[TowerTalk] Balanced wire feed line

[Jim Brown]

On Sun,6/19/2016 9:36 AM, Hans Hammarquist via TowerTalk wrote:
> The problem is that the feed line works as part of the antenna and there is CM current along the feed line starting at the antenna.
Right.
>   That current dies out once it reaches the shack and will cause no problem inside my shack.

Wrong.  It only becomes small when it reaches the shack if it encounters 
a high common mode impedance at the shack between it and other 
conductors (like a ground, or connections to other equipment). That high 
common mode impedance could take the form of an effective common mode 
choke or an ordinary 2-winding transformer with low capacitance between 
the windings. And it can still be large at the top and along the line.

> It's just like an extra wire from the antenna down towards my shack and it is radiating all the way down with an influence to the radiation pattern.
Right.
> I don't know how big of a problem it is in my case as I have nothing more than my tower next to it.

Your tower is another conductor, and is probably grounded. The common 
mode current on the line will couple to it, and BOTH can radiate. 
Remember, that tower acts like an antenna too -- it's a conductor with 
some electrical length.

> I can, lively, imaging if it passes a TV, telephone or a HIFI setup on its way down.

This is radio -- the feedline doesn't have to "pass" these devices to 
radiate energy that is received when wires connected to those devices 
act as receiving antennas, because most of those devices have Pin One 
Problems.

73, Jim K9YC