[RFI] Ladder Line and RFI/Noise

[W2RU - Bud Hippisley]

How do you know your wide-spaced OWL isn't picking up noise that is then radiated by your 135-ft doublet and re-picked up by your 40-m inverted V to be sent down your 52-ohm coax to your receiver?  In that case, your choke balun on the 40-m coax cable has nothing to do with anything.

Bud, W2RU

On Oct 19, 2010, at 4:27 PM, dalej wrote:

> 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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