[TowerTalk] Fwd: current balance in ladder line?

[Jim Brown]

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 
Chokes.  http://audiosystemsgroup.com/RFI-Ham.pdf

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