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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