Directional antennas
To make their signals more effective, some amateurs use directional antennas. Directional antennas, such as Yagis and quads, direct most of the power output in a particular direction, making the signal seem more powerful. They are also more sensitive to receiving signals from a particular direction. This feature makes them useful for reducing interference. All you have to do is turn the antenna away from the source of interference.
QUESTION: Which HF antenna would be the best to use for minimizing interference? (G9C11)
ANSWER: A directional antenna
The “gain” of a directional antenna is the relative increase in power radiated in the direction in which the antenna is pointing. The gain is usually specified in decibels, or dB. Look at this specification very carefully, because the gain may be specified in relation to either an isotropic antenna (dBi) or in relation to a dipole (dBd). dBi = dBd + 2.15, so if an antenna specification uses the dBi value, which is really just a theoretical value, the manufacturer is making the antenna look better than it really is.
QUESTION: What is meant by the terms dBi and dBd when referring to antenna gain? (G9C15)
ANSWER: dBi refers to an isotropic antenna, dBd refers to a dipole antenna
QUESTION: How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna? (G9C04)
ANSWER: dBi gain figures are 2.15 dB higher than dBd gain figures
A characteristic related to the antenna gain is the “front-to-back ratio.” The “front-to-back ratio” of a Yagi antenna is the ratio of the power radiated in the forward direction (the main or major lobe) to the power radiated off the back of the antenna.
QUESTION: What does “front-to-back ratio” mean in reference to a Yagi antenna? (G9C07)
ANSWER: The power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction
QUESTION: What is meant by the “main lobe” of a directive antenna? (G9C08)
ANSWER: The direction of maximum radiated field strength from the antenna
Yagis are perhaps the most common type of directional antenna. A Yagi antenna consists of a driven element, a reflector, and one or more directors. The reflector and directors are called parasitic elements. The approximate length of the driven element of a Yagi antenna is ½ wavelength. The reflector is about 5% longer than the driven element, and the first director is about 5% shorter than the driven element.
What is the approximate length of the driven element of a Yagi antenna? (G9C02)
ANSWER: 1/2 wavelength
QUESTION: How do the lengths of a three-element Yagi reflector and director compare to that of the driven element? (G9C03)
ANSWER: The reflector is longer, and the director is shorter
By changing the physical characteristics of the elements and the spacing between the elements, you can change the characteristics of the antenna. For example, if you increase the diameter of the aluminum tubing that most Yagis are made from, you can increase the antenna’s bandwidth. By increasing the boom length and the number of directors, you can increase the gain of a Yagi antenna.
QUESTION: Which of the following would increase the bandwidth of a Yagi antenna? (G9C01)
ANSWER: Larger diameter elements
QUESTION: How does increasing boom length and adding directors affect a Yagi antenna? (G9C05)
ANSWER: Gain increases
QUESTION: Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna? (G9C10)
ANSWER: All of these choices are correct
- The physical length of the boom
- The number of elements on the boom
- The spacing of each element along the boom
While a Yagi antenna is a great antenna, you can improve the performance of this antenna by stacking one on top of another. You get about a 3 dB gain by stacking two, three-element, horizontally polarized Yagi antennas 1/2 wavelength apart vertically. Another advantage is that it narrows the main lobe in elevation, meaning that you get a lower angle of radiation, which can be an advantage in making long-distance communications.
How does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi? (G9C09)
ANSWER: Approximately 3 dB higher
QUESTION: What is an advantage of vertical stacking of horizontally polarized Yagi antennas? (G9D05)
ANSWER: It narrows the main lobe in elevation
Although the driven element of a Yagi antenna is similar to a dipole, the other elements cause the feedpoint impedance to be significantly lower than 50 ohms. To increase the feedpoint impedance to 50 ohms, so that we can use 50-ohm feed line, many Yagis use a gamma match. One advantage of using a gamma match is that the driven element need not be insulated from the boom.
QUESTION: Which of the following is an advantage of using a gamma match with a Yagi antenna? (G9C12)
ANSWER: It does not require that the driven element be insulated from the boom
Another type of matching device used with Yagi antennas is the beta match, also called the hairpin match. A hairpin match is simply a coil connected across the coax to increase the feedpoint impedance.
QUESTION: What is a beta or hairpin match? (G9C16)
ANSWER: It is a shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
You can also make directional antennas using square loop elements. The driven element loop is a full wave long, while the reflector is a little bit bigger. One advantage of a quad antenna over a Yagi is that it can be physically smaller for the same amount of gain. The forward gain of a two-element quad antenna is about the same as the forward gain of a three-element Yagi antenna.
QUESTION: Approximately how long is each side of the driven element of a quad antenna? (G9C13)
ANSWER: 1/4 wavelength
QUESTION: What configuration of the loops of a two-element quad antenna must be used for the antenna to operate as a beam antenna, assuming one of the elements is used as a reflector? (G9C06)
ANSWER: The reflector element must be approximately 5 percent longer than the driven element
QUESTION: How does the forward gain of a two-element quad antenna compare to the forward gain of a three-element Yagi antenna? (G9C14)
ANSWER: About the same
Specialized antennas
In addition to the dipole, vertical, Yagi, and beam antennas, there are many other types of antennas that you may wish to use in your station. Some of these antenna have characteristics which you may find useful in your situation.
