Topband: Common mode filters...help

[Tom Rauch]

> Question: This seems to be simply an RF choke on the shield of the coax =
> (same as winding the coax into a loop).....therefore, would it not =
> accomplish the same thing to just wind a high impedance RF choke on a =
> toroid and insert it at the feedpoint between the coax shield and the =
> antenna?

When the frequency is somewhat low it is almost always more 
cost effective to use a toroid rather than a string of 
beads...assuming the target is a high impedance.

That's because impedance increases by the square of the turns in 
a multiturn choke and only at a direct proportional rate with length 
of the choke for beads. 

As a secondary benefit capacitance across the "choke" increases 
the effective impedance, unless frequency is well above the point of 
parallel resonance.

One thing you have to watch for is making sure the common mode 
impedance at the point where the choke is installed is not highly 
capacitive. If the impedance is capacitive the choke can make 
things worse. The best thing to do with most antennas is use an 
isolated primary and secondary and ground the feedline to a 
separate ground a short distance from the feedpoint. Then, after 
that point (towards the house) you can add a choke and be sure it 
is effective.
 
73, Tom W8JI
W8JI@contesting.com 


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