On Thu, 22 Dec 2005 04:17:11 GMT, John Ferrell
wrote:


It is true that reducing SWR for a given line does reduce the loss if
the line is long enough. (There are some scenarios where a short line
with high VSWR has less loss than matched line of the same length.)

I will have to take your word for it, I cannot think of any examples.

John,

It was really a bit of an aside, a lead into the more important point
that followed, however...

In most practical lines at HF, loss is dominated by the series
resistance.

In the regions of a current minimum on a line with high VSWR, the I^2R
losses are less than for the same net power on a flat line, and
conversely, in the region of a current maximum on a line with high
VSWR, the I^2R losses are more than for the same net power on a flat
line. Over a half wave of line, the total losses are higher than a
flat line, but a short line in the regions of a current minimum may
have losses less than the matched line loss.

Whilst that is not often to our benefit as we rarely have relatively
short lines with high Z loads, the converse is true of the more common
situation of a short line with a low Z load. For example, 3m of RG58
with a 5+j0 load (eg mobile antenna) on 3.5MHz has a matched line loss
of 0.08dB, and an actual loss of 0.66dB. Many charts and formulas
would predict the mismatched loss to be only 0.39dB, but it is worse
because the line is short and in the region of a current maximum.

But is matched line the real goal?

If low loss is the goal, there are often cost effective lower loss
solutions possible with lower loss line operated at high VSWR.

A mismatched transmission just MIGHT work OK. If there is any
possibility of generating interference, especially TVI, it will. The

Why? How is TVI "generated" by line mismatch?


I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.

If you can't explain the mechanism by which SWR causes TVI, perhaps
they correlate by some other cause. For example, an antenna may
develop loose oxided connections which both change the load impedance
(and hence VSWR), and create intermodulation causing TVI.

If VSWR *does* cause TVI, surely someone will be able to explain how?

Lots of people operate feedlines at high VSWR by design, and they do
not necessarily cause TVI.

When you dismiss the TVI myth, you get closer to understanding how the
transmission lines work and perform, and that for example, operating a
feedline at high VSWR can be part of an efficient and effective
multiband HF antenna. Such a solution should not be dismissed out of
hand because of high VSWR alone.

Owen
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