Hi Mauri,
<A plate choke must "resonate" well above the operating frequency when
installed and connected to the amplifier circuits. Choke <anomalies of such
kind are easily visibile tracking for PI return loss, looking from the PA
output and loading the tube with a suitable <resistor. Often, rewinding a
critical plate choke with a smaller diameter wire the problem fixes, (who
knows why ?!), although the coil
Respectfully I must disagree. My normal design proceedure in a multiband
amplifier that must cover 160 - 10 meters is to intentionally place the
series resonances outside the bands used. In the Ameritron amplifiers the
chokes have series resonances at 11.8MHz. The second order mode falls at
26.5MHz. You can see there is no harmonic relationship. The "series"
resonance at 12 and 26 does not hurt in-band performance. If you operate an
Ameritron amp on 27 MHz or on 12Mhz, they will either melt a few turns of
the choke or arc a few inches from the center of the choke to the chassis
(many many kilovolts). Without doing that, the choke would have to be
switched.
(Some designs eliminate the plate choke, but that is not related to this
discussion.)
The series resonances are formed where the choke behaves like two or another
even number of back-to-back L networks with the inductance at outer ends
working against stray capacitance someplace in the winding center. The high
series inductance and low shunt capacitance steps the impedance up to an
extreme value, and then back down. The result is very high voltages in the
center and high currents at the ends of the choke, and a low impedance from
end-to-end.
Removing turns in the area(s) of peak voltage shifts the series resonance up
in frequency while having minimum effect on other parameters, so by picking
diameter of the form and spacing of gap areas you can set these resonances
where you want.
The second series-resonance has a LOW impedance at choke center, and a peak
about 1/2 distance to either outer end. Because the stray C affecting the
second resoance is at a different location, it does not fall on or track
changes in the first resonance. This effect causes the higher order
resoances to move independently of low order.
Every large inductor has this problem. Even if you short turns at one end
(like in a roller) the lowest order series resonance moves at a different
rate and direction as the second point of series resonance. The high order
can be moving down as the low is moving up!
Physically large air-core baluns are no different at all. It would be
possible to park low impedances on unused areas but the balun has to be used
in an environment where stray capacitance is not added to the center of the
coil. If you do it will shift the first series resonance.
Sinisa VA3TTN detailed descriptions reflect **exactly** what I measure here.
Designers fight this problem constantly in equipment designs that cover wide
frequency ranges and that have somewhat large components, so we must measure
such things if we want to do a good design.
HNY and 73,
Tom
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