In message , Owen Duffy
writes
Ian Jackson wrote in
news

The question is, at which end of the feeder is the most absolute power
lost per unit length?

In response to your two posts...

You might be informed of the traditional graphs (or simple underlying
formulas) of "Additional loss due to VSWR" that appear in some
publications including several ARRL publications.

The formulas on which these graphs usually depend rely on a set of
assumptions that are often, if not usually, not stated.

You could be forgiven for interpreting these things to mean that loss per
unit length (in dB) is constant along a transmission line independent of
VSWR.

The traditional RLGC model of a transmission line does give constant loss
per unit length when VSWR=1.

For most practical lines at HF, most of the loss is due to conductor loss
(the R term in RLGC) and very little due to dielectric loss (G in RLGC).
The result of that is that line loss in any incremental length is very
dependent on current, higher in regions of higher current, and lower in
regions of lower current.

Consider the loss in 1m of RG58 at 3.5MHz with a 500+j0 load. It is a
very short line electrically so current is almost uniform. VSWR=10 and
current is approximately 1/3 of that for a matched line for the same
transferred power.

Why should loss be greater than a matched line when I^2 is one tenth the
matched line case? Well, it isn't. The formulas and graphs do not apply
because the (often unstated) underlying assumptions are not met.

Regarding the dummy load, it might seem intuitive that loss is highest
where the VSWR is worst, but that is not correct. The correct answer is
found by calculating the loss along the line.

If you consider the case of 100m of RG58C/U with O/C termination as a
dummy load for 1296MHz...

Lets section the 100m into 10+90m.

Using TLLC (http://www.vk1od.net/calc/tl/tllc.php), the input impedance
of the last 90m is 50.00-j0.01.

Now lets look at the first 10m of our 100m, it has a termination of
50.00-j0.01 and the loss is 8.5dB, only 14% of the input power flows into
the last 90m, 86% if the input power is lost somewhere in the first 10m.

By the same method, you could find that almost 20% of input power is loss
in the first metre, even though the VSWR in that section is almost
perfect.

That is indeed very interesting - and illuminating. I've always assumed
that, for electrically long feeders, most of the power got lost at the
input end - but I didn't realise that the effect was so dramatic. I must
do a few calculations for myself!

And as for electrically short feeders, I do see how the greatest power
loss is where there are current nodes etc (although, presumably,
averaged over a halfwavelength, the power loss will still be greater
towards the TX end). But I now recall that some time ago, there was a
lengthy discussion (in this NG, I'm sure) about how a matched
low-impedance feeder could actually have more loss than a mismatched
relatively high-impedance feeder.
--
Ian