[TowerTalk] Best coaxial to make Chokes

[Roger (K8RI) on TT]

On 11/12/2013 9:14 PM, Jim Brown wrote:
> On 11/12/2013 5:28 PM, Peter Voelpel wrote:
>> Any choke is inductive.
>> If not it is no choke at all.
>
> WRONG.  A proper common mode choke inductively couples loss from the 
> ferrite core. A parallel resonance is formed by the inductance of the 
> winding (the wire passing through the core) and the stray capacitance 
> between the winding. The coupled resistance is what forms the common 
> mode choke, and because the core material is very lossy, the circuit Q 
> is quite low (typically on the order of 0.5), so the resonance is 
> quite broad.
>

I may be wrong, but I've always looked at the solenoid coil as if it 
forms two resonances. The choke itself forms a parallel resonant 
frequency with a rather high Q So it may work on one or two bands and it 
also forms a second resonant frequency with the overall coax length 
which can be worse than no choke at all.  Even when one works and does 
not create a problematic resonance with the overall line length, it is 
no where near as effective as the choke wound on the ferrite core(s). 
Using the small coax rated for high power lets one make a compact choke 
for the tribander while a couple more cores and turn (based on your 
tutorial) will take you down to 160.  You can wind a choke large enough 
to be effective on 160 by running 6 turns of the LMR-400 feedline  
through 5 or 6 2.4" #31 mix cores.  OTOH using the smaller coax you can 
easily add several more turns with out crowding.

These are essential as antennas like a center fed, half wave, sloping 
dipole for 75 can be far from being a balanced load and difficult to 
keep the common mode voltage on the coax down to reasonable values.  It 
may even take two chokes to keep the RF "out there" instead of lighting 
up every thing in the shack.

73

Roger (K8RI)


> A single turn through a ferrite core resonates in the 150 MHz range. 
> Like any coil, winding multiple turns multiplies L by the square of 
> the turns, capacitance increases approximately linearly with the 
> number of turns, the resistance, because it is coupled inductively is 
> multiplied by the square of the turns. This moves the resonance to the 
> part of the HF spectrum where choking action is needed, and it is the 
> RESISTANCE that provides the most reliable choking action.  A proper 
> choke wound on a suitable core material can provide resistive choking 
> Z in the range of 3K - 8K Ohms over a 2 octave frequency range 
> (because the circuit Q is so low).
>
> Again, any choke that is not strongly resistive at frequencies of 
> interest will resonate with the line (which behaves as an antenna) at 
> frequencies which depend on the length of the feedline and the 
> reactance of the choke.
>
> For a thorough development of these concepts, see my RFI tutorial, 
> cited in my prior email.
>
> 73, Jim K9YC
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