John Lyles wrote:
> 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.
I would argue you need to do this if you want better than 10-15%, not 5%. Also
remember the Bird is (or at at least was) rated at +/- 5% of FSD. So if you are
measuring at the half scale, Bird would claim your measurement would be within
10%, not 5%. But given Birds specs are at least a factor of 2 optimistic, I'd
reckon you have a 95% certainty of being within 20% if measuring at half the
full scale.
> 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.
From the point of view of a laboratory standard, any attempt to feed this
directly from a mains water supply and measure the flow rate would be
inaccurate, as the flow rate would depend on who has has just flushed the loo,
or turned a tap on. It would never be a constant, so could not be used for a
standard.
The one in the standards lab used a constant head of water, so the pressure was
always the same. With the pressure a constant, the flow rate should never
change.
The whole thing was in a temperature controlled room of course.
I suspect information about all this are published in papers by NIST, NPL etc.
I'm looking at this from the point of view of a primary standard. As I've made
clear, the Bird 43 is fine for normal amateur use. Between a primary standard
and a Bird 43, there are of course a whole range of other options. I believe
the
directional coupler and lab power meter is the most practical for hams that
want
something better than a Bird.
> 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.
I suspect, but do not know, that if you designed the amplifier with this method
of power measurement in mind (rather than try to do it on a random amplifier),
then it could be made quite accurate.
> 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.
If a calorimeter is designed as a standard (i.e. your *only* concern was best
accuracy), and things like cost, size, response time were irrelevant), it would
be better than the power meter/coupler. Those sensors on the HP power meters
ultimately depend for their accuracy on a water calorimeter. Or at least they
did, 25 years ago!
Dave
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|