Tech

Antenna Stuff

Dan Romanchik, KB6NU

Want to lean about antennas?

Here’s a series of study guides from which you can learn a lot! 2020 Extra Class study guide: E9E – Matching: matching antennas to feed lines; phasing lines; power dividers For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline or dissipated in the feedline. The ratio of the amplitude of the reflected wave to the amplitude of the incident wave, or the waver that you’re transmitting, is called the reflection coefficient, and it is mathematically related to SWR. QUESTION: What parameter describes the interactions at the load end of a mismatched transmission line? (E9E07) ANSWER: Reflection coefficient To match the impedance of the feedline to the impedance of the antenna, we use a variety of different techniques. The delta matching system matches a high- impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center. It’s called a delta match because when connected this way, the feedline and antenna look like the Greek letter delta. QUESTION: What system matches a higher-impedance transmission line to a lower- impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center? (E9E01) ANSWER: The delta matching system The gamma match is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center. The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. The gamma match is an effective method of shunt feeding a grounded tower so it can be used as a vertical antenna. QUESTION: What is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center? (E9E02) ANSWER: The gamma match QUESTION: What is the purpose of the series capacitor in a gamma-type antenna matching network? (E9E04) ANSWER: To cancel the inductive reactance of the matching network QUESTION: Which of the following is used to shunt-feed a grounded tower at its base? (E9E09) ANSWER: Gamma match The stub match is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point. What the stub does is to add reactance at the feed point. By varying the length of the stub, you can change the reactance that the stub provides to whatever value is needed. QUESTION: What is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point? (E9E03) ANSWER: The stub match Many directly-fed Yagi antennas have feedpoint impedances of approximately 20 to 25 ohms. One technique often use to match these antennas to 50-ohm coaxial cable is the hairpin match. To use a hairpin matching system to tune the driven element of a 3-element Yagi, the driven element reactance must be capacitive. QUESTION: How must an antenna’s driven element be tuned to use a hairpin matching system? (E9E05) ANSWER: The driven element reactance must be capacitive Lengths of 75-ohm coax can also be used to match impedances. For example, inserting a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line. Note that this only works on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band. QUESTION: Which of these feed line impedances would be suitable for constructing a quarter-wave Q-section for matching a 100-ohm loop to 50-ohm feed line? (E9E06) ANSWER: 75 ohms QUESTION: Which of these choices is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line? (E9E10) ANSWER: Insert a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable Another use for coaxial cable is as a phasing line for antennas that have multiple driven elements. The theory here is that by feeding the driven elements out of phase with one another, you can create a directional radiation pattern. A common application for phasing lines is a phased, vertical array. QUESTION: What is the primary purpose of phasing lines when used with an antenna having multiple driven elements? (E9E11) ANSWER: It ensures that each driven element operates in concert with the others to create the desired antenna pattern Finally, unless you’re going to be doing microwave work, you probably won’t need to know about Wilkinson dividers, but here’s the information anyway. Wilkinson dividers divide power equally between two 50 ohm loads while maintaining 50 ohm input impedance. They’re used mainly in microwave systems. QUESTION: What is a use for a Wilkinson divider? (E9E08) ANSWER: It is used to divide power equally between two 50-ohm loads while maintaining 50-ohm input impedance. 2020 Extra Class study guide: Yagi antennas; parabolic reflectors; circular polarization; loading coils; top loading; feedpoint impedance of electrically short antennas; antenna Q; RF grounding Posted: 05 Mar 2020 06:57 AM PST Yagi and parabolic antennas When designing a Yagi antenna, you might think that the most important parameter is forward gain. What usually occurs if a Yagi antenna is designed solely for maximum forward gain, though, is that the front-to-back ratio decreases. In other words, the antenna becomes more bi-directional than simply directional. QUESTION: What usually occurs if a Yagi antenna is designed solely for maximum forward gain? (E9D05) ANSWER: The front-to-back ratio decreases On the VHF and UHF bands, Yagi antennas are operated horizontally for weak- signal work and vertically for FM operations. In some cases, such as operating satellites, circular polarization is desirable. By arranging two linearly-polarized Yagi antennas perpendicular to each other with the driven elements at the same point on the boom and feeding them 90 degrees out of phase you produce circular polarization. The disadvantage to this approach is, obviously, that you need two antennas, instead of just one to achieve circular polarization. QUESTION: How can linearly polarized Yagi antennas be used to produce circular polarization? (E9D02) ANSWER: Arrange two Yagis perpendicular to each other with the driven elements at the same point on the boom fed 90 degrees out of phase Parabolic antennas are often used at microwave frequencies to direct a signal in a particular direction. The bigger the dish, the higher the gain for a given operating frequency. The gain of an ideal parabolic dish increases by 6 dB when the operating frequency is doubled. The beamwidth is narrower as well. QUESTION: How much does the gain of an ideal parabolic