[TowerTalk] Common-mode choke

[Jim Thomson]

Date: Mon, 19 May 2014 14:08:17 -0700
From: Jim Lux <jimlux at earthlink.net>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Common-mode choke

On 5/19/14, 9:07 AM, Steve Hunt wrote:
> That's right - to get meaningful results you need to understand the
> stray capacitance added by the test structure.
>
> Steve G3TXQ
>
>
> On 19/05/2014 15:08, Jim Brown wrote:
>> I don't trust any impedance measurements of chokes wound on #61,
>> including my own, simply because the Q is so high. Even the slightest
>> stray capacitance moves the resonance, and it's the resonance that we
>> need.
>
> ___


Beyond that, if the thing is so sensitive to surrounding structures, how 
close is your measurement fixture to where it will actually be used? 
You might optimize performance for your fixture, and it's worse in the 
real installation.

I think the take home thing here is that when it's a "choke" you're 
looking for, then you want fairly broadband and lossy.  If it's a "notch 
filter" you want, then resonance is fine.  I would guess that most 
antenna applications are more the "broadband choke" than the "narrowband 
notch".

##  use 2 foot of the large diam type 43 beads  slid over  213u  or  393 coax..and be done with it. 
1 inch OD  x 1.2 inch long x aprx .515 inch ID.      I also found some  2 inch long beads, also type 43
that will just slide over the  393 coax  nicely.... which has a .390 inch OD  on the coax.   213 U  has a .405
inch od..and these 2 inch long beads wont slide over. 

##  The advantage of the large diam beads is they are simple..and no worry about detuning  to nearby metal. 
type 43 works better at freqs greater than 5 mhz...so I use it on 40-6m.   You wont blow these things up anytime soon either.

Jim  VE7RF