Before we put this thread to bed, one final comment on the ARRL's test
methodology.
Knowing that an 80m dipole fed with openwire can present an impedance of 2K
to 3K Ohms to the matchbox when trying to match the antenna on 40m, I was
curious as to why the ARRL only tested these two high power Balanced Antenna
Tuners up to 800 Ohms -ESPECIALLY since some of the matchboxes tested in
previous tests were tested all the way up to 3200 Ohms.
So I asked ARRL. (I asked Joel, who forwarded my request to Bob Allison.)
Bob Allison replied today with a logical answer.
Here is his reply:
"Our test fixture, specially built for us by our Lab consultant and QST
author, Phil Salas, AD5X, works up to 800 Ohms (resistive) without stray
coupling from surrounding objects. Higher value loads tends to add error to
our measurements, especially on 10 meters. I wish we could test higher value
resistive loads effectively for efficiency, but the price tag on such a
specialized piece of test gear would be prohibitive."
IN THE PAST, in some of the tests, there have been statements to the effect
that testing at higher impedance/frequencies was not very accurate. You
have problems with stray capacitance, etc.
The ARRL has improved its test methodology and can now test more accurately
than before, but so far, only up to 800 Ohms.
THIS IS NOT AS BIG OF A PROBLEM AS YOU MIGHT THINK.
IN FACT, THIS IS BETTER THAN YOU MIGHT REALIZE.
Two reasons.
1. Matchboxes generally have their highest losses at very low impedances.
It is important to know how they behave down there, because that presents
one of the biggest challenges when attempting to use a matchbox on many
bands. These types of problems occur when trying to match a relatively
short antenna on the lower bands (e.g. 160 or 80m). While some matchboxes
have less than 10% loss here, other matchboxes can have up to 50% loss!
2. A typical problem often incurred with high impedance (i.e. 2K+ Ohms on
40m with your 80m dipole) can be easily fixed by adding just 13 feet of
openwire feedline to the antenna system. It's a nuisance, but not a
problem. MUCH cheaper than trying to address the problem with more hardware
inside the matchbox!
Bottom Line: Again the ARRL has improved its testing methodology and is
providing us with even more useful information than before.
73
Rick, DJ0IP
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