Well, let me ring back in with a point or two. Pete and I are on the
same page, but I think we are pronouncing some of the words
differently! <:}
First, if you are measuring SWR with an accurate meter that is
responding only to the true SWR on the line, then the only change in
SWR reading that a change in line length can provide is to make the
reading lower as you add more line due to added line loss. "True" SWR
is determine solely by the ratio of the antenna driving point
impedance to the Zo of the line, not the length of the line.
"Measured" SWR will be affected by line length because of the added
loss, if any.
Second, line length changes aimed at improving impedance matching do
not directly attempt to change SWR but rather are efforts to change
the line input impedance to a value the tuner or transmitter can
handle. Usually, adding only a few feet of line that brings the input
Z into a range the rig can handle will not show up on the SWR meter
since the added loss will be so low.
So, the basic thoughts are that in the absence of extraneous effects
like common-mode current upsetting SWR meter operation and the like,
SWR is a function solely of the relationship between the antenna
driving point impedance and the line Zo. But in the real world, lines
have some loss so that the "measured" SWR at the antenna end will be
higher than at the line input end due to the effect of line loss.
The impedance changes with line length will have little or no effect
on SWR reading - again if they are accurate and not corrupted by
common-mode current effects - since the line input impedance does not
enter into determining SWR on the line. It is easy to confuse these
two points - SWR along a line and impedance along a line - so we have
to be careful to keep them straight.
SWR along the line changes only because of line loss effects.
Impedance changes along the line only because of the transformer
action of the line when it is terminated in other than its Zo.
Many years ago, a classic article came out in QST by By Goodman, W1DX
(I think I remember this right!) called "My Feedline DOES Tune my
Antenna." It was a classic for its time since it tried to demonstrate
how line input impedance will vary with line electrical length, Zo,
etc. for a given load that does not match the line Zo. He showed that
by choosing an appropriate length of line, one could find an input Z
that the transmitter tank circuit could accommodate. What he didn't
say was that the effect he was describing was due to conjugate
matching of the system, a topic which Walt Maxwell, W2DU, so ably
presented first in his QST series in the 70's and then later in
"Reflections" and "Reflections II."
Remember, in those days, transmitters used tank circuits in the final
amplifiers that had to be tuned. But, they offered the advantage that
they could work with a wide range of line input impedances by acting
as impedance transformers. Today's rigs demand 50+j0 ohms for a load
and use fixed tuned output circuits. As a consequence, if impedance
transformation is required, it now appears in a separate box called
an antenna tuner. Sometimes the tuners are placed inside the rig to
the tune of a few extra $$$.
If we can manage to provide an antenna/line combination that presents
the transmitter with a load "close enough" to 50 ohms resistive, then
no additional tuner is required. In the more usual situation, we may
be unable to prune the antenna and the line to provide that "close
enough" 50 ohm resistive load, so we have to resort to an external
"tuner." Without a tuner, we have two variable to work with: antenna
length and line length. To a large extent, we are restricted to
single-band operation once a workable combination has been found, but
not always.
One other point that Pete brought up - an external SWR meter and one
in the rig itself - deserves a caution: when adjusting a tuner or
pruning a line, the meter to watch and believe is the one in the rig,
not the external one. The reasons can be complex, but the easy way to
look at it is this:
1. the inside meter is NOT actually reading an SWR value but is
showing how far the actual load to the transmitter departs from 50+j0
ohms. The meter scale is calibrated in SWR for convenience in
interpretation.
That is the meter you want to satisfy since the goal of the
tuning/pruning is to provide the rig with its required load.
2. the external meter in the line has an effect on the line itself. It
will not in general have a Zo of its own that exactly matches that of
the line - Bird 43 meters are probably an exception but the typical
reflectometer-type of meter is not . Placing the external meter in the
line thus introduces an impedance discontinuity and some degree of
loss. The transmitter no longer sees the same impedance as when the
meter is absent.
Thus, since the external meter is separated from the internal meter by
the length of the intervening coax and circuitry, etc. and it is not
transparent in the line, there can be a difference in the reading of
the two instruments, even though they are otherwise perfectly
accurate, due to the impedance transformation action of the line.
Remember that the internal meter is NOT reading SWR like the external
meter is. ( Purists argue that neither is the external meter, but that
is another story!). It is reading departure from ideal load resistance
and thus is primarily sensitive to impedance changes and not the
actual SWR in the intervening cable.
Hope this clarifies to some extent these frequently confused aspects
of line length, SWR. impedance, etc. To hear it from the lips of the
prophet, read "Reflections II."
Sorry for such a long posting . . .73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489 IC-765 #2349 IC-756 PRO #2121 IC-756 PRO2 #3235
----- Original Message -----
From: <AC5E@aol.com>
To: <tentec@contesting.com>
Sent: Wednesday, January 29, 2003 3:22 PM
Subject: [TenTec] Jupiter SWR, and VSWR generally
> This question has come up many times and always reminds me of my old
> professor deponing "If you don't like the VSWR, change the length of
the
> transmission line."
>
> Essentially, the indicated SWR changes with feedline length. Within
reason,
> some length of transmission line will provide an INDICATED match to
the
> transmission line regardless of how mismatched the transmission line
to load
> match may be.
>
> Back in the days when George and I were young, one of the most
popular home
> brew accessories was a box with short lengths of RG8 and toggle
switches.
>
> If you couldn't get the antenna to match straight through, you could
switch
> in 1, 2, 4, 8, 16, and 32 foot chunks of cable, stretching your coax
1, 2, 3,
> 4, 5, 6, 7, 8 feet, etc.. Some combination would provide a good
match to your
> transmitter.
>
> And that being the case, it's no surprise that the SWR in a
transmitter and
> the SWR in an outboard meter are different; unless the actual SWR at
the load
> is really 1:1. The only surprise would be if the meters read the
same.
>
> 73 Pete Allen AC5E
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