[TowerTalk] Balun Recommendation

[Jim Brown]

On 4/14/2012 8:49 AM, Hank Garretson wrote:
> I gently suggest that a more rigorous test is in order.

The most effective common mode choke is a low Q resonant circuit 
operated near resonance, so it is predominantly resistive device at 
frequencies where it is used. The most rigourous test of a common mode 
choke is one that applies the greatest common mode voltage across it.  
The common mode voltage will be proportional to the imbalance in the 
system as a whole, and will be strongly dependent on the length of the 
transmission line into which it is inserted.  Off-center-fed antennas 
can place very high common mode voltages across a choke, and are 
notorious for destroying them. :)

The most rigourous test that I've been able to set up was to use a choke 
as the end insulator of an end-fed dipole that used the coax shield as 
one half of the dipole, then transmitting at 1.5kW until I started 
seeing degradation of SWR that indicated overheating.  There's a 
description of that antenna and those tests in the Power Point on Coax 
Chokes on my website.  It also includes an NEC analysis of the 
dissipation that can occur with an off-center-fed antenna.

73, Jim Brown K9YC