As usual, an attempt is under way to reduce the solution to a complex problem
to a one-liner. The closest I can get to that is : Stainless, or any material
with a conductivity significantly less than Copper, should not be used in any
current-carrying paths within the amplifier. Knowing how to apply this rule
may require some serious judgement. So, as usual, the one-liner is probably
not going to help the average guy.
But I came bearing gifts. One comment passed by here which I would like to
offer some response to, in the form of hard data. It relates to the effect of
nearby conductive surfaces (i.e. metal sheets or structures) on the Q of tank
circuits. Like the writers of those comments, I have always tacitly accepted
this advice about keeping the coil away from the cabinet. Now the issue seems
to be further fragmenting into a consideration of whether that wall is highly
conductive, poorly conductive, ferromagagnetic, etc. I believe that some
tabled data on this exists somewhere, but I can't put my finger on it right
now.
Again, I re-iterate the concept that ALL engineering decisions are based on
'the numbers'. In theory, everything affects everything and the only way to
sort things out is to rule out as many effects as possible using engineering
calculations - this is just Occam's razor as practiced on the street.
Enough preaching. Here is what I did to help, not further obfuscate, this
issue. I made a simple setup, in my lab, to measure the Q of a typical tank
coil in open-air. The coil I selected was typical for 20 or 40 Meters in a
typical amplifier. The Q-setup utilizes a low-loss variable C, and the
parallel impedance is measured with a Vector Impedance meter. The coil is
kind of suspended by a plastic shelf which allows access all around the coil.
Here is what I found.
The measured Q of the coil is around 125 at ca. 20MHZ. I then watched the Q
change as I brought various grounded sheets of metal closer to the coil. I
oriented the perturbing surface both parallel to the coil, and perpendicular
to it endwise.
The detuning effect of the sheet was very small, almost unnoticeable. But
just to be certain, I constantly readjusted the frequency to maintain the
resonant state.
Generally speaking, the degradation in Q was not even noticeable until the
sheet got a distance from the axis of the coil about equal to the coil
diameter (actually, slightly less, but let's not split hairs.) The Q then
started changing, more rapidly as we got closer. In order to drop the Q by
50%, the sheet had to get quite close, like under 1/8 inch. This pattern was
the same for ALL metals which I had available for the test: Copper, Brass,
Steel, Manganin, Phosphor Bronze, Stainless, Aluminum, and Kryptonite. Only
with the sheet laying right up against the coil could I detect the effect of
the material conductivity. The greatest Q-reduction was, as expected, due to
the highest resistance material (Stainless). But in order to see it produce a
serious perturbation, I had to create a pathological situation.
Conclusion: For walls separated by at least one coil diameter, no Q
degradation.
I believe that the issue basically ends here.
I would speculate that for spacings of 1/2-1 coil diameters, the Q
degradation is low enough to be ignored. I would be glad to measure, table
and post a specific set of data points for close-spacings if requested, but
I don't think it is necessary. I have never seen anyone with any sense build
an amplifier with the coil so close to the walls that arc-over is actually a
concern.
But if you insist on putting a wall(s) within a quarter of an inch of your
tank coil, you are going to have a problem, whether you use high-rho material
(Copper or Al) or a low-rho material
(Stainless,Nichrome,Nickel,Solder,Galvanized Steel,etc.)
73
Eric von Valtier K8LV
P.S. I was just kidding about the Kryptonite. I used it all up months ago in
a secret new parasitic suppressor that I am working on.
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