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
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