On 7/11/2013 2:56 PM, Robert Mcgraw wrote:
Add a complex Xc or Xl plus R load and they aren't matched.
I prefer a 1:1 current balun rated for at least 10 times the expected
power. They produce less heat and smoke that way.
Let's call a spade a spade. I think we're talking about a common mode
choke, right?
The differential mode dissipation of a ferrite common mode choke is
negligible. Dissipation is due to common mode voltage and current across
the choke, where P = E*I = I sqrd R = Esqrd /R There choking impedance
must be high enough to reduce common mode current to a very small
value. So that's a DESIGN problem, both the design of the choke and the
design of the antenna system (where the feedline and the choke are part
of that system).
Example -- we can use a simple bifilar choke wound on a single core like
those shown in my RFI tutorial for legal limit to a well-balanced dipole
if the feedline is not close to an odd multiple of quarter waves (Vf of
the feedline ~ 0.98 because we're talking common mode). Move the
feedpoint off center by 20% and the choke will fry, because the common
mode voltage is MUCH higher, which increases the common mode current. In
this condition, you could use two of those chokes in series to double
the choking Z, which would cut the current in half and cut the power in
each choke by a factor of 4.
73, Jim K9YC.
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|