I don't see the original posting here on rec.radio.amateur, but there
are a few misconceptions in the followups which should be addressed.

Lancer wrote:
On Tue, 28 Jun 2005 19:13:35 GMT, james wrote:


On Tue, 28 Jun 2005 18:31:58 GMT, Lancer wrote:


To have the measured SWR change with coax length, means you have
current flowing on the outside of the coax. Your coax then becomes
part of the antenna, so changing its length is changing the antenna
length. This would change the feedpoint impedance and the SWR.

That's correct, except that coax loss will also cause the SWR to change
with coax length. Loss will cause the SWR at the antenna (load) to
always be greater than at the transmitter (source).


Unless the line is carrying common mode currents that affect antenna
impedance, changing coax length won't change the SWR, even if the
antenna isn't matched.

Again correct except for overlooking the effect of coax loss.

But there's a real problem in communicating this. If you hook a 50 ohm
SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance
other than 50 ohms, the meter reading won't be the SWR on the
transmission line. That can mislead people into thinking that the SWR is
changing with line length when it actually isn't.


********

BS

Common mode currents on the shield of coaxial cables do not alter the
feed impedance. Repeat ofter me. Common mode currents on the shield of
coaxial cables do not alter the feed impedance.

Why repeat it if it isn't true? The explanation given by Lancer was
correct. If you change the length of the antenna, the feedpoint
impedance will change. When you have common mode current flowing on the
feedline, the feedline is part of the antenna; changing its length is
changing the antenna's length.


The feed impedance of an antenna is solely determined by its physical
length and any load impedances within the antenna structure. Load
impedances can be stray capacitance with ground via metal objects
within the near field of the antenna or even a building.

You have to realize that a radiating feedline (one carrying common mode
current) IS part of the antenna structure.


The "Magic" of an electrical halfwave transmission line is at a
precise frequency, the reflection of the load to the transmistter is
equal to the characteristic impedance of the transmission line
irregardless of what impedance it is terminated with.

This is true only of a lossless line. If the load impedance isn't far
from the line's characteristic impedance (i.e., the line's SWR is low),
a small amount of loss won't make much difference. However, if the line
SWR is high, even a small amount of loss can make a major change in the
impedance seen at the line's input. The effect is to skew the impedance
toward the line's Z0.

Other lengths
have the load impedance reflected back and transformed by the length
of the coax. The coax then acts as a transformer. It will either step
up or step down the impeadnace of the load depending on the load
itself and the electrical length of the coax.

It's a little more complicated than that. The line doesn't simply
multiply or divide the impedance by a constant, like a transformer --
except in the special case of a quarter electrical wavelength line or
odd multiples thereof. In other cases, the line does transform the
impedance, but in a complex way in which the resistance and reactance
are transformed by different factors. And reactance can be present at a
line's input even when the load is purely resistive. A Smith chart is a
good visual aid in seeing what happens. Assuming a lossless line, the
impedance traverses a circle around the origin. The radius of the circle
corresponds to the line's SWR. With the chart, you can see all the
combinations of R and X which a given line can produce with a given load
by changing its length. Incidentally, loss causes the impedance to
spiral inward toward the origin as the line gets longer, showing how
loss skews the input impedance toward Z0.


All a tuner does is electrically lengthen or shoten the coax by
introducing a lumped LC constant that helps present a resistive load
to the transmitter. The SWR at the feedline does not change. By
placing various different lengths of coax inline, you do the same
thing a tuner does, add a lumped LC constant.

As can be seen from the Smith chart, you can produce only particular
combinations of R and X by changing the length of a line which has a
given load impedance. Unless you're unusually lucky or have planned
things carefully, none of these combinations will result in 50 + j0
ohms, the usual goal, at the line's input. In contrast, a tuner is able
to adjust both R and X to produce, if designed right for the
application, 50 + j0 for a wide range of load impedances.

It requires at least two adjustable components to achieve an impedance
match from an arbitrary load impedance, because there are two separate
quantities, R and X or impedance magnitude and phase, which have to be
adjusted. Changing the line length is only one adjustment, so it can't
be guaranteed to provide a match. If you could also change the line's
Z0, for example, or the length of a stub, you'd have two adjustments and
you could guarantee a match providing you have enough adjustment range.


james


So thats all my tuner does, lengthen or shorten the coax?

Are you sure about that?

Rest assured, that's not all it does.

Roy Lewallen, W7EL