - the title of which I cannot recall but perhaps is M3001, on the totalling of uncertainties and a true reference document. Then there is the power reference uncertainty. HP did some very valuable work in their AN 64 et al (IIRC) but is available as a PDF.. Another factor is that HP sometimes cannot fully calibrate their own products without return to the manufacturing division in the states. For example check the spec of their 3458A against their accredited capability in the UK.

Although I do nut run anything like the 300W the OP has to measure, I used a HP 778D coupler and 20 dB attenuator to provide ~42dB of coupling and a 8481A/436A power meter/sensor to measure powers in the region of 100W at 70 cm. Of course, one has to calibrate the coupling factor of the directional coupler first. As you rightly point out calorimeter type power meters are probably the most accurate systems available, but there is/was a whole industry built up around those things - one rule, the more power you have to measure, the more accurately you can do it.


Geoff Blake,   G8GNZ    JO01fq:   Chelmsford,  Essex,  UK
< geoff@...>    or   < melecerties@...>
Using Linux: Ubuntu 14.04 on Intel or Debian on UltraSparc
and  Apple  OS  X  10.10  Yosemite  on  my  Macbook  Pro.    
                Avoiding Micro$oft like the plague.

On 26 February 2015 at 10:35, 'Dr. David Kirkby (Kirkby Microwave Ltd)' drkirkby@... [ukmicrowaves] <ukmicrowaves@...> wrote:
On 26 February 2015 at 09:19, Richard Baker perwick@...
[ukmicrowaves] <ukmicrowaves@...> wrote:
> Keith
>     I use a Coaxial Dynamics 82075 950-1300 MHz 1000 watt slug in a Bird Type 43 meter for routine measurement.
> I have just put a Bird 1000E 400-1000MHz slug in the same equipment on 1296.250MHz and here is the comparison.
> 82075 slug 380 watts indicated.
> 1000E slug 520 watts indicated.
>  The true power is unfortunately closer to the lower reading based on thermal measurements.
> I got the Coaxial Dynamics slug because it was going cheap and good enough for my purposes.
> 73s
> Richard

I tested 100's of Bird elements at a placement in a standards lab as
part of my sandwich degree. I forget the uncertainty of the
measurements, but they were referenced backed to a water calorimeter.
Some guys had spent 20 yeas of their working life improving the
accuracy which it was possible to measure RF power.

I know the elements are not as accurate as claimed. Many were out of
spec the when new and many customers would send the meters in with
elements, and a sticker which said "10% of FSD is acceptable". That
was because the failure rate was so high, they could not use the
meter! Most, but not all would pass at 10% of FSD.

But lets be honest, for most measurements, 10% is good enough.

I bought a brand new element that was supposed to cover 432 MHz from
Aspen Electronics in the UK who were, and I believe still are, a Bird
distributor. I took it into work, and it was out of spec! So I took it
back to Aspen Electronics, told them it was out of spec, and also said
I only cared what it was like at 432 MHz. A few minutes later the
technician adjusted the element. I then took it into work, and it was
within spec at 432 MHz.

I don't know if the Birds elements have improved in the last 30 years,
or whether the Coaxial Dynamics ones are any better, but I know the
meters are very over-rated by hams. Some see them as a gold standard,
but personally I see them as a piece of ****.

The poor quality of the elements is one reason I would look to use an
HP power meter and sensor, along with a directional coupler and
attenuator. The HP power sensors have an uncertainly of less than 1%.
I've never actually sat and worked out the uncertainty you would
achieve, but it would certainly be a lot less than 10-15% of a Bird
and depending on the sensor you buy, you also have the ability to
measure down to very low powers.


Posted by: "Dr. David Kirkby (Kirkby Microwave Ltd)" <drkirkby@...>


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