Topband: FW: Commond mode choke for Beverage

Tom W8JI w8ji at w8ji.com
Mon Oct 12 19:56:23 EDT 2015


>
> I tried 7 & 8 turns, but it resolved too high in freq.
>
>
>
> Use 9 TURNS on a core-stack of five (5) of  P/N 2631803802 cores. That’s 
> the Mix #31 core.


The attenuation of any choke significantly depends on the common mode 
impedance, and impedances from shield to ground. Many times just a few ohms 
is enough, some cases even 10,000 ohms will not help.

It is a system that varies from place to place. For example, if you have a 
small floating loop the common mode impedance of that loop is already so 
high a choke would have no effect at all. If you have a cable between two 
fairly well grounded points, if it even needs a choke, the choke requirement 
would be low.

It is almost like a pi network filter. There is a shunt impedance at one end 
of the choke, the choke impedance, a series line common mode impedance, and 
another shunt impedance.

A dipole is another example. If the feed cable to a dipole is 1/8th to 3/8th 
wave long with a cable suspended away from things, and it is grounded at the 
earth, it is just as effective as using a balun. If you added a common mode 
choke near ground, it would make common mode significantly worse.

Before going wild with impedance goals, it is a good idea to look at the 
particular system.

My Beverage RX antennas at the antennas are isolated from the cable shield 
because I have just a few short ground rods. I don't want the shield being 
be a ground for those poor ground rods, because the shield would couple in 
unwanted signal. My RX verticals out in a field each have 8 long radials and 
buried feed cables, and the hub is well grounded. They have no common mode 
isolation at all, and do not need any.  Any contribution of signal by the 
coax would be no different than just adding another radial.


73 Tom



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