In article ,
Paul Burridge k wrote:

I have at 2400mhz too! Down around DC (sub 30mhz) your "pin"
is a short and the effects are mostly (though measurable) a shorted
coax with all the effects as expected. As you get up there in
frequency the "short" as described doesn't behave as it did at DC.

The problem is similar to another thread concerning real world
components where the discussion finally recognized that like other
real world components a short is not always what it may look like.

Sigh...

I can't differentiate between you and that other chap on this issue.
You both cite perfectly legitimate grounds and come to entirely
separate conclusions. You can't both be right, but neither of you seem
to be wrong!

Can we focus down on *one* issue to avoid disappearing up our own
backsides: as far as the tx is concerned, is the portion of the feed
line beyond the pin relevant at all or does it effectively cease to
exist, as would be the case at VLF/DC?

It's really a question of the problem domain - that is, what are the
frequencies involved, and what are the sizes of the feedline and the
"width" of the shorting bar/pin? Allison and I have been talking
about rather different sets of test conditions. I think we're
actually in "violent agreement" about what actually goes on.

At upper-UHF and microwave frequencies, I agree that Allison is correct.
The length of the pin is a significant fraction of a wavelength, and
it thus does not behave as a true short circuit - rather, it's an
inductor of significant value shunted across the transmission line.
At these frequencies, in this problem domain, you have to consider the
shunt combination of two non-zero impedances. "Shorting" the feedline
with this 'straight pin' inductor will probably have a significant
effect on the impedance seen by the transmitter, but it won't be as
simple as I had portrayed.

At HF (and, I think, VHF up through the 2-meter frequency range) a
shorting pin of perhaps 1/4" in length is a negligible fraction of a
wavelength long. Whatever small amount of inductance it introduces
will have a reactive impedance whose magnitude is far below that of
the 50-ohm load, and its very high admittance will swamp the lower
admittance of the load. Hence, the load impedance will be of
negligible importance in deciding what the transmitter "sees" at these
frequencies, under these conditions... the transmitter "sees" only the
impedance of the short itself, transformed by however much line is
between transmitter and short.

If I can find a scrap BNC male connector, and make a shorting-plug out
of it, I'll run the coax-and-T experiment I suggested, and post some
actual numbers for the systems's behavior at those frequencies I can
coerce out of my MFJ-269.

--
Dave Platt AE6EO
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