Re: [IRCA] Antenna question

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

> The question: When I attach the feedline at the end of the long wire, I
will
> solder the coax center wire to the antenna wire. But what does one do with
> the braid or shield at the same location adjacent to the antenna wire?
> Should it be attached to anything, sealed over with black electrician's
> tape, or what? If I was putting up a dipole the answer would be easy, but
> I've never fed a long wire with coax before.

One thing to consider is the impedance match.  A wire usually has a pretty
high resistive value, so a transformer is a good way to handle it.  That
also breaks up the ground currents and can reduce noise.

In the wire side, one side of the transformer primary goes to the long wire,
the other side to a ground rod or small radial setup.  The secondary of the
transformer goes to center and shield of the coax.

The transformer itself can be bought at a number of places or home-made.  I
usually use a ferrite toroid of #77 material such as an Amidon FT240-77.  I
like larger cores, as they are simpler to work with and resistant to core
saturation.  The primary might be 35-45 turns of #22 wire.  The secondary to
the coax would be maybe 5 turns of #16 or #14 wire.  I use much bigger wire
so it's rugged and easy to build.  And, I usually put the core in a sealed
box with two screw terminals for the primary and a UHF coax socket to the
receiver cable.  There are commercial transformer setups available, such as
the RF Systems MLB balun sold by Universal Radio.  I have never used that
item, but it seems close to what I build.

Craig Healy
Providence, RI

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