dansawyeror wrote:
My first claim is a tuner at the source does not materially improve what
is happening in the coax. That is a tuner does not recreate the
condition above where the coax is functioning as a properly matched and
terminated transmission line. All the tuner does is match the impedance
at the coax source back to some known, usually 50 Ohm, value.

No matter what the voltages and currents are, if they are balanced,
the transmission line won't radiate (much). If the SWR is 100:1 and
the currents are balanced, the transmission won't radiate (much). If
the SWR is 1:1 and the currents are unbalanced, the feedline is likely
to radiate.

My second claim is when the mismatch condition at the coax destination,
i.e. antenna that may result in significant radiation from the coax itself.

Please understand it is not impedance mismatches that cause radiation
from the feedline. It is unbalance in the feedline currents that causes
feedline radiation. Current imbalance and impedance mismatches are not
necessarily related.

Current imbalance in a matched system can cause feedline radiation.

Impedance mismatches can exist with negligible feedline radiation.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil,

To this point I do not know where the radiation comes from. I suspect it is from
the antenna. The easiest case is to model a poorly balanced dipole directly feed
by coax. It radiates significant power back down the shield.

After the response of the majority of posters to this concept I plan to find out
more.

Dan

Cecil Moore wrote:
dansawyeror wrote:

My first claim is a tuner at the source does not materially improve
what is happening in the coax. That is a tuner does not recreate the
condition above where the coax is functioning as a properly matched
and terminated transmission line. All the tuner does is match the
impedance at the coax source back to some known, usually 50 Ohm, value.


No matter what the voltages and currents are, if they are balanced,
the transmission line won't radiate (much). If the SWR is 100:1 and
the currents are balanced, the transmission won't radiate (much). If
the SWR is 1:1 and the currents are unbalanced, the feedline is likely
to radiate.

My second claim is when the mismatch condition at the coax
destination, i.e. antenna that may result in significant radiation
from the coax itself.


Please understand it is not impedance mismatches that cause radiation
from the feedline. It is unbalance in the feedline currents that causes
feedline radiation. Current imbalance and impedance mismatches are not
necessarily related.

Current imbalance in a matched system can cause feedline radiation.

Impedance mismatches can exist with negligible feedline radiation.

dansawyeror wrote:
To this point I do not know where the radiation comes from. I suspect it
is from the antenna. The easiest case is to model a poorly balanced
dipole directly feed by coax. It radiates significant power back down
the shield.

Actually, a perfectly balanced dipole can also radiate from the coax
shield because coax is unbalanced. Take a look at www.w2du.com and
www.eznec.com for information on baluns and chokes which tend to
reduce feedline radiation.

It appears to me that you may be confusing "balance" with "matching".
Those words have very different definitions.

A balanced 50 ohm antenna fed with coax and having an SWR of 1:1 is
matched but unbalanced and is likely to radiate from the feedline.

A balanced 50 ohm antenna fed with 600 ohm balanced line and having
an SWR of 12:1 is unmatched but balanced and is not likely to radiate
much from the feedline.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

SNIPPED

It appears to me that you may be confusing "balance" with "matching".
Those words have very different definitions.

A balanced 50 ohm antenna fed with coax and having an SWR of 1:1 is
matched but unbalanced and is likely to radiate from the feedline.

A balanced 50 ohm antenna fed with 600 ohm balanced line and having
an SWR of 12:1 is unmatched but balanced and is not likely to radiate
much from the feedline.

Yep!! Love that high VSWR! Tuned feeders for 50+ years and they still
work fine.