Topband: 160m noise

[Jim Brown]

On Sun, 14 Nov 2004 13:45:12 -0500, Sinisa Hristov wrote:

>It's great if one can afford to trade suppression for bandwidth.
>
>For those who cannot: excellent chokes can be built
>with resonance at 3.5 MHz, which maintain very high
>suppression from 1.8 to 7 MHz. And that's not a
>"single frequency" academic exercise.

The point is that it is almost never necessary to do so if you can 
maximize the suppression that can be achieved without resonance, 
as well as the trouble you can get into when you DO achieve series 
resonance at the frequency of an interfering signal.  

Remember this VERY important fact -- virtually all detection is by 
means that follow square law, and occur above some threshold. So 
a 10 dB reduction in RF level at the point of detection results in a 
reduction of AT LEAST 20 dB in the amplitude of the detected RF. 

I am firm subscriber to the KISS theory (Keep It Simple, Stupid).  A 
simple choke of the right material and optimally wound is KISS. 

Note also that a wound ferrite choke, by virtue of its stray 
capacitance, already has a broad, low Q resonance. Good 
engineering practice generally dictates that we USE a ferrite choke 
at frequencies above that natural resonance, and in the range 
where it has a relatively high value of resistance. I assume that you 
mean to move this resonance down in frequency by adding a fixed 
capactor across the choke. I fail to see the benefit of this approach, 
because, in effect, the capacitor shorts out the resistive component 
of the choke on the high side of resonance. A far better approach 
is to match the number of turns required to the operating frequency 
of the choke.  


Jim Brown  K9YC