[Amps] More parasitic choke questions

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Fri, 30 Jul 2010 17:01:48 -0400, Roger <sub1 at rogerhalstead.com>
wrote:

>
>Hmmm...That is not what I get out of the statement.  The job of the 
>coil/inductor is to provide enough reactance at the frequency of the 
>parasitic to "quench" it, yet not provide enough reactance at the 
>fundamental that the resistor has to carry too much current.

REPLY:

Remember, reactance is lossless. Reactance alone can not "quench"
energy, in the sense of suppressing or absorbing it. Only resistance
can do that. 

Think of the coil and resistor as a simple low pass filter. At the VHF
parasitic frequency, the coil has relatively high impedance so the
resistor is the primary current path and thereby de-Qs the parasitic
tank. At the lower HF frequencies, the coil now becomes the lower
impedance path and simply bypasses the HF energy around the resistor. 

Of course, this is NOT a sharp cutoff filter and that's why balancing
the values of L and R can be hard to get just right. If it was a sharp
cutoff type, one could just set the cutoff frequency half way between
ten meters and the parasitic frequency and be done. Too bad it doesn't
work that way. 

73, Bill W6WRT