On 12/20/2014 10:56 AM, Wayne wrote:


"Jerry Stuckle" wrote in message ...

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.



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

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

# 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.

Well, I was talking about "lossless" line. Otherwise, the constant VSWR
circle will spiral inward to the center as the line length is increased.

If you use a crappy lossy line of sufficient length, you can make an
open circuit look like 50 ohms.

Same for a short circuit. The load condition never gets back to the source.