I have 135 ft doublet fed with 100 ft of wide spaced open wire line to a
Johnson KW match box. I also have a 40 m. inverted V fed with 70 ft of 52 ohm
coax and choke balun, both are suspended at the same point on my tower and
oriented in the same direction. I hear my neighbors light dimmer equally well
with both antennas tuned to 40 meters. I am not convinced that in my case,
coax would work better than open wire feeders regarding the noise problem.
73
Dale, K9VUJ
On 19, Oct 2010, at 12:31, Jim Brown wrote:
> 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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