Monday, February 14, 2011

Center-Fed Half-Wave Dipole


A Center-Fed Half-Wave Dipole is probably the simplest of antennas to construct and use. It is usually suspended between two supports, from it's end insulators, and has the feedline hanging from the center. The drawing below shows the esential parts of a dipole. A good wire to use is a #14 or #16 stranded copper wire, for flexability and to minimize weight. Needless to say, the end-insulators, center-insulator, and the wire need to be fairly strong, especially when you are dealing with low frequency antennas due to their length.

The recommended height for a dipole is 1/2 wavelength above ground. Finding two supports at the recomended height may be fairly easy for higher frequency antennas (15 and 10 Meters), but may present a problem at lower frequencies (80 and 40 Meters). At low frequencies, like 40 Meters, you would need two vertical supports that are 65 Feet (20 Meter) high to meet the recomended height. But, don't let this bother you too much. Lower heights will reduce the feed impedance and change the radiation angle, diminishing the overall effectiveness of the antenna, but the antenna will still be very usable.

The 1:1 Balun, pictured in the drawing, is a device that transforms a balanced transmission line to an un-balanced transmission line. The feed point of the antenna is balanced but the coax is un-balanced. Connecting the coax directly to the antenna feed point will work but you may experience feedline ratiation and have trouble obtaining a good match at your transmitter.

The simplest of Baluns is known as a Choke Balun. It doesn't really do a good job of transforming from a balanced to an un-balanced condition but it does provide a good degree of isolation to keep the feedline from radiating. This type of balun is constructed from 8 to 10 loops of coax with a diameter of about 8 to 10 inches. Bind the turns together with electrical tape or UV stabilized tie wraps.

Choke Balun

Ferrite Balun

There are also commercially available Baluns. These are usually very well made and provide for strain relief of the radiator elements. Some provide an attachment at the top that can be used for hanging. Internally, these are usually made from several turns of wire wrapped around a ferrite core. The usable bandwidth is very large making it useful from 3 to 30 MHz. The picture on the left is only and example of one type of commercial balun. They come in a variety of shapes and sizes.

Depending on your height above ground, you may not be able to obtain a 1:1 SWR, however, properly adjusted it should be some where between 1:1 and 1.5:1. Don't worry about a couple of tenths in your SWR, just get it as low as possible, and use it. The SWR will change anyway as you move across the band. The real point here is to just get the dipole up as high as you can.

The dipole antenna is really only good for use on one frequency band. Sometimes you can use it on it's third harmonic, in a pinch. For example, a 40 Meter Dipole is usable on 15 Meters, but the SWR may not be as good. For multi-band operation see the sections on Trap Dipoles or Fan Dipoles.

The formulas on the right can be used as a starting point for a center-fed half-wave dipole. It's a good idea to cut your wire a few inches longer than the calculated values to allow for securing to the insulators and final adjustments. The length can then be changed incrementally for the best match. Enter your required center frequency in the box on the right.

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