A recent post implied, incorrectly, that measurements of a HF PA are
difficult to make. There is no basis for such claims. These aren't
GHz band measurements of complex systems. They don't require SMA
connections. Even BNC's and SO-239's are perfectly;y acceptable
through VHF.
The general rule is a one degree long impedance bump can be tolerated
with no ill effect on impedance measurements. That's over one foot
on 160 meters, and 1/4 inch on two meters. It's pretty darned easy to
use BNC's and homemade fixtures to get into the 500 MHz range with
excellent accuracy. On 160 meters you could use a Radio Shack clip
lead!
Even a slight error isn't a problem for generalized discussions, and
a four degree guideline would apply.
I personally own an HP-4191A Impedance test set, as well as two
network analyzers. I have multichannel RF vector voltmeters, as well
as selective and sweep generators. I'm sure others have nice equipment
also.
All I need to do is attach a calibrated test probe to the anode, and
another to the point I want to measure. Pressing a few buttons and
entering a few keystrokes provides many hundreds of data points over
any frequency range between 100 kHz and 1 GHz that are sorted by the
instrument according to what parameter I am interested in, and
displayed as a list of measurement points or as a graph.
With even modest equipment someone could make very accurate
measurements at home. The big bucks of fancy equipment has only
bought me time and traceable calibration that is accurate beyond
anything necessary for this discussion.
73, Tom W8JI
w8ji.tom@MCIONE.com
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