dish antenna change when the operating frequency is doubled? (E9D01) ANSWER: 6 dB Antenna efficiency, shortened and mobile antennas Designing an efficient mobile HF antenna is perhaps the toughest job for a radio amateur. More often than not, they are operated below their resonant frequency. From a practical point of view, the antenna’s radiation resistance decreases and the capacitive reactance increases as the operating frequency decreases. That’s why most mobile HF antennas use a loading coil to provide a 50 ohm match. The loading coil cancels the capacitive reactance. In effect, loading coils to make the radiator of a short vertical antenna look electrically longer. QUESTION: What happens to feed-point impedance at the base of a fixed length HF mobile antenna when operated below its resonant frequency? (E9D10) ANSWER: The radiation resistance decreases and the capacitive reactance increases QUESTION: What is the function of a loading coil used as part of an HF mobile antenna? (E9D09) ANSWER: To cancel capacitive reactance Unfortunately, the loading coil can’t increase the radiation resistance, and as a result, short vertical antennas are inherently inefficient. To minimize losses and make them as efficient as possible is to use a high-Q loading coil. That is to say a coil with a high ratio of reactance to resistance. Another thing that you can do is to place the high-Q loading coil near the center of the vertical radiator. QUESTION: Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance? (E9D04) ANSWER: To minimize losses QUESTION: Where should a high Q loading coil be placed to minimize losses in a shortened vertical antenna? (E9D03) ANSWER: Near the center of the vertical radiator One disadvantage of using a loading coil with a short vertical antenna is that it decreases the SWR bandwidth of the antenna. This means that it has to be retuned more frequently than an antenna that doesn’t need a loading coil. Not only that, as the Q of an antenna system increases, the SWR bandwidth decreases. QUESTION: What happens to the SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna? (E9D06) ANSWER: It is decreased QUESTION: What happens as the Q of an antenna increases? (E9D08) ANSWER: SWR bandwidth decreases One way that some amateurs improve the radiation efficiency of a short vertical antenna is to use a technique called top loading. This is most often accomplished by using a “capacitance hat” on top of the vertical element. QUESTION: What is an advantage of using top loading in a shortened HF vertical antenna? (E9D07) ANSWER: Improved radiation efficiency RF grounding While much has been written about station grounding, one thing’s for sure. A station’s safety ground is not adequate as an RF ground. The reason for this is that conductors present different impedances at different frequencies. Perhaps the best conductor for minimizing losses in a station’s RF ground system is wide flat copper strap. The main reason for this is that RF tends to be conducted near the surface of a conductor. The more surface area there is, the lower the impedance to ground, and copper strap normally has many small conductors braided together to maximize surface area. QUESTION: Which of the following conductors would be best for minimizing losses in a station’s RF ground system? (E9D11) ANSWER: Wide flat copper strap To minimize inductance, it’s best to keep the RF ground connection as short as possible. An electrically-short connection to 3 or 4 interconnected ground rods driven into the Earth would provide the best RF ground for your station. QUESTION: Which of the following would provide the best RF ground for your station? (E9D12) ANSWER: An electrically short connection to 3 or 4 interconnected ground rods driven into the Earth. 2020 Extra Class study guide: E9A Basic antenna parameters: radiation resistance, gain, beamwidth, efficiency, effective radiated power Posted: 02 Mar 2020 08:25 AM PST Antenna gain is one of the most misunderstood topics in amateur radio. There are several reasons for this, including:  Antennas don’t really have gain in the same way that an amplifier has gain. When you use a linear amplifier, you get more power out than you put in. Since transmitting antennas are passive devices, there’s no way to get more power out than you put in.  It’s not easy to measure antenna gain. There is no antenna gain meter that you can simply hook up to an antenna to measure its gain. So, what is meant by antenna gain? Antenna gain is the ratio of the radiated signal strength of an antenna in the direction of maximum radiation to that of a reference antenna. What this means is that when you talk about antenna gain, you have to know what kind of antenna you’re comparing it to. When talking about antenna gain, antenna engineers often refer to the “isotropic antenna.” In practice, an isotropic antenna is a theoretical antenna that has no gain in any direction. That is to say it radiates the power input to it equally in all directions. QUESTION: What is an isotropic antenna? (E9A01) ANSWER: A theoretical, omnidirectional antenna used as a reference for antenna gain Let’s take a look at a practical example. The 1/2-wavelength dipole antenna is the most basic amateur radio antenna. The dipole actually has some gain over isotropic antenna. The reason for this is that it is directional. The signal strength transmitted broadside to the antenna will be greater than the signal strength transmitted off the ends of the antenna. The gain of a 1/2-wavelength dipole in free space compared to an isotropic antenna is 2.15 dB. Sometimes, you’ll see this value as 2.15 dBi, where dBi denotes that an isotropic antenna is being used for this comparison. Since the isotropic antenna is a theoretical antenna, some think it’s better to compare an antenna to a dipole antenna. Let’s look at an example: QUESTION: How much gain does an antenna have compared to a 1/2-wavelength dipole when it has 6 dB gain over an isotropic antenna? (E9A12) ANSWER: 3.85 dB You obtain this value by simply subtracting 2.15 dB from the 6 dB figure: Gain over a dipole = gain over