Measuring power accurately is one of the most difficult things to do. There
are so many things that come into play.
I have a Boonton power meter that is the equivalent of the HP 436. Some
expensive fixed attenuators and some good step attenuators, and an HP 3586C
level meter that reads out in .01Db increments.
I built a coupling device to use with a good bird dummy load to try and
measure power.
I even have a 200 watt bird 30 Db attenuator that I have trouble determining
its exact attenuation.
I thought that if I could get 2 or 3 different methods of measuring to agree
with each other that I could get in the ball park on power measurements.
Trying to calibrate that stuff to get repeatable readings with different
references is a challenge. I don't think that I can come much closer than a
bird 43 meter, trying to measure at the 1kw level.
One of the problems is if an attenuators impedance happens to be of just a
little it will throw a kink in the measurements that is not easily realized.
Unless you send ALL of the equipment, including cables, to a good lab and
have it calibrated at the frequency you are trying to measure you might just
as well use a bird meter.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of John Lyles
> Sent: Wednesday, December 02, 2009 3:22 PM
> To: amps@contesting.com
> Subject: [Amps] power meters for HF/VHF
>
> The Cantenna in question (70 ohms) is no good. As others said, replace
> the element, someone cooked it too hard.
>
> Dr. Dave K. is spot on, Bird 43 is not a laboratory instrument. Just a
> rough indicator. Certainly useful when trimming antennas to watch
> reflected power, or to tune the input of an amplifier, or the output for
> maximum. When measuring efficiency, gain, anything where you do need
> better than 5%, use directional couplers and attenuators, and bench
> power meters like the old HP 436/437/438 series. Other manufacturers
> such as Boonton also make decent meters. These can be gotten on epay for
> reasonable cost. If you have access to a network analyzer, even the
> TenTec kit, or a good gain/loss set, you can check and calibrate your
> own couplers and pads at the exact frequency being tested.
>
> Good power metering is obviously needed if you are making commercial
> rigs and amplifiers. Calorimetric dummy loads can be fashioned from
> standard plumbing, if you can find an accurate flow meter and a pair of
> thermometers in wells. But working with only a few deg C rise in water
> temperature requires that all systematic errors be removed first. Like
> offsets between thermometers, recording this value with no RF applied.
>
> And if the load resistor that is being cooled has flat response down to
> DC or 60 Hz, you can apply power at these ranges and measure the applied
> power using ammeter and voltmeters, to do a transfer calibration check
> with your RF/calorimeter measurement.
>
> I'd guess that the cooling air measurement would be fraught with errors.
> We used small temperature sensors in the exhaust of the Broadcast
> Electronics cavity amplifiers for FM, that would readout on the screen
> and give you a warm feeling about how you were tuning the PA, for best
> efficiency or far off of normal. But nothing was accurate enough to
> calculate power.
>
> Using calorimetric water loads, meters on the plate DC power, and
> directional coupler measurements at work, I can usually get within 5%
> on power balance, which is considered good. I trust the couplers and
> power meters the most, as I can measure the coupling of a line section
> to better than 0.02 dB using a network analyzer. And the power meters
> can be checked, send out for calibration. This is acceptable to the
> gov't.
>
> 73
> John
> K5PRO
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|