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. 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. 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 pattern, and 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.
QUESTION: What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase? (E9C01)
ANSWER: A figure-8 oriented along the axis of the array
QUESTION: What is the radiation pattern of two 1/4 wavelength vertical antennas spaced 1/4 wavelength apart and fed 90 degrees out of phase? (E9C02)
ANSWER: Cardioid
QUESTION: What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2 wavelength apart and fed in phase? (E9C03)
ANSWER: A Figure-8 broadside to the axis of the array
How and where you install an antenna affects its radiation pattern. For example, a vertically polarized antenna has a lower angle of radiation when it is mounted over seawater versus rocky ground.
QUESTION: How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus soil? (E9C11)
ANSWER: The low-angle radiation increases
Placement also affects horizontally-polarized antennas. For example, the takeoff angle of the main lobe of a horizontally-polarized antenna mounted on the side of a hill will decrease in the downhill direction when compared with the same antenna mounted on flat ground. The radiation pattern of a horizontally polarized 3-element beam antenna will also vary as you change its height above ground. What happens is that the main lobe takeoff angle decreases with increasing height. A low takeoff angle is desirable for making long-distance contacts as it will generally increase the distance that a signal skips off the ionosphere.
QUESTION: How does the performance of a horizontally polarized antenna mounted on the side of a hill compare with the same antenna mounted on flat ground? (E9C14)
ANSWER: The main lobe takeoff angle decreases in the downhill direction
QUESTION: How does the radiation pattern of a horizontally polarized 3-element beam antenna vary with increasing height above ground? (E9C13)
ANSWER: The takeoff angle of the lowest elevation lobe decreases
Wire antennas
There are many different kinds of wire antennas. Some of the more popular are the long-wire antenna, the folded dipole antenna, the off-center fed dipole antenna, the Zepp antenna, and the G5RV antenna.
The long-wire antenna, as the name implies, is just a long piece of wire, as long as you can make it, strung up as high as you can string it. An antenna tuner is almost always required to match the impedance at the end of the long wire to the 50 Ω output of the transmitter. The radiation pattern of a long-wire antenna is unpredictable, but the lobes align more in the direction of the wire.
QUESTION: What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased? (E9C04)
ANSWER: The lobes align more in the direction of the wire
The folded dipole is a variation on the basic half-wave dipole antenna. A folded dipole antenna is a dipole consisting of one wavelength of wire forming a very thin loop. The approximate feed point impedance at the center of a two-wire folded dipole antenna is 300 ohms. This antenna was very popular when 300 Ω twinlead was used for TV antennas. The reason for this is that 300 Ω was widely available and you could use it for both the antenna and the feedline.
QUESTION: What is a folded dipole antenna? (E9C08)
ANSWER: A half-wave dipole with an additional parallel wire connecting its two ends
QUESTION: What is the approximate feed point impedance at the center of a two-wire folded dipole antenna? (E9C07)
ANSWER: 300 ohms
Another popular variation on the half-wave dipole is the off-center fed dipole, or OCFD. An OCFD antenna is a dipole fed approximately 1/3 the way from one end with a 4:1 balun to provide multiband operation. The reason this antenna has become so popular is that it not only provides a good match on its fundamental frequency, but on harmonically-related frequencies as well. An OCFD antenna cut for the 80 m band, for example, will also present a good match on 40 m, 15 m, and 10 m bands.
QUESTION: Which of the following is a type of OCFD antenna? (E9C05)
ANSWER: A dipole fed approximately 1/3 the way from one end with a 4:1 balun to provide multiband operation
Zepp antennas are also a half-wavelength long. A Zepp antenna is an end fed dipole antenna. Because the impedance is very high at the feedpoint, the Zepp antenna uses a quarter-wave length of transmission line to transform the impedance to something closer to the 50 Ω transmitter output impedance. An extended double Zepp antenna is a center-fed 1.25 wavelength antenna (two 5/8 wave elements in phase.
QUESTION: Which of the following describes a Zepp antenna? (E9C10)
ANSWER: An end-fed dipole antenna
QUESTION: Which of the following describes an Extended Double Zepp antenna? (E9C12)
ANSWER: A center-fed 1.25-wavelength antenna (two 5/8-wave elements in phase)
An example of a wire antenna that is not a half-wavelength long is the G5RV antenna. A G5RV antenna is a multi-band dipole antenna fed with coax and a balun through a selected length of open wire transmission line. The G5RV was originally designed as a single-band, 20m antenna that provided some directionality. Amateurs soon found that they could easily tune this antenna on other frequency bands, and this multi-band capability has made it very popular.
QUESTION: Which of the following describes a G5RV antenna? (E9C09)
ANSWER: A multi-band dipole antenna fed with coax and a balun through a selected length of open wire transmission line
Rhombic antennas are large wire antennas that have four sections, each one or more wavelengths long, arranged in a rhombic shape, and fed at one end of the rhomboid. When the end opposite the feedpoint is open, the antenna is bidirectional. Putting a terminating resistor on a rhombic antenna, however, changes the radiation pattern from bidirectional to unidirectional.
QUESTION: What is the effect of adding a terminating resistor to a rhombic antenna? (E9C06)
ANSWER: It changes the radiation pattern from bidirectional to unidirectional
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