From: "K7ITM" on Thurs 26 May 2005 13:32

snip of good stuff

In summary, the assumption that the (unloaded) Q drops drastically is
incorrect for practical coils commonly used in transmitter tank
circuits. But it does suggest that you should understand what the
coupling is, and insure that the shorted part of the coil really isn't
coupled too strongly with the unshorted part: if the coil is very
short compared with its diameter, you'd get into trouble shorting
turns.

Good stuff there, Tom. The COEFFICIENT OF COUPLING isn't
considered enough by others (in my opinion) but makes all
the difference as far as losses.

Toroidal power transformers are going to be in for a hot
time in the old town if they are mounted with a non-
insulating strap. That's the "shorted turn effect" with a
toroidal form having a high coefficient of coupling due to
concentration of flux lines. The conductive mounting strap
will suddenly become very hot being a one-turn winding that
has very low resistance (always insulate such mountings).

Air-core coils having a high length to diamerter ratio will
have a lower possible coefficient of coupling...which
actually varies somewhat depending on the position of the
coupling location along the length...lowest at the ends.
The flux density is very low, spread all over as compared to
a toroid where the flux density is concentrated IN the torus.
"Pie-wound" multi-layer coils have an intermediate coefficient
of coupling, less than toroids but greater than high L
solenoidal windings.

There will be some losses due to that "shorted turn effect"
but that hasn't unduly bothered high-power transmitter
designers and users for over six decades. I got started
in HF communications in the Army in the fifties and one of
my tasks was changing the shorting link assemblies on a 15
KW Press Wireless transmitter running FSK RTTY. Two tank
coils about four feet long and about a foot in diameter using
tubing about 3/4" in diameter (memory may be short on the
exact dimensions, I didn't take notes due to having to QSY
as quickly as possible when so ordered). Those got thermally
HOT sometimes but then the final amplifier compartment was
generally thermally hot all over despite forced air cooling.
At least 20 KW of the AC electrical feed of about 45 KW
was wasted in various forms of HEAT on those critters.

Shorting bars were ALWAYS placed beginning at the bottom end
of the coil. A similar shorting system was used on the
old BC-340 power amplifiers (10 KW CW, also running FSK).
Those beasties were operating 24/7 and cranked out the RF
continuously until another QSY was required. BC-340 was
water-cooled with the water flow going through the tank
coil tubing from the final amplifier tube jacket. Was
thermally cooler than the PW-15.

For those who are interested in details and comparisons with
actual result numbers can consult with almost any old text
for ways of measuring Q, coefficient of coupling, etc.
Hours of drudgery to derive all that...as lots of EE students
can testify to. :-)