Difference between revisions of "Wire Antenna"

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(Added more text. Ultimately I think the content of this section doesn't belong under "Wire", since it applies to all antennas. We should think about moving it.)
(Cleaned up text. Removed references to impedance mismatching, which is too complex for a general article and not limited to wire antennas. We need to move that to a separate detail article.)
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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 half-wave center-fed [[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.  
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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 half-wave center-fed [[dipole]] with [[coax]] 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 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.  
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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.  
 
The feedpoint impedance of a horizontal [[dipole]] various dramatically depending on the electrical height above ground.  
 
For example an [[80 metres|80m]] horizontal [[dipole]] 132 feet long and 66 feet high (1/4 [[wavelength]] above ground) has a feedpoint [[impedance]] of 84 ohms. This would create a mismatch of 1.68:1 [[SWR|VSWR]] if fed with 50 ohm [[coax]].  The same antenna installed only 18 feet above ground (7 percent of a [[wavelength]]) has an [[impedance]] of 45 ohms with a mismatch of only 1.1:1 [[SWR|VSWR]].  (However, the radiation pattern of the lower antenna would put more signal at high elevation angles, suitable for local [[QSO|QSOs]], but it would be inferior for [[DX]].)
 
  
 
Refer to the article on [[impedance matching]] for more details on connecting transceivers to [[feedline]] and [[feedline]] to antennas.
 
Refer to the article on [[impedance matching]] for more details on connecting transceivers to [[feedline]] and [[feedline]] to antennas.

Revision as of 18:15, 20 May 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 half-wave center-fed dipole with coax 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.