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Re: [TowerTalk] Fwd: current balance in ladder line?

Subject: Re: [TowerTalk] Fwd: current balance in ladder line?
From: Jim Brown <>
Date: Fri, 21 Jan 2011 09:41:58 -0800
List-post: <">>
On 1/20/2011 12:54 AM, Steve Hunt wrote:
> For me, distortion of the radiation pattern of a simple dipole or loop
> is the very least problem that CM currents can cause.
> Much more significant is the potential for RFI, and increased noise
> levels on Rx, because the feedline has become an active element of the
> antenna system. It's usually easy to spot if you have the RFI problem;
> but the Rx noise problem is more difficult to identify, and many folk
> are unaware that they could potentially lower their noise levels by
> addressing the CM issue.

I completely agree. The first observation about this that I saw (about 
2006) was from  W1HIS, in an app note on using ferrite chokes for this 
purpose.  It's a very good piece, except that he didn't know how to 
measure the impedance or performance of the chokes he was building, so 
his winding instructions were very wrong.

On 1/19/2011 5:00 PM, Steve, W3AHL wrote:
> The current flowing in each element of a dipole is a result of its impedance. 
>  If the impedance is not balanced, neither will the current be.
> To demonstrate this use EZ-NEC to create a simple dipole and make it 
> unbalanced by moving the feed point off center or by tilting one side closer 
> to the ground.  Using an ideal point source to feed it, look at the currents. 
>  They will not be balanced.  It is NOT dependent on the feed line, although 
> that can also cause additional imbalance.

YES!  A year or so ago I did exactly this, and summarized the result in 
a presentation to the Antenna Forum at Pacificon this fall.  What I was 
specifically looking at was the dissipation in the choke as the result 
of imbalance. I also looked at the noise reduction. I used a 40M dipole 
with the feedpoint offset by an amount that varied from 5 ft to about 20 
ft, and with a feedline having a common mode length of a quarter wave or 
a half wave. The Power Point is on my website. Select the one on Coaxial 

That presentation also shows several bifilar chokes for the HF bands 
wound with AWG #14 THHN (house wire) that should perform very well in a 
balanced line. The model (cited above) showed that these chokes will 
safely handle relatively small imbalances (5-10 ft offset) at maximum 
legal power, but that greater imbalances are likely to fry them for 
feedline lengths that place the greatest common mode voltage across the 
choke (near a half wave, or multiples of a half wave).

Note that while these chokes are a short section of parallel wire line 
wound a round a toroid, with a characteristic impedance on the order of 
100 ohms. They're a relatively small fraction of a wavelength at HF, so 
the additional loss they introduce is negligible, and the small mismatch 
is easily compensated by the antenna tuner.

GM3SEK -- Ian, I built and measured these chokes in response to your 
need for low cost chokes!

73, Jim Brown K9YC


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