> I read and then re-read the two messages. I suppose the easiest thing is to
> simply say you are both right. The quotes from Walter's book are OK, but
> what Tom has said is OK too. I think you guys are just slightly out of synch
> in pursuing this thread. Since I have no idea who started the thread I find
> it impossible to guess who is "out of phase" with whom.
>
> Tod, K0TO
Tod,
The out of phase condition arises because the citations from Walt Maxwell
refer to SWR. Tom's note refers to the impedance to be measured.
The SWR (ignoring line losses) does not change. But, except for a 1:1
condition, the impedance does change along the line, repeating itself
every 180 degrees (1/2 wl, allowing for velocity factor of the line).
Since the problem was the impedance at the antenna terminals and the
difficulty of measuring it right at that point, the advice to use a
prechecked and validated 1/2 wl line to the impedance (not SWR) measuring
instrument is sound advice.
If anyone wishes to get some idea of how impedance does change along a
transmission line with mismatches, there is a program in the HAMCALC
collection that will calculate the value every 5 degrees for 180 degrees
to make a table. A small collection of these will familiarize the reader
with what happens when loads are reactive in either the inductive or
capacitive direction. The program will also calculate for specific line
lengths. If you do not know the voltage and current at the load, just use
a power setting. The figures are for lossless lines. To calculate
specific line lengths for lines with losses, use TL by N6BV, available at
the ARRL site or on the disk that accompanies the Antenna Book. To make a
table, you will have to run multiple times.
Remember that the simplified formula for determining SWR that simply takes
the ratio of load impedance to the Zo of the feedline works ONLY if the
load is purely resistive. If the load is reactive to any significant
degree, that formula is not even a good approximation. The actual
equation is much more complex (although there are simplified methods for
reaching the SWR number) and factors in the resistive and reactive
portions of the load impedance.
In a case like the beam, where the resistive impedance may be in the 20-30
ohms range, and the required capacitive reactance for a beta match may be
also in the 20 to 30 ohms range, the SWR will be well over 2:1, and the
impedance along the 50-ohm line will change its R and X values fairly
rapidly. Hence, the need for a well-calibrated 1/2 wl line to determine
the load values accurately.
Hope this is useful.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://web.utk.edu/~cebik/radio.html
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