[TowerTalk] homebrew 1:1 current balun

[Jim Brown]

On Mon,11/30/2015 3:11 PM, Jim Lux wrote:
> On 11/30/15 9:17 AM, Jamie Tolbert wrote:
>> What tpe beads do I need, 43 or 73? length? 4 ft, 100 beads seems 
>> awfully long.....
>>
>> ____________
> I use 31 mix for this kind of choke application.

#73 is the ONLY material suitable for use in a "string of beads" choke 
because it is the only material with a single turn impedance that is 
predominantly resistive at HF. Unfortunately, the largest #73 beads just 
barely fit RG58 and other specialty miniature coaxes.

>
> what is your operating frequency?
>
> http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf
> see pages 22-25
> 73 is better in general, 43 is ok for low frequencies
> 31 isn't great for bead baluns..

#43 and #31 are NOT acceptable for "string of beads" HF chokes (that is, 
a lot of cores on the coax line), because they are inductive at HF.  To 
be effective, the choke must be strongly resistive at the operating 
frequency. A single turn through these core materials resonates around 
150 MHz, far too high to be useful at HF. We can use  #31 and #43 
material by winding multiple turns through the core(s) to lower the 
resonance to the part of the HF spectrum where we want to use the choke. 
Luckily, it's a very low Q resonance (typically around 0.5), so the 
resonance is quite broad. This allows a choke to cover 3-4 adjacent bands.

My best tutorial on this is k9yc.com/RFI-Ham.pdf   It's worth studying. 
I did the work to learn (and support it) beginning around 2003 and 
continuing to around 2010. It is, I believe, the basis of all modern 
thinking about how common mode ferrite chokes work.

73, Jim K9YC