GC - Chapter 7, Antennas
007-9) Figure 7.8 Two Element Yagi Antenna
Two element Yagi antennas using a single parasitic element: a) At "A" the PARASITIC element acts as the DIRECTOR. b) A "B" the PARASITIