Re: [IRCA] AC twin lead or speaker wire for lead in.

[Rick Kunath <k9ao@xxxxxxxxxxx>]

Patrick Martin wrote:
> Since I use plastic boxes for my matching transformers, I only fitted
> those with banana jaxs, so I use banana plugs on the end of the coax.
> That would be hard to terminate those.

That's why you have ingress. Using banana jacks on the coax does not 
isolate the inner and outer surfaces of the coaxial cable shield.

Either you need to put the matching transformers inside a metal box, 
with a coax jack mounted to the box, and the antenna input through small 
holes in the box with the transformer inside the box, this will create 
the isolated condition on the shield surfaces, or else add a really high 
impedance choke-type balun sleeve over the antenna end of the coax. The 
balun sleeve won't work as well as a shielded metal box. The sleeve 
choke and metal box would work better together than either one alone.

> I have a regular coax fitting on
> the receiver end screwed into a shilded SO239 to the back of the
> receiver.

This end is good.

> By the way, isn't the matching transformer supposed to isolate the
> antenna from the coax anyway? 

Nope. It will perform impedance and balanced to unbalanced 
transformation, but without the shielded box, it won't isolate the inner 
and outer coax shield surfaces.

A sleeve-type choke balun places a high impedance on the outer surface 
of the shield of the coax, making it hard for the currents on the rest 
of the coax shield from making the transition to the inner surface.

> So does a stray signal on the coax really
> make any difference with a load on it? 

Yes. The outer surface of the shield becomes a part of the antenna.

Rick Kunath
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