A common variation of the dipole antenna is the inverted-V antenna. Instead of supporting the dipole at both ends of the antenna, you use a single support at the center of the antenna and stretch out the legs, forming the inverted V. The advantage of this antenna is that it requires only a single support and less horizontal space than a dipole. The sloping elements also make the feedpoint impedance closer to 50 ohms than a horizontal dipole.
QUESTION: What is the common name of a dipole with a single central support? (G9D12)
ANSWER: Inverted V
Another HF antenna that’s become popular is the horizontal loop antenna. This antenna is a wavelength or more on the lowest band it will be used on and normally fed with some type of parallel transmission line. An antenna tuner matches it to the transmitter, and because you’re using a tuner, it can be used on multiple bands.
QUESTION: What is the combined vertical and horizontal polarization pattern of a multi-wavelength, horizontal loop antenna? (G9D13)
ANSWER: Virtually omnidirectional with a lower peak vertical radiation angle than a dipole
The log-periodic antenna is one such antenna. It is called this because the length and spacing of the elements increases logarithmically from one end of the antenna to the other. It’s a directional antenna, like the Yagi, and it even resembles the Yagi, although it has more elements. Its gain, however is less than the Yagi. Its main advantage is that it has a much wider bandwidth than the Yagi antenna.
Which of the following describes a log periodic antenna? (G9D07)
ANSWER: Element length and spacing vary logarithmically along the boom
Which of the following is an advantage of a log periodic antenna? (G9D06)
ANSWER: Wide bandwidth
Another interesting antenna is the near vertical incidence sky save, or NVIS antenna. It’s designed to have a very high angle of radiation to make short-skip contacts, usually during the day. NVIS antennas are usually dipole antennas mounted close to the ground. Ground reflection give the NVIS antenna its high angle of radiation.
QUESTION: Which of the following antenna types will be most effective as a Near Vertical Incidence Skywave (NVIS) antenna for short-skip communications on 40 meters during the day? (G9D01)
ANSWER: A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
Beverage antennas are a very specialized type of antenna. They are long, low, directional antennas used for receiving on the low HF bands. Beverage antennas are not used for transmitting because they have high losses compared to other antennas.
QUESTION: What is the primary use of a Beverage antenna? (G9D09)
ANSWER: Directional receiving for low HF bands
Many antennas are designed for a single band, but in many cases, putting up an antenna for each band you want to operate is impractical. So, many amateurs put up antennas that will work on more than one band. These are called multiband antennas. Because some of the amateur radio bands are harmonically-related, one disadvantage of a multiband antenna is that it will radiate a signal’s harmonics just as well as its fundamental frequency.
QUESTION: Which of the following is a disadvantage of multiband antennas? (G9D11)
ANSWER: They have poor harmonic rejection
One type of multiband antenna is the trap vertical. Antenna traps block RF energy in a certain frequency band. This makes the antenna look shorter than it really is at that frequency. Traps are also used on Yagi antennas, so that elements can be used on multiple bands.
QUESTION: What is the primary purpose of antenna traps? (G9D04)
ANSWER: To permit multiband operation
For mobile and portable use, amateur radio operators have designed many innovative antennas. One of these is the end-fed antenna. Its main advantage is that it is easy to set up and operate when operating portable. It’s biggest disadvantage is that it requires some kind of matching unit. The reason for this is that the feedpoint has a very high impedance.
QUESTION: What is the feed-point impedance of an end-fed half-wave antenna? (G9D02)
ANSWER: Very high
Another antenna that many amateur radio operators like to use in portable operations is the magnetic loop antenna. This type of antenna has a diameter less than one-third wavelength in circumference, so it’s relatively easy to transport, and with a tripod or similar kind of stand is self-supporting. And, there are nulls in the radiation pattern broadside to the loop that you can use to reduce interference.
QUESTION: In which direction or directions does an electrically small loop (less than 1/3 wavelength in circumference) have nulls in its radiation pattern? (G9D10)
ANSWER: Broadside to the loop
For mobile operation, some amateurs use a “screwdriver” antenna. The screwdriver antenna has a motorized tuning assembly that varies the inductance of the loading coil to tune the antenna to the operating frequency.
QUESTION: How does a “screwdriver” mobile antenna adjust its feed-point impedance? (G9D08)
ANSWER: By varying the base loading inductance
For weak-signal VHF/UHF work, some amateur radio operators use a halo antenna, which is a horizontally polarized, omnidirectional, half-wavelength dipole antenna. It’s called a halo because the elements are bent into a loop with a small gap between the elements opposite the feed point. It’s a small, yet very effective antenna.
QUESTION: In which direction is the maximum radiation from a portable VHF/UHF “halo” antenna? (G9D03)
ANSWER: Omnidirectional in the plane of the halo
Dave New, N8SBE says
“It is called this because for a log periodic antenna because the length and spacing of the elements…” –> “It is called this because the length and spacing of the elements…”