[RFI] where to buy FairRite?

[Jim Brown]

On Mon, 27 Apr 2009 19:56:27 -0400, Roger (K8RI) wrote:

>I don't know about you, but I want a lot of head room when running a KW 
>when I get near the band edges.  I've blown out more than one balun 
>albeit, never a choke balun. <:-))

My apologies if I insulted an honest man. My common mode choke designs are 
pretty conservative, but I have no meaningful way of testing them at 
greater than 1.5kW. The only way that I've been able to get one of my 
chokes warm is to use it as the end insulator of an end-fed vertical 
dipole. See the Power Point on Coaxial Chokes for details, including 
photos. As Dan correctly observes, the failure mode is excessive 
dissipation that relates to voltage, current, and their resistance. 

The key to not overheating them is to make the total resistance in the 
common mode equivalent circuit very large, so that the current is very 
small. In the case of the vertical dipole, which was really a method of 
trying to push the choke hard enough to cause overheating, a single choke 
did not provide enough resistance to achieve that, but two chokes in series 
did. And remember, this test is FAR more severe than anything the choke 
would see on an antenna that wasn't seriously broken!  In my test, it's the 
end insulator of a resonant half wave dipole that's transmitting 1.5kW. 

73,

Jim Brown K9YC