>
>It might be a good idea if I explain the choke resonance problem in
>detail, because it is often misunderstood. Looking at the first series
>resonance frequency, although this can be extended to higher
>order resonances, here is what really happens....
>
>Some section of the choke to a central electrical point, forms two
>inductances to either end of the choke.
>
>At the junction of those two inductances, there is stray
>capacitance to the outside world.
>
>Each inductance, along with that capacitance, forms an L network
>with a very very high impedance at the choke electrical center. The
>voltage at that point is very high because the L/C ratio is very high.
>
>Since the step-up network in one direction is connected to an
>"opposite facing" step-down L-network in the other direction, the
>result is from end to end the system has very low impedance.
>
>Now you can take those two inductors and move them anywhere
>you like, even in separate rooms, and you will STILL have the same
>situation. Mutual coupling is NOT a requirement. That's why if you
>stick a "good" ten meter choke in series with a large choke that
>has a "series" resonance on ten meters, the system will still have
>problems. You will still have an abnormal amount of voltage from
>the high impedance point of the system to the chassis, you only
>move the problem to a new lower frequency!
>
>The more compact the system (decreasing capacitance by
>keeping the winding in a straight line), the higher the resonance
>that will ALWAYS occur. They key, no matter what you do, is to
>park the unwanted resonances outside the operating bands. If you
>have enough choke for 160 with a moderately high impedance tube
>impedance, you will always have a resonance in the HF range.
>
>The same is true for roller inductors or tanks that do not short the
>center of the tank to one end, eliminating one of the "inductances".
>
>When you look at what really happens, it makes a good solution
>easier to see. Since the capacitance is low and inductance high,
>you are much better off removing wire in a gap at a point where a
>voltage maximum occurs than anything else. That's because you
>are changing the capacitance a large amount, and the small
>inductance reduction also helps a bit.
>
>On the other hand if you split that choke into two halves, you have
>the same basic two L sections except you'd add extra capacitance
>from the connection between the two sections. The result is you
>lower the problem frequency.
>
>Either solution works, depending if you want to move the
>resonance up or down, but even adding a simple "hanging wire" at
>the choke center will move the resonance down with less work. In
>every case, the result is the same.
>
pontification
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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