Topband: CAT5 for Flag Antenna Feedline ?

[Bob K6UJ]

Jim,

Your hams guide to RFI is my bible for mitigating RFI issues.  My feedline chokes are all designed from 
this great article.  I probably have optimized my feedline to the flag in regards to common mode noise
but after reading the post I was curious about CAT5.  Before possibly wasting my time experimenting 
with CAT5 for feeding my flag I wanted to learn more about it.  Below is a paragraph from the Topband 
reflector  thread titled "high performance receive antenna at T6LG" that sparked my interest.  They were
discussing the delta shaped flag they used which was used  before on the FO0AAA dxpedition.  

Bob
K6UJ

The parts for the antenna was very simple, a 9"1 balun and a 910 ohms
resistor, and a 100 to 75 ohms BALUN to feed the preamp. The key component
here was the CAT 5 single twisted pair to feed the flag antenna without any
common mode noise pickup (it is necessary to strip the CAT5 and separate
each of the 4 pairs), a coax cable won't work in high noise environment,
even with a killer choke the ground does not help to stop the common node
noise. That was not the first time a twisted pair saved the day, two years
ago  I suggested Rolf PY1RO a similar antenna fed with twisted pair that
worked very well, bringing the noise to zero in a s9+20 noise environment.

On Sep 8, 2013, at 11:12 PM, Jim Brown wrote:

> On 9/8/2013 10:30 PM, Bob K6UJ wrote:
>> I got the impression from the post
>> that CAT5 is immune to common mode noise.
> 
> I've not heard that, nor can I think of a theoretical basis for saying so. CAT5 is very good twisted pair, with a high twist ratio, Zo = 100 ohms, and its loss is low enough to make it work fine on 160M.  The primary advantage of twisted pair is that it rejects DIFFERENTIAL mode noise. Rejection is greatest when both ends of the line are balanced.
> 
> One possible source of your confusion may be reading, but not fully understanding, my work, and Neil Muncy's work, on SCIN, whereby certain deficiencies in the construction of a cable shield convert common mode current to a differential voltage on the signal pair. I have on several occasions observed that high quality unshielded twisted pair, like CAT5/6/7, would have much better noise rejection than that sort of cable.  Read about it in several tech papers and tutorials on my website. No need to burden the list with it.
> 
> 73, Jim K9YC
> 
> _________________
> Topband Reflector