Topband: Un-Un

[Tom Rauch]

> windings don't move.  The first one I tried worked fine at 
> low power but
> when I turned on my amp I think the forces between the 
> wires caused some
> motion of the wires and generated enough reflected power 
> to trip the
> detector in my amplifier. After I put on enough electrical 
> tape to keep
> everything in place it seems to work quite well.

Many cores conduct, and if the wire is right against the 
core the wire can arc over to an adjacent turn through the 
core.

I virtually never use un-uns for matching of antennas. By 
making the antenna slightly off resonance a simple inductor 
or capacitor can be used to transform impedance, and I 
prefer that method (or an L network).

We have to be a little careful with magnetically soft iron 
core selection. Cores can be non-linear...although usually 
the most common problem is they dissipate enough power to 
overheat and reach curie temperature. Just a fraction of a 
dB loss can produce a big heat problem.

73 Tom