Topband: Balun Discussions

[i4jmy at iol.it]

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 runs warm (may be hot hot) because of the DC current flow. When a thinner wire solution is not applicable, it’s best to switch a set of different plate chokes, depending on the used frequency. But this is another story, and another revealed "trick".

Again, the mechanism of an antenna choke it's not that of a transmission line. Behaviour of a transmission line (when not resistively terminated into it’s own characteristic impedance) is to cyclically show high and low impedances, and repeatedly change in reactance sign, with an EXACT harmonical relation. 

We can agree that not only capacitance between turns has to be taken in account. Expecially when it’s small, sourrounding objects may be dominating as the parasitic capacitances. Circuit resonance anyway occurs when the reactances are equal and opposite in sign, and you can't find out more than a single resonance in an antenna choke between its terminals, whose thing is finally what we need. If still in doubt, place a capacitor that's larger than parasitic across the choke and you'll see that's just a conventional parallel resonant circuit.

Experience tells that it is perfectly possible to get a reasonable high impedance without dealing to reach a resonance. Self resonance is hardly a stable condition, unless the environment is controlled (closed) and weatherproofed.

73 & HNY
Mauri I4JMY


> I'd take this as said in good faith, but even good faith errs occasionally.
> 
> Everything that happens inside the choke is visible
> in it's outside appearance - impedance.
> 
> Please go to my site to see multiple alternating parallel and
> series resonances of an ordinary HF amplifier choke:
>   http://www.geocities.com/va3ttn
> 
> 
> Thank you for the opportunity to discuss the matter.
> 
> 
> 73,