[RFI] RF getting into 4-port wirelss router.

[Jim Brown]

On Wed,1/14/2015 12:28 PM, Roger (K8RI) wrote:
> It appears from what you have said that there may be Knob and Tube 
> connected to regular wiring.  The biggest problem with Knob and Tube 
> is one of polarization. 

Probably a much larger issue with RFI is the wide spacing between the 
conductors. This increases the area of the magnetic loop, and 
magnetically induced current is proportional to the loop area. In the 
near field of antenna, the magnetic field is dominant, so magnetic 
coupling can be a very big deal.

At my Chicago QTH, I fed my 80/40 dipole with some old 75 ohm KW 
twinlead, but I also tied the two sides of the feedline together to feed 
it as a top-loaded quasi-vertical on 80 and 160M. This put a current 
peak in the shack, and the magnetic coupling to the parallel wire RS232 
cable for the K2 caused the CW generation from the logging program to 
lock up with as little as 15W. I replaced that parallel wire serial 
cable with unshielded CAT5, using one pair for each signally circuit, 
and with the returns going to the DB9 shell on both ends. This allowed 
me to run my Titan at full power.

As an experiment, I loaded that antenna to full power all the way up to 
10M, with no RFI issues until I got to 15M. At 15M and above, I needed 
shielded twisted pair. (I never USED that long wire on the higher bands 
-- I was simply doing this to learn the conditions under which 
un-shielded cable was good enough. In that shack, the cable from the 
computer, a Thinkpad, ran within several feet of the antenna.

73, Jim K9YC