[AMRadio] antenna tuners


Phil Galasso k2pg at worldnet.att.net
Thu Apr 20 21:13:37 EDT 2006


----- Original Message -----
Subject: [AMRadio] antenna tuners


> I don't believe in them.
>
> If you can't resonate your antenna, then what are you doing?

There is an antenna that will require tuning components at the feedpoint
even at perfect resonance: the folded unipole. This is a vertical antenna
that is commonly used in AM broadcasting. It is physically shorter than a
standard 1/4 wavelength series-fed vertical and consists of a grounded tower
with a skirt of wires around it. The wires are held away from the tower
members with insulating spacers that are about 1 meter long. A ring of wire
connects the three skirt wires together and this is then connected to the
tower as the "commoning" ring. The bottoms of the skirt wires are also
joined by a ring of wire, but this is kept insulated from the tower and
serves as the feedpoint. If the commoning ring is placed at the proper spot
along the skirt, the resistance will be 50 ohms. But a tuning network (or at
least a capacitor) will be needed at the feedpoint, as folded unipoles have
a lot of inductive reactance. This is an efficient antenna that provides
excellent bandwidth.

Come to think of it, even a standard series-fed vertical will need some
impedance matching, as a 1/4 wave vertical antenna working against a good
ground system will have an impedance of 35 ohms. Modern rigs require a load
of 50 j0 ohms.

Electrically short antennas are less efficient than their full-sized
counterparts. But city dwellers and others who, for various reasons, cannot
put up a standard antenna, will get decent results with short antennas and
"antenna tuners".

Phil K2PG




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