an isotropic antenna – 2.15 dB = 6 dBi – 2.15 dBi = 3.85 dBd Sometimes, the gain over a dipole is denoted as dBd. Effective radiated power When you use an antenna that has gain, you are increasing the effectiveness of the power input to it in the direction the antenna is pointing. We call this the effective radiated power, but it is not just the transmitter’s output power times the gain of the antenna. You also have to take into account losses in other parts of the antenna system. This is especially true for VHF and UHF repeater systems, where losses in the feedline, duplexer, and circulator can be significant. The power that reaches the antenna may be substantially lower than the power output of the transmitter. Let’s look at an example. Say that your repeater station had a transmitter output power of 150 watts, a feed line loss of 2 dB, 2.2 dB duplexer loss, and 7 dBd antenna gain. To calculate the effective radiated power, you have to first subtract the losses from the gain, as expressed in dB to get the total gain of the system: total system gain = 7 dB – 2 dB – 2.2 dB = 2.8 dB. Now, recall that 3 dB corresponds to a power ration of 2:1, as shown in the table below. 2.8 dB would then be slight less than that. In fact, 2.8dB corresponds to a power ratio of approximately 1.905, so the effective radiated power is the transmitter output power times the total system gain: effective radiated power = 150 W x 1.905 = 268 W. QUESTION: What term describes station output, taking into account all gains and losses? (E9A13) ANSWER: Effective radiated power QUESTION: What is the effective radiated power relative to a dipole of a repeater station with 150 watts transmitter power output, 2 dB feed line loss, 2.2 dB duplexer loss, and 7 dBd antenna gain? (E9A02) ANSWER: 286 watts Let’s look at another example. In this example, your repeater station has 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain. The total gain of the system is, therefore, 10 dB – 4 dB – 3.2 dB – 0.8 dB, or 2.0 dB. 2.0 dB corresponds to a power ratio of approximately 1.585, making the effective radiated power 200 W × 1.585 = 317 W. Note that in this system, the high feedline and duplexer losses almost completely negate the benefit of using a high gain antenna. QUESTION: What is the effective radiated power relative to a dipole of a repeater station with 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain? (E9A06) ANSWER: 317 watts Here’s a third example. Notice that in this example we comparing the effective radiated power to an isotropic antenna, not a dipole. In this example, the repeater station has 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain. The total gain of the system is 7 dB – 2 dB – 2.8 dB – 1.2 dB, or 1.0 dB. 1.0 dB corresponds to a power ratio of approximately 1.26, and the effective radiated power equals 200 W × 1.26 = 252 W. QUESTION: What is the effective isotropic radiated power of a repeater station with 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain? (E9A07) ANSWER: 252 watts Feedpoint impedance Other antenna parameters are also important, of course. One of the most basic antenna parameters is the feedpoint impedance. The reason that the feedpoint impedance is important is that you want the feedpoint impedance to match the impedance of the feedline that you use and the output impedance of the transmitter. When these are all equal, we say that the system is “matched,” and it will radiate the maximum amount of energy. Many factors may affect the feed point impedance of an antenna, including antenna height, conductor length/diameter ratio and location of nearby conductive objects. For example, we say that the feedpoint impedance of a half-wavelength, dipole antenna is 72 Ω, but that’s only really true if the antenna is in free space. When it’s closer to the ground than a quarter wavelength, then the impedance will be different. That’s why you have to tune the antenna when you install it. QUESTION: Which of the following factors affect the feed point impedance of an antenna? (E9A04) ANSWER: Antenna height Radiation resistance Another antenna parameter that’s frequently bandied about is radiation resistance. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. In the case of an antenna, however, that power isn’t being turned into heat, but rather turned into radio waves. The total resistance of an antenna system includes the radiation resistance ohmic resistance. QUESTION: What is the radiation resistance of an antenna? (E9A03) ANSWER: The value of a resistance that would dissipate the same amount of power as that radiated from an antenna QUESTION: What is included in the total resistance of an antenna system? (E9A05) ANSWER: Radiation resistance plus loss resistance If you know the radiation resistance and the ohmic resistance of an antenna, you can calculate its efficiency. Antenna efficiency equals the radiation resistance divided by the total resistance. QUESTION: What is antenna efficiency? (E9A09) ANSWER: Radiation resistance divided by total resistance Vertical antennas, bandwidth Vertical antennas are sometimes criticized as being inefficient antennas. The main reason for this is the lack of a good radial system. Installing a good radial system will improve a quarter-wave vertical’s efficiency. Another for poor vertical performance is poor soil conductivity. If soil conductivity is poor, ohmic resistance will be high, and the antenna’s efficiency will be low. QUESTION: Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna? (E9A10) ANSWER: Installing a radial system QUESTION: Which of the following factors determines ground losses for a ground- mounted vertical antenna operating in the 3 MHz to 30 MHz range? (E9A11) ANSWER: Soil conductivity Antenna bandwidth is the frequency range over which an antenna satisfies a performance requirement. Normally, the performance requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this parameter referred to as the 2:1 SWR bandwidth. QUESTION: What is antenna bandwidth? (E9A08) ANSWER: The frequency range over which an antenna satisfies a performance requirement.