BUILDING AND USING DIPOLE ANTENNAS

Perhaps the most common amateur antenna in use today is the dipole. It is inexpensive to construct, and it provides good results for local and DX communications if it is erected high above ground. 

A dipole antenna is a 1/2-wavelength conductor that is fed at the electrical center. Gain types of directional antennas, such as the log periodic dipole array consists of a group of dipoles. In other words we can combine a number of dipoles to form a wideband log-periodic beam antenna.

A single dipole antenna has no gain, but it does exhibit a bidirectional radiation pattern if it is a half wavelength or greater above ground, and if it is erected for horizontal polarization (parallel to the earth). The radiation pattern for this type of dipole resembles a figure below. Maximum radiation is off the broad side of the antenna.

We learned that a dipole antenna loses its direct-ional characteristics when it is placed close to ground (less than approximately 0.5 wavelength). The resultant radiation pattern for a dipole that is, say, 1/20 wavelength above ground (e.g., 53 feet for 3.7 MHz), is pretty much circular. In other words, the signal is radiated almost equally in all directions. Also, the radiation angle will be very high, which is not ideal for long-distance communications.

Dipoles may be erected horizontally or vertically. They may also be configured as inverted Vs. This is a popular format because only one tall support pole or tower is needed. The center of the dipole is held aloft by the tall supporting structure, and the halves of the dipole droop toward ground at approximately 45 degrees. You may also use your dipole as a sloper (sloping dipole). In this example , see figure 1, we tie one end of the dipole to a tall mast or tower and slope the entire antenna toward ground at approximately 45 degrees. The lower end of the antenna (as with an inverted V) is just a few feet above ground-in a typical case where the tower or mast height is 50-60 feet.

Radiation angles and patterns vary in accordance with the manner in which we erect our dipole antennas. A vertical dipole, for example, has an omnidirectional radiation pattern, and it exhibits a relatively low radiation angle. Sloping dipoles and inverted Vs with an enclosed angle of 90 to 100 degrees will also radiate a vertically polarized signal. The sloper will have a unidirectional pattern off the slope of the antenna (directivity rather than gain) if it is supported by a metal Mast or tower: 

the metal support device acts somewhat as a reflector, If the sloper is supported on a wooden pole, the radiation pattern will be omnidirectional. 

Inverted-V antennas also have an omnidirectional pattern if they are erected in a symmetrical manner, respective to the tower and placement of the antenna legs.

Fig 1. Examples of how half-wave dipoles may be erected. A figure-8 radiation pattern results from the arrangement at A, Antenna B has an omni- directional pattern and vertical polarization. Antennas C and D also have vertical polarization and omnidirectional patterns assuming the enclosed angle at C is 90-100 degrees, and when antenna D slopes approximately 45 degrees toward ground from a nonconductive mast

The feed impedance of the antennas in above figure, ranges from approximately 35 to 75 ohms. The actual value is dependent upon the height above ground, and in the case of antennas C and D, the angle of the wires respective to mast. 

For most amateur operation you may use 50-ohm coaxial cable to feed these antennas, provided the feeder length is 100 feet or less. The losses from the moderate SWR (standing-wave ratio) at high frequency will be minimal if quality cable is used, such as RG-8 line. 

The antennas in Figure 1 are for single band use when coaxial feed line is employed. Multiband operation is possible if you use tuned feed line and a Transmatch.

See Figure 2  for this hookup. If you wish to take advantage of the multiband concept with your dipole, you should cut it to length for the lowest operating frequency of interest. By way of illustration, your dipole should be dimensioned for 80 meters if you plan to use it from 80 through 10 meters. 

The formula for half-wave dipoles is given in Fig 1. Thus, a dipole for 3.7 MHz will be roughly 126 feet, 6 inches long, overall. I said "approximately" because the final length is determined by the installation. Height above ground and nearby conductive objects will affect the initial 468/f(MHz) dimension somewhat. You may need to cut a few inches of wire off the ends of the dipole halves in order to obtain minimum SWR An SWR bridge is used when making the final adjustments.

Fig 2- Example of a multiband dipole that has balanced, tuned feeders. The feed-line impedance is not critical provided it is between 300 and 600 ohms. TV ribbon line may be used, or the line may be 450-ohm ladder type. Homemade open-wire feed line is the best in the interest of minimum RF power loss. The Transmatch (transmitter to feed-line matcher) should be designed for balanced feed line. If not, a 4:1 balun transformer must be used between the balanced feeder and the Transmatch. The SWR is adjust- ed for a 1:1 condition while observing the SWR meter (minimum reflected power) for each frequency of operation,

source : Novice Antenna Notebook

               by DeMaw, Doug