[TowerTalk] 2000 Antenna Forum Speakers

[Eric Gustafson]

>Date: Thu, 4 May 2000 15:57:01 -0400
>From: Bill Coleman AA4LR <aa4lr@radio.org>
>
>On 5/3/00 8:57 PM, Eric Gustafson at n7cl@mmsi.com wrote:
>
>>The only difference is that some want to ascribe the additional
>>line loss due to mismatch to the fact of the line being
>>mismatched.  And others (rightly) point out that if the line
>>had no loss in the first place, the mismatch would be
>>irrelevant and cause no additional loss of its own.  So the
>>loss is really due to the lossiness of the line not the
>>mismatch condition.  The mismatch condition merely exacerbates
>>the bad characteristics of the line.  It really just depends on
>>how one _wants_ to look at it.
>
>Except for one thing. You can do experiments to show that if you
>substitute a less lossly feedline, the losses decrease, or a
>more lossly feedline they increase.
>
>In this case, the mismatch condition still exists, the only
>thing changed was the loss of the feedline. If the mismatch were
>the cause of the loss, one would think it would remain constant
>when the feedline is substituted.
>


Hi Bill,

Actually, in my case, you are preaching to the choir.  But taking
the devil's advocate role here for a few sentences, one could
also say the following:

"Except for one thing.  You can do experiments to show that if you
substitute a less severe mismatch, the total losses decrease, or
a more severe mismatch, the total losses increase.

In this case, the inherent line loss still exists, the only thing
that changed was the severity of the mismatch.  If the line loss
was the cause of all the losses, one would think it would remain
constant when the degree of mismatch was varied."

The confusion comes when mixing total loss with losses which are
separately identifiable due to different causes.  And to a large
degree on what is taken to constitute a "significant" loss.

Of course, where the argument falls apart is in the limit as line
loss approaches zero.  When the line loss is zero, then there are
no additional losses introduced by a mismatched condition.  Under
the zero line loss condition, varying the mismatch does _not_ vary
the total loss.  This is the thing that prevents me from wanting
to think of the mismatch as the cause of the loss.

But for certain, a severe mismatch will increase the effects of
the inherent cable loss.  However, when the line loss is very
near zero for practical purposes (air line at HF with a mere 10:1
or even 20:1 SWR at the load), the fact that there is a
vanishingly small additional loss because the load is mismatched
is of little or no practical consequence.  And, at HF using
reasonably short runs of reasonably good coax with a moderate
amount of load mismatch (4:1 or even 5:1) does not present a huge
problem either.  Unless, of course, you want to argue about the
significance of a 0.5 dB change in ERP on an HF circuit where
signal levels due to propagation variations minute to minute are
going up and down by 30 dB or more.

73, Eric  N7CL

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