On Sun, 28 Aug 2005 09:38:52 -0700, 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.

Hi Dan,

As true as that may be, the results run the gamut from trivial to
considerable as has been already discussed in this thread.

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.

This mismatch could arise for any number of reasons, and not all
contribute to radiation from the coax. Wes has already demonstrated a
deliberate mismatch at the end of a cable that exhibits absolutely no
radiation from the coax. This is because he has contrived to contain
the fields from emerging and coupling to the outside of the coax
shield. You should be aware that the shield does support currents on
the inside and outside that are wholly unaffected by each other -
except at the drive point where the two conduction paths are joined.

When you drive a dipole with a coax, the exterior conductive path of
the shield (a separate circuit from the interior conductive path of
that same shield) is in parallel with one arm of the dipole. This
means you have a third radiator that has a length and termination that
is undefined. It is THAT radiator that both causes a higher SWR AND
radiation that is not a normal condition for an otherwise tuned
antenna. Given that the length of the line's external conductive
path, and its termination is largely undefined (unless you take great
care to both measure and characterize such issues), the occurrence of
mismatch and radiation is highly variable. Thus, anecdotal accounts
of antennas being poor or good when they are driven by a simple coax
are suspect (barring the reporter also supplying the conditions of the
external path).

To eliminate the effects of this third path, a choke is added to the
drive point. The purpose of the choke is to add impedance to this
path to reduce Common Mode current. Common Mode current is the
current that flows due to an unbalanced system (the unanticipated
third radiator does that in spades). It flows in two wire
transmission lines too when the unbalance occurs for other reasons
(and those are plentiful as well).

73's
Richard Clark, KB7QHC