[AMPS] Choke series resonance clarification

[Hans Goldschmidt]

0-12-09 19.35, skrev Tom Rauch på w8ji@contesting.com följande:

> 
> 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.
> 
> 
> 
> 
> 
> 
> that are not effectively coupled because of flux leakage in the c
> has a step up impedance ratio
> 
> 
> 73, Tom W8JI
> w8ji@contesting.com
> 
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> 
Interesting Tom. Why not discuss  how  the coil form (bobin?) affects the
capacitance turn to turn? I have a ceramic bobin where the wire only touches
the ceramic only at a few points each turn. There is a lot of air beetween
the turns. Low self capacitance then?

??? de Hans SM5KI


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