[TowerTalk] 40M rotary dipole and CM current

[Jim Brown]

On 9/22/2017 5:31 AM, Wes Stewart wrote:
> This thread started with a question about whether to just coil up some 
> coax or to wind a balun at the feed point to "keep RF off the coax."
> This sounded like a valid transmitting concern to me. 
It IS a valid concern. Coiling some coax at the feedpoint IS a common 
mode choke. Winding a "balun" at the feedpoint IS a common mode choke.

> It then morphed into something else completely when the salesmen for 
> CM chokes hijacked it. 

My contribution to the thread was to correct misconceptions about how 
these methods work (or don't work).  hardly  'hijacking, and I don't 
sell anything, I'm simply giving away what I've learned.

The fundamental principle of common mode chokes to "keep RF off the coax 
" is that pure inductance (that is, just winding turns of coax) is a 
lousy solution because L can resonate with the rest of the feedline in 
the common mode circuit at frequencies where that feedline is 
capacitive. This makes its effectiveness strongly dependent on feedline 
length and geometry.

Ferrite chokes work NOT by virtue of their inductive reactance, but by 
virtue of that inductive reactance forming a low-Q resonance at 
frequencies of interest with the parasitic capacitance of the choke, 
resulting in a high resistive Z in the common mode circuit. The 
resistance in the circuit is coupled from the ferrite core, and appears 
in series with the inductance, and that series combination is in 
parallel with the capacitance. In the octave or so around resonance, the 
circuit devolves to parallel RLC. That "coil of coax" (with no ferrite 
core) is a high Q inductor, with self-resonance far above the frequency 
where it is used.

All of this is discussed in the two links I posted much earlier in this 
thread. I read your stuff, find it useful, and have said so on many 
occasions. Maybe you can take the time to read mine!

73, Jim K9YC