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

[Richard (Rick) Karlquist]

A long time ago, Heathkit made a balun like this.  I have
one I bought on ebay, more as a curiosity than anything
I would ever actually use.

Rick N6RK

hanslg at aol.com wrote:
>  Hi,
> 
> I used a rather large, 4" diameter, 12" long solenoid with two parallell windings as a common mode choke for the ladder line to my windom antenna. Worked OK for my purpose. Kept the RF out of the shack. Used #14 wire with a little space between the wires. Have no idea how it affected the "SWR" in/on the line, but who cares. It is a ladder line after all.
> 
> I don't see a practical solution to put a ladder line through a toroid core unless it is incredible large. It should be possible though even if it will be heavy. :-)
> 
> 73 de N2JFS Hans
> 
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> 
> -----Original Message-----
> From: jimlux <jimlux at earthlink.net>
> To: Jim Brown <jim at audiosystemsgroup.com>
> Cc: chas <chasm at texas.net>; towertalk <towertalk at contesting.com>
> Sent: Tue, 5 May 2009 10:27 pm
> Subject: Re: [TowerTalk] Ladderline  - what are the facts??/
> 
> 
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> 
> 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.)
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