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
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