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
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|