> Have a look at http://www.vcnet.com/measures
>
> This will tell you all you need to know about plate chokes and more. I'm
> currently re-building my 4-250 amp and have hit the very same problem, so
> at this moment I've redesigned the plate choke and added a secondary choke
> in series at right angles.
That moves the series resonance, but doesn't eliminate it at all. It
moves it because you change the capacitance from the choke
"center" to the ends of the coke and surrounding objects.
The choke problem is cause by the choke acting like two back-to-
back L networks. At the inductive middle of the choke or chokes,
the stray C forms with the inductance either direction forms an L
section.
No matter what the magnetic coupling between those sections, the
series resonance will occur. The "thing" you think accomplish by
placing two chokes at an angle really isn't even as effective as
removing turns at the area of a voltage peak in the choke.
In either case, you are reducing inductance by reducing mutual
coupling...but if you remove the TURNS at the voltage peak of the
choke (at the problem frequency) you also reduce capacitance!
That moves the series resonance higher at a much faster rate than
simply breaking the choke in half and placing two chokes at right
angles.
For a given inductance at the lowest frequency required, you
ALWAYS want the most compact in-line distance possible with
minimum distributed capacitance.
If you simply install the choke and look at the impedance at the top
as you sweep frequency upwards from the lowest expected
operating frequency, you will find a dip in impedance. Watching
that point, you can move a screwdriver blade near the choke at
right angles to the choke down the choke side. When you get to
the "sweet spot" where the resonance changes the most, that is
where you need to remove turns.
Peel turns back there and the series resonance, for a minimum
amount of change in overall inductance, will move upwards the
most rapid amount. You simply park that resonance, with the
choke in its final location and all sheetmetal installed, out of the
way of any operation.
When you find the next higher resonance, you do the same.
When you use this approach, you will have absolutely the
maximum possible inductance possible.
Using two choke, or two chokes at any angle, does the same
thing....but straight in line is the most efficient in terms of
minimizing stray capacitance and retaining maximum inductance.
I've tried the multiple choke thing many times, and never found it
better than the approach above. It all makes sense when you
understand the problem is one of back-to-back L networks, and not
voodoo.
73, Tom W8JI
w8ji@contesting.com
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