Extra Class question of the day: Wire and phased vertical antennas

There are many ways to put up antennas that are directional. Yagis are directional antennas, but they require a structure, such as a tower, to get them high in the air. One way to get directionality without a tower is to use phased vertical arrays.

In general, the phased vertical array consists of two or more quarter-wave vertical antennas. The radiation pattern that the array will have depends on how you feed the vertical antennas.

So, for example, the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase is a figure-8 oriented along the axis of the array. (E9C01) The radiation pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase is a cardioid. (E9C02) The radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase is a Figure-8 broadside to the axis of the array. (E9C03)

A rhombic antenna is often used for receiving on the HF bands. A basic unterminated rhombic antenna is described as bidirectional; four-sides, each side one or more wavelengths long; open at the end opposite the transmission line connection. (E9C04) The disadvantages of a terminated rhombic antenna for the HF bands is that the antenna requires a large physical area and 4 separate supports. (E9C05) Putting a terminating resistor on a rhombic antenna changes the radiation pattern from bidirectional to unidirectional. (E9C06)

Figure E9-2: Elevation Pattern

The type of antenna pattern over real ground that is shown in Figure E9-2 is an elevation pattern. (E9C07) The elevation angle of peak response in the antenna radiation pattern shown in Figure E9-2 is 7.5 degrees. (E9C08) The front-to-back ratio of the radiation pattern shown in Figure E9-2 is 28 dB. (E9C09) 4 elevation lobes appear in the forward direction of the antenna radiation pattern shown in Figure E9-2. (E9C10)

How and where you install an antenna affects its radiation pattern. For example, the far-field elevation pattern of a vertically polarized antenna is affected when it is mounted over seawater versus rocky ground. What happens is that the low-angle radiation increases. (E9C11) The main effect of placing a vertical antenna over an imperfect ground is that it reduces low-angle radiation. (E9C13) When constructing a Beverage antenna, remember that it should be one or more wavelengths long to achieve good performance at the desired frequency. (E9C12)

Speak Your Mind

*