"Pipex News Server" wrote in message
...
The flux at the center of the coil is highly concentrated. This flux then
leaves one end of the coil and returns to the other end via an external
path, just as it is supposed to do, but at a much smaller value of flux
*density* in the external path. This external flux can induce "eddy"
currents in a shield that can decrease coil inductance and Q, usually
only a small amount if the shield is not too "close".
=====================
The above sparks-off my following question:

If an antenna matching unit (some prefer calling it a tuner) has air
/ceramic/polystyrene wound inductors ,to which extent will a metal
cabinet affect the Q of the said inductors ?
I have read somewhere that to maintain the best possible Q , the distance
between the inductor(s) and other metal parts should be not less than the
diameter of the inductor. Does this make sense ?

TIA for any response

Frank GM0CSZ / KN6WH

I prefer the term "tuner" because the unit very seldom actually "matches"
impedances. Instead, it "transforms" the impedance at the sending end of
the transmission line, coax or whatever, to the 50 ohms resistance that the
transmitter is usually (these days) designed for. The term "tuner" is common
usage and it's OK.

The coil in a CLC tee-type tuner can have a Q as high as 400, and stray
coupling to the metal cabinet or ground plane can easily cut the Q in half.
What effect that has depends on the load impedance of the antenna
feedpoint. If the load is highly reactive (high X, low R) the coil can get
quite hot.

I believe it is important that the open ends of the coil should be at least
one coil diameter away from any metal surface. The sides of the coil are
less critical, but the mechanical design should do a pretty reasonable job
of reducing that stray coupling to a low value also.

Stray capacitance from coil to ground or elsewhere can be a problem
sometimes.

I have been thinking about my previous inputs to this thread, and I am not
entirely satisfied with them. I will try to improve them sometime today.

Bill W0IYH

William E. Sabin wrote:
....

The coil in a CLC tee-type tuner can have a Q as high as 400, and stray
coupling to the metal cabinet or ground plane can easily cut the Q in half.
What effect that has depends on the load impedance of the antenna
feedpoint. If the load is highly reactive (high X, low R) the coil can get
quite hot.

I believe it is important that the open ends of the coil should be at least
one coil diameter away from any metal surface. The sides of the coil are
less critical, but the mechanical design should do a pretty reasonable job
of reducing that stray coupling to a low value also.

....
Hi Bill,

In practical tuner applications the unused part of the coil is usually
shorted. The reasoning is that it prevents the generation of high RF
voltages. Shorting part of the coil should ruin the Q quite a bit.
Any comment on this practice?

Thanks, Peter

Some years ago I found that leaving the unused turns open-circuit caused
very serious interactions with the remaining part of the coil at certain
frequencies, and actually made the tuner untunable at those frequencies. I
was sure that the open turns and the capacitances involved with those turns
was resonating, causing "suckouts". If the unused turns were shorted, the
spurious resonances in the coil disappeared, a well as I could determine.

Assume the shorting bar is perfect (lossless). Then:

The equivalent circuit just for the coil alone is the used part with its
resistance, magnetically coupled to the shorted part with its resistance.
The coupling from the shorted part to the used part adds an inductive
reactance plus some resistance in series with the used part. The shorted
turns have nearly the same ratio of inductive reactance per inch of coil
length to resistance per inch of coil length as the rest of the coil, so the
Q should not be degraded by this coupling. In other words the net loss is
the same as that of the entire coil operating alone. The perfect short does
not add power loss into the coil.

Assume the shorting bar is not perfect (not lossless). Then:

Some additional power loss is added to the shorting bar and the Q should
decrease.

Bill W0IYH

"Peter Orban" wrote in message
...


William E. Sabin wrote:
...

The coil in a CLC tee-type tuner can have a Q as high as 400, and stray
coupling to the metal cabinet or ground plane can easily cut the Q in
half. What effect that has depends on the load impedance of the antenna
feedpoint. If the load is highly reactive (high X, low R) the coil can
get quite hot.

I believe it is important that the open ends of the coil should be at
least one coil diameter away from any metal surface. The sides of the
coil are less critical, but the mechanical design should do a pretty
reasonable job of reducing that stray coupling to a low value also.

...
Hi Bill,

In practical tuner applications the unused part of the coil is usually
shorted. The reasoning is that it prevents the generation of high RF
voltages. Shorting part of the coil should ruin the Q quite a bit.
Any comment on this practice?

Thanks, Peter

"William E. Sabin" wrote in message
news:9rSbe.21106$WI3.17494@attbi_s71...

Assume the shorting bar is perfect (lossless). Then:

The equivalent circuit just for the coil alone is the used part with its
resistance, magnetically coupled to the shorted part with its resistance.
The coupling from the shorted part to the used part adds an inductive
reactance plus some resistance in series with the used part. The shorted
turns have nearly the same ratio of inductive reactance per inch of coil
length to resistance per inch of coil length as the rest of the coil, so
the Q should not be degraded by this coupling. In other words the net
loss is the same as that of the entire coil operating alone. The perfect
short does not add power loss into the coil.

Assume the shorting bar is not perfect (not lossless). Then:

Some additional power loss is added to the shorting bar and the Q should
decrease.

Bill W0IYH

A followup to my previous input:

I connected an old roller coil with short circuited turns to my ancient
Boonton 260A Q meter.

Over most of the inductor range at 10 MHz the Q remained "fairly" constant.
At the low values of inductance the Q (about 250) dropped about 15%. I
suspect changes in shape factor of the unshorted turns are involved. The
winding pitch of the coil is greatly reduced at low L values to try to
maintain shape factor and improve Q. At low L values, inreasing the
frequency to 15 MHz helped to restore most of the Q.

The detailed analysis of all that is going on in the roller coil is a
time-consuming chore that I don't want to get into. I believe my previous
explanation is OK under ideal textbook conditions (Kraus "Electromagnetics"
4th edition page 235, Eq 2) but of course that is just an approximation to
the actual coil, as could be expected.

On to other things.

Bill W0IYH

You certainly have your mind strolling down paths I have wandered...
Yes, it does seems so--is that found to be true when carried into actual
construction practices--well, yes and no, but not to any extent which poses
a "real" problem...

Or, simply put, I no longer lose sleep over the fact my L-Tuner shorts turns
when tuning the coil...

Regards,
John