On 12/19/2014 8:08 PM, Jerry Stuckle wrote:
On 12/19/2014 12:10 PM, Wayne wrote:


"Jerry Stuckle" wrote in message
...

On 12/19/2014 2:33 AM, Jeff wrote:
By changing the length of the coax you can make the antenna
appear as being resonant or non resonant, even though the
antenna may or may not be a perfect 50 ohm load.

Incorrect, changing the length of the feeder will not change the
SWR beyond any extra loss in the cable. It will change the phase
of the mismatch that is all; (rotate around a constant VSWR
circle on a Smith Chart).

Jeff


# Incorrect. The basic VSWR meter measures the voltage, not the
power. # And if the SWR is other than 1:1, this voltage will change
depending on # the distance to the mismatch.

But isn't it still Vmax over Vmin, regardless of where that happens
on the feedline?


Yes, but that changes along the coax, depending on the length from
the mismatch.


# Additionally, a shorted coax 1/2 wavelength long shows a short (0
ohms). # But a shorted 1/4 wavelength coax shows an open (infinite
impedance). # Somewhere in between (I'm not going to bother to
figure out exactly # where because it's not that important) it
will show an effective 50 ohm # impedance.

# Coax length is unimportant when you have a 1:1 SWR, but if you
don't, # the coax will act as a matching network. And length will
affect the # overall system.

I always understood the VSWR to be constant with the feedline
length moving the parameters around the Smith chart constant VSWR
circle.

Thus it is possible by changing length to provide an antenna tuner
with R and X values that the tuner can handle better.




Let me take this one piece at a time.

No, VSWR is not constant along the length of the feedline.

Yes - it is for lossless lines, and as Jeff wrote, for lines with
attenuation, the SWR spirals inward. Consider either a very lossy or a
very long line. No matter what is attached at the load end, the source
will see the line characteristic impedance called "surge impedance" of
the line.

The variation that you see when you change the location of your
instrument is due to common mode current on the outside of the shield of
your coax. It will give you a false indication.

A transmission line connected to a load of a different impedance
will act as an impedance transformer.

Yes, it will.

The actual impedance at the source will be different than that at
the load, depending on the load impedance, the transmission line
impedance and the length of the transmission line.

Of course.

This is why changing the length of the coax allows the tuner to work
better.

Also since impedance is a function for the R and X values, when
these change, the impedance changes.

Impedance IS the R and X values which we commonly write as R+jX in what
is called complex notation. When the R and X values change the complex
will change, but not necessarily the absolute value which, unfortately
is also called impedance. Consider that |Z| = sqrt(R^2 + X^2) and note
that this is the MAGNITUDE of the vector and carries no information
about phase. What two values can you put under the radical and have the
same |Z|?

So, do you mean the impedance R+jX or |Z|? They are not the same
although one is derived from the other using Pythagorean's theorum.
Please specify.

Yes, changing the length of the coax does move the parameters around
the Smith chart in a circle, but it is not a constant VSWR circle.
Plot it out and you will see the impedance changes, depending on the
length of the coax.

I have done it many times and see very little difference using common
transmission lines. As mentioned by Jeff, Wayne, and possibly others, it
diminishes as a function of the transmission line length and loss
(commonly called alpha).

If you want to experiment with Smith charts rapidly and easily, then get
the Smith V3.10 from:

http://www.fritz.dellsperger.net/

If you learn to use it, it will open a world that you will become
addicted to.