Topband: Balun Discussions

[i4jmy at iol.it]

Hi Tom,

the procedure of parking choke resonances out of bands can't be used in a 2-30 MHz continuous coverage unit, and I don't like to have chokes like that in my own amplifiers. When plate choke wire size doesn't allow to have the lowest resonance above 30MHz, wahtsoever shape You chose for it, I (and many others) switch two or more different plate chokes. 

Using the method I described for bench testing an amplifier PI, analyzer and traking generator, reflectometric bridge, etc etc, you can see also the plate choke resonances displayed as little notches. There's no need to go exactly tuning the amplifier on the choke resonance to have troubles, it's enough the two notches, one the PI, and the other the choke, appear side by side "touching" slightly, may be one MHz apart. In such a case, the insurance for getting problems (from a low efficiency to failures) if operating in that around is maximal. 
The plate choke self resonance is a very critical equillibrium, very much connected with external environment. It's enough approaching with one hand or changing its location in the cabinet, or symply if the amplifier has the upper cover removed for some reason. In those cases the choke resonance might shift away, may be a little, may be a lot, from the wanted "parking spot".

The problem of multiple resonances appears when an inductor becomes very long compared with its diameter and can be thought more as a combination of different inductors in series, rather than a single one. The parasitic capacitances (mostly external) among different portion of such an inductor, and the series inductance of the next section, determine different resonances combinations.
Seeing the choke as a transmission line because of this effect is stretching things far away, beyond reality.

Coming back to the original topic, as per object:  
Under the resonance, inductive reactance rises continuously and the choke fully behaves as a parallel LC.

73 & HNY
Mauri I4JMY


> 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.
> 
> HNY and 73,
> Tom