>
>
>> 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.
>
Better check it out for yourself with a dipmeter.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|