[TowerTalk] Ladderline - what are the facts??/

[jimlux]

Jim Brown wrote:
> On Tue, 21 Apr 2009 00:14:50 -0500, chas wrote:
> 
>> WHERE are the facts about ladderline??
> 
> Ladderline does not radiate differential mode current (the power it 
> carries from the transmitter to the antenna). Ladderline DOES 
> radiate COMMON MODE current. So does coax. 
> 
> Common mode current is the result of imperfect balance at either end 
> of the feedline. Nearly all practical ham antennas are unbalanced by 
> their surroundings. Common mode current causes radiation, and 
> antenna imbalance causes common mode receive current.
> 
> An important difference between coax and ladder line is that coax 
> can be CHOKED to kill common mode current, but ladder line cannot. 



Just wondering out loud here..  If I put a twisted pair (unshielded) or 
ladder line through the center of a ferrite toroid, would it not choke 
the common mode currents? I can see this being somewhat impractical for 
ladder line, although with some sort of appropriate spacers to make sure 
it's perfectly centered, and the toroid would have to be big enough that 
it didn't intercept much of the differential mode field.

(that is, with 1" wide ladder line, you'd probably need a ferrite with a 
6-8" hole )

(hmm, one could design a common mode choke for a balanced transmission 
line.  they're pretty common for switch mode power supply inputs, so the 
challenge is designing one for HF.)