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The Marconi Antenna
The Marconi Antenna is one of the more
popular antenna types. It's general form is a ground mounted
quarter wave radiator.
It is the antenna used by most broadcast
radio stations. (frequently it is not an exact 1/4 wave length for
other reasons) It can be grounded at the base and shunt fed,
or it can be isolated from ground (the tower sits on an insulator)
This antenna depends on an effective counterpoise system to radiate efficiently.
The counterpoise can be radial wires, a screen (radial wires with connecting
concentric wire rings, or a evenly spaced connected cross hatch), sheet
metal strips or a body of water, highly conductive salt water have been
used. The wires can be buried, lay on the ground or they could be
elevated, even overhead. The base (bottom of the vertical radiator)
needs to be above the counterpoise if it is elevated. A isolated
radiator on a good counterpoise has an impedance just under 40 ohms without
reactance. Poor conductivity in the counterpoise causes increased
"ground losses" which increases the base impedance, making it a good match
for 50 ohm cable. At reduced efficiency.
Ron KA4INM email@example.com