On 7/30/2010 10:21 PM, Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Fri, 30 Jul 2010 17:01:48 -0400, Roger<sub1@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"
>
As reactance is lossless that is just how it quenches. Quench does not
mean to absorb, it means to put out, stop, or even prevent. IOW it
prevents the parasitic from getting started. Another way of phrasing the
quenching, or preventing, is to say it "chokes off" the parasitic and
prevents the oscillation. If the parasitic choke is designed and working
properly It doesn't have to absorb anything as it prevents said parasite
from taking life.
Just go back to the one line of quoted text from the "Radio Handbook"
20th ed. "Just enough turns should be used to suppress the parasitic
oscillation,
and no more."
Suppress may be a better term that quench, but it does not absorb, it
prevents.
73
Roger (K8RI)
> 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.
It's more like a broad band stop 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
>
>
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