[TowerTalk] Balun for Force 12 6 Element 20 Meter Monobander

[jimlux]

On 7/21/20 4:40 PM, Jim Brown wrote:
> Hi Jim,
> 
> Choking impedance is driven by both noise and power handling. The 
> fundamental principle on which my designs are based is that the choke 
> should have a high value of RESISTIVE impedance at the frequency(ies) of 
> interest. Reactive impedance can be cancelled by the common mode 
> impedance of the feedline.
> 
> Because the choke is resistive, it dissipates power due to the common 
> mode current. And because power is proportional to the resistance but to 
> the square of voltage or current, dissipation falls twice as fast as 
> choking Z. So Z must be high enough to minimize the dissipation. That's 
> why some of my designs for higher power consist of two chokes in series.

Aha.. that's the other design consideration, then - not just "no current 
on outside of coax" but also "don't melt the choke"  And as you say big 
R helps.


> 
> I have learned, through extensive work in the lab, that Fair-Rite #31 is 
> the only ferrite material suitable for use at HF. The reason is, that 
> because it is a very special MnZn mix, the dimensional resonance 
> component of its impedance combines with the circuit resonance to make 
> the resonance much broader than with NiZn materials like #43, #52, and 
> #61. It is the higher Q resonance of the NiZn, combined with the wide 
> tolerance of ferrite parts, that every single choke must be measured to 
> find it's resonance, AND that the resonance is too narrow to cover 
> adjacent bands. In other words, the wide tolerance of the ferrite 
> material moves the resonance too much for a cookbook like mine to be 
> published.

So that allows broadband chokes - as I was winding two at lunch time 
today to cover 10-80, following your cookbook designs.




> 
> 73, Jim K9YC
> 
> On 7/21/2020 3:30 PM, jimlux wrote:
>> And that leads to a "choking impedance should be around 5k" which is 
>> 100x.
>>
>> But if it's 4k, it's still good. If it's 3k, it's still good.
>>
>> I think the value of Jim's cookbook is not so much the choking 
>> impedance, per-se, but that he's
>>
>> a) bounded the core variability (designed for worst observed case 
>> performance of the core)
>> b) also taken into account side effects like resonance and leakage 
>> capacitance
>>
>> The latter is MUCH less affected by core properties (I think, Jim can 
>> correct me if I'm wrong).
> 
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