On Tue, 12 Aug 2008 11:59:22 EDT, HansLG@aol.com wrote:
>You may utilize the coax as radiator if you add a trap at the "right
>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.
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.
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
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