----- 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
