I've noticed that the various tunable inductors (and transformers) in a metal
can are much better shielded than I might have initially guessed, based upon
the premise that most all of the flux from the coil in concentrated inside the
coil which is 'visible' through the hole in the can (so that the thing can be
tuned!). In thinking about this, I've pretty much convinced myself that the
shielding works as well as it does because electrically it still "looks"
pretty much contiguous at the frequencies you're typically operating the coil
at (e.g., tens of MHz for a 1/4" hole), in a similar manner to how perforated
enclosures make an effective shield so long as the holes are small enough
(...and how you can even calculate the attenuation based on "waveguide below
cutoff" formulas).

Is that correct?

If so, it seems that if you tune the slug so far "out" that it protrudes from
the top of the can, you're probably starting to seriously degrade the
shielding... right?

Thanks,
---Joel Kolstad

In a solenoidal, helically wound coil, most of the magnetic flux is aligned
with the center of the coil and flows along the axis of the coil.
Relatively little flux is outside the coil because of cancellation effects.
For a single turn, current in different parts of that turn tend to partially
cancel flux in regions that are outside that turn.

To see this, draw a circle of wire and show a current at one point flowing
in one direction (out of the page). At the corresponding point
diametrically opposite, that same current flows in the opposite direction
(into the page). Inside the turn the two magnetic fluxes that encircle the
wire add in-phase, but outside the turn these same fluxes cancel in opposite
phase. This effect is more pronounced in a coil of small diameter.

Coils of that kind are pretty well self-shielding for regions beyond the
*side* of the coil.

But if a copper disc is placed near the *end* of the coil a large
circulating current is induced around the edge of the disk and a flux is
created that opposes the flux inside the coil (Lenz's law) and a significant
reduction of the inductance of the coil can occur. This effect has been used
to "tune" coils.

The completely enclosed metal shield helps to create a closed path for
leakage current that makes the shield much more effective.

The shield around the coil can also reduce capacitive coupling to adjacent
circuitry. This is often important.

Bill W0IYH

"Joel Kolstad" wrote in message
...
I've noticed that the various tunable inductors (and transformers) in a
metal
can are much better shielded than I might have initially guessed, based
upon
the premise that most all of the flux from the coil in concentrated inside
the
coil which is 'visible' through the hole in the can (so that the thing can
be
tuned!). In thinking about this, I've pretty much convinced myself that
the
shielding works as well as it does because electrically it still "looks"
pretty much contiguous at the frequencies you're typically operating the
coil
at (e.g., tens of MHz for a 1/4" hole), in a similar manner to how
perforated
enclosures make an effective shield so long as the holes are small enough
(...and how you can even calculate the attenuation based on "waveguide
below
cutoff" formulas).

Is that correct?

If so, it seems that if you tune the slug so far "out" that it protrudes
from
the top of the can, you're probably starting to seriously degrade the
shielding... right?

Thanks,
---Joel Kolstad

"William E. Sabin" wrote in message
news:Roybe.17552$WI3.12208@attbi_s71...
In a solenoidal, helically wound coil, most of the magnetic flux is
aligned with the center of the coil and flows along the axis of the coil.
Relatively little flux is outside the coil because of cancellation
effects. For a single turn, current in different parts of that turn tend
to partially cancel flux in regions that are outside that turn.

To see this, draw a circle of wire and show a current at one point flowing
in one direction (out of the page). At the corresponding point
diametrically opposite, that same current flows in the opposite direction
(into the page). Inside the turn the two magnetic fluxes that encircle
the wire add in-phase, but outside the turn these same fluxes cancel in
opposite phase. This effect is more pronounced in a coil of small
diameter.

Coils of that kind are pretty well self-shielding for regions beyond the
*side* of the coil.

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".

This is in addition to the things mentioned before.

Bill W0IYH

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

"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