[TowerTalk] CM Choke Cookbook

[Jim Brown]

On 9/24/2017 1:06 AM, Ian White wrote:
>
> The dielectric of the coax which affects the choke resonance is only
> that of the outer jacket.

Right. Thanks for clarifying what I didn't say well enough. :)
> Along with the o.d. of the coax and the precise layout of the turns, the
> dielectric of the outer jacket will affect the capacitance between
> turns, and three of those factors will affect the choke resonance.  That
> is a very good reason to:
>
> * Use "cookbook" designs from someone like K9YC, G3TXQ or myself, who
> has actually measured those resonant frequencies (which requires some
> special techniques in addition to owning a VNWA)

Actually, my VNA (the DG8SAQ design  built and sold by SDR Kits in the 
UK) is NOT the most accurate system for measuring ferrite chokes, and I 
still don't know why.  My test setup is documented several places in my 
application notes and tutorials.

> .and.
> * Choose a broadband design which will be more tolerant of small
> differences in winding layout and the unavoidable stray reactances when
> you actually install the choke
> .and.
> * Copy that cookbook design EXACTLY - including the coax and the layout
> of the turns.
>
> I agree with K5GO about normally preferring a cable with a stranded
> copper center conductor in preference to silver-plated solid steel, if
> available.

I agree, but I'm also pretty happy with solid copper center.  IMO, not 
enough attention is paid to the shield construction.

> But if this is different from the original cable in the
> cookbook design, it needs to be a very close alternative with the same
> OD and outer jacket, and you should still aim for exactly the same
> layout of turns.

The key point is to apply fundamental principles to these chokes, 
realizing that control of resonance is key to them working the way we 
want them to.

73, Jim K9YC