Wire Antenna: Difference between revisions
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Many amateur radio antenna systems use a simple wire to carry the RF current in such a way as to radiate. One of the simplest is the dipole. When a dipole oscillates current, in sync with the radio's RF output during a transmission, the magnetic field generated around the wire expands and contracts very quickly, in most cases millions of times per second. It is the outer most part on the field that is radiated away. | Many amateur radio antenna systems use a simple wire to carry the RF current in such a way as to radiate. One of the simplest is the dipole. When a dipole oscillates current, in sync with the radio's RF output during a transmission, the magnetic field generated around the wire expands and contracts very quickly, in most cases millions of times per second. It is the outer most part on the field that is radiated away. | ||
The shape of a dipole resembles the letter "T". The middle leg connects the radio to the center of the upper, horizontal legs. There are several ways to make the connection. The simplest has the coax shield connected to one side and the center conductor connected to the other side. This works but there are losses at the connection due to impedance mismatch. A better way is to make or purchase a "balun". A balun matches the impedance of the radio to the impedance of the dipole more closely. This increases the power actually transferred to the upper legs of the dipole. (Balun stands for BALanced to UNbalanced.) | The shape of a dipole resembles the letter "T". The middle leg connects the radio to the center of the upper, horizontal legs. There are several ways to make the connection. The simplest has the coax shield connected to one side and the center conductor connected to the other side. This works but there are losses at the connection due to impedance mismatch. | ||
The feedpoint impedance various dramatically depending on the electrical height above ground. | |||
For example an 80m horizontal dipole at 66ft. equal to 1/4 wavelength above ground has a feedpoint impedance of 84 ohms. This would create a mismatch of 1.68:1 VSWR. Now take the same antenna and install it only 18ft above ground, equal to 7 percent of a wavelength, and it now has an impedance of 45 ohms with a mismatch of only 1.1:1 VSWR. | |||
A better way is to make or purchase a "balun". A balun matches the impedance of the radio to the impedance of the dipole more closely. This increases the power actually transferred to the upper legs of the dipole. (Balun stands for BALanced to UNbalanced.) | |||
Revision as of 04:52, 8 February 2008
Many amateur radio antenna systems use a simple wire to carry the RF current in such a way as to radiate. One of the simplest is the dipole. When a dipole oscillates current, in sync with the radio's RF output during a transmission, the magnetic field generated around the wire expands and contracts very quickly, in most cases millions of times per second. It is the outer most part on the field that is radiated away.
The shape of a dipole resembles the letter "T". The middle leg connects the radio to the center of the upper, horizontal legs. There are several ways to make the connection. The simplest has the coax shield connected to one side and the center conductor connected to the other side. This works but there are losses at the connection due to impedance mismatch.
The feedpoint impedance various dramatically depending on the electrical height above ground.
For example an 80m horizontal dipole at 66ft. equal to 1/4 wavelength above ground has a feedpoint impedance of 84 ohms. This would create a mismatch of 1.68:1 VSWR. Now take the same antenna and install it only 18ft above ground, equal to 7 percent of a wavelength, and it now has an impedance of 45 ohms with a mismatch of only 1.1:1 VSWR.
A better way is to make or purchase a "balun". A balun matches the impedance of the radio to the impedance of the dipole more closely. This increases the power actually transferred to the upper legs of the dipole. (Balun stands for BALanced to UNbalanced.)