[TowerTalk] Fwd: Fwd: OT: Exterior Ethernet Cable

[Jim Brown]

On Tue, 12 Aug 2008 11:59:22 EDT, HansLG at aol.com wrote:

>You may utilize the coax as radiator if you add a trap at the "right 
point"  
>on the coax to stop the current from reaching you shack.

There are other good reasons for blocking feedline current. First, the 
feedline radiates if it is carrying common mode current -- into your 
neighbor's stereo rig, for example. Second, antennas are reciprocal, so if 
the feedline radiates, it will also RECEIVE noise from your neighbor's 
computer rig, the power supplies that charge his drill, and the DC power 
supplies for his low voltage lighting. Both of these are VERY good reasons 
for killing feedline current. 

The most effective way to kill common mode current on coax is with a multi-
turn ferrite choke. 

See http://audiosystemsgroup.com/RFI-Ham.pdf  

Hans used the word "trap," implying a parallel resonant circuit. As I have 
shown in published research that led up to the tutorial, the equivalent 
circuit of a ferrite choke is a parallel resonant circuit. The most 
effective ferrite chokes are resonant in the center of the range where they 
need to suppress current, have very high impedances and very low Q. 

The tutorial shows you you can measure these chokes far more accurately 
than with most commonly used methods, AND come up with actual values of R, 
L, and C that make up the parallel resonant circuit. You can tnen plug 
those values into NEC to model the effect. 

On Tue, 12 Aug 2008 16:49:05 +0100, Ian White GM3SEK wrote:

>When there is current on the outside of the coax, the radiating system 
>consists of both the antenna and feedline. If you suppress the feedline 
>surface current by inserting a choke, or if you change the length of the 
>coax as Hans suggests, then you have changed the entire radiating 
>system. The new system will almost certainly have a different feedpoint 
>impedance - so yes, you *can* expect the VSWR to change.

YES!  

BTW -- the reason that I'm pounding on these concepts is that you can't 
solve antenna problems, RFI and noise problems, until you understand what's 
REALLY going on.  

Jim Brown K9YC