[TenTec] OCF antennas evolution

[Jim Brown]

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.