Technical Blog

Wire Antennas

The following Dipole description was taken from the General Class license manual, pg 7-2 and 7-3 with permission from the ARRL.

 

“Dipoles are the most fundamental antenna.  It is a straight conductor that is ½ wavelength long with its feed point in the middle.

 

A dipole radiates strongest broadside to its axis and weakest off the ends as shown in the diagram below.  The current in a half-wave dipole is highest in the middle and zero at the ends.  Voltage along the dipole is highest at the ends and lowest in the middle.

 

The feed point impedance (the ratio of RF voltage to current) of a center-fed dipole in free-space is approximately 72 ohms, but it varies widely depending on its height above the ground.

 

In free space, ½ wavelength in feet equals 492 divided by frequency in MHz.  At resonance, a ½ wave dipole made of ordinary wire will be shorter than the free-space ½ wavelength for several reasons.  First, the physical thickness of the wire makes it look a bit longer electrically than it is physically.  The lower the length-to-diameter (l/d) ratio of the wire, the shorter it will be when it is resonant.  Second, the dipole’s height above ground also affects its resonant frequency.  In addition, nearby conductors, insulation on the wire, the means by which the wire is secured to the insulators and to the feed line also affect the resonant length.  For these reasons, a single universal formula for a dipole length, such as the common 468/f, is not very useful.  You should start with a length near the free-space length and be prepared to trim the dipole to resonance using an SWR meter or antenna analyzer.”

 

Learn more at the February 17, 2017 Tech Demo from 7-9 pm at the Olathe City Hall, or check out these “how to” information resources.  Either way, enjoy learning about simple wire antennas.

 

The ARRL web site has a nice “For the Beginner” article on how to build a dipole.  You can find it at http://www.arrl.org/files/file/Technology/tis/info/pdf/0683033.pdf.  And, of course, you can find most anything on the Internet.  Here’s one http://www.wikihow.com/Build-Several-Easy-Antennas-for-Amateur-Radio.