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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.
Related wiki pages: [[Wire comparison tables]]


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.  
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 half-wave center-fed [[dipole]] with [[Coaxial Cable]] feedline. When a radio-frequency oscillating current is applied to a dipole, 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 feedpoint impedance various dramatically depending on the electrical height above ground.  
The shape of a [[dipole]] resembles the letter "T". The middle leg, which consists of the [[coax]] feedline, 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.  However, this can cause [[feedline radiation]] due to [[common-mode currents]].  It is often recommended that a [[balun]] be attached at the feedpoint, but many operators have good success with dipoles that have no baluns.


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.
The feedpoint impedance of a horizontal [[dipole]] various dramatically depending on the electrical height above ground.  


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.)
Refer to the article on [[impedance matching]] for more details on connecting transceivers to [[feedline]] and [[feedline]] to antennas.
 
 
{{antennas}}

Latest revision as of 17:56, 8 April 2009

Related wiki pages: Wire comparison tables

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 half-wave center-fed dipole with Coaxial Cable feedline. When a radio-frequency oscillating current is applied to a dipole, 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, which consists of the coax feedline, 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. However, this can cause feedline radiation due to common-mode currents. It is often recommended that a balun be attached at the feedpoint, but many operators have good success with dipoles that have no baluns.

The feedpoint impedance of a horizontal dipole various dramatically depending on the electrical height above ground.

Refer to the article on impedance matching for more details on connecting transceivers to feedline and feedline to antennas.


Antennas
Design Beam * Dipole * Dish or Parabola * DDRR * Log Periodic (LPDA) * Loop * Mobile and portable * Omnidirectional * Panel * Quad and Quagi * Screwdriver * Small tuned loop * Vertical * Yagi-Uda * Wire and random wire antennas
Installation Antenna Tuners * Capacity hats and loading coils * Cavity filters * Coaxial Cable * Feedlines * Rotators * Towers and Masts * VK2ACY - G5RV coupler
Theory Front-to-back ratio * Impedance matching * SWR * Tower design * Vertical Antenna efficiency * Wire comparison tables
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