[RFI] Ladder Line and RFI/Noise

[Jim Brown]

  On 10/19/2010 8:35 AM, Howard Lester wrote:
> Gene right
> off asked me if I was feeding my antenna with ladder line. I replied "Yes,
> and it runs through the wall and connects directly to the transmatch on my
> desk." He said that was the problem.

Yes. Folks have bought into the notion that ladder line is a cure for 
all ills, that you can put up any wire and feed it with no problems. 
Nothing could be further from the truth. The monster hole in this 
approach is NOISE and RFI. The developers of telephone communications 
learned more than a century ago that twisted pair was at rejecting noise 
and crosstalk, and that parallel wires were bad.

Why this is true is pretty simple. Voltage is induced in both wires, and 
if it is equal, the receiver will see zero voltage. But if the voltages 
are slightly different, the receiver will see the difference. If the 
noise source is relatively close, it will be closer to one conductor 
than the other, so the voltage will be slightly different. With twisted 
pair, one conductor will be closer at one point on the line and the 
other will be closer a fractional inch down the line, so the induced 
noise is much closer to zero, and the cancellation is more nearly perfect.

This discussion applies to differential coupling. There's also common 
mode coupling, of course, and antennas fed with ladder line rarely have 
a common mode choke. But they NEED a common mode choke, because nearly 
all ham antennas have at least SOME unbalance by virtue of asymmetry in 
their surroundings, their length, etc.  Yes, coax has more loss and it 
costs more, but we can choke it, and there's no differential coupling at 
proper connections.  And all of this works in reverse, of course, by 
reciprocity, so ladder line puts more RF in your neighbor's A/V system 
and computer speakers too.

100 years later, we use parallel wires for speakers and power wiring. El 
dumbo grande. Replacing them with twisted pair solves a LOT of noise and 
RFI problems. And ladder line is a TERRIBLE idea if you have neighbors. 
Or any noise generators in your home.

73, Jim Brown K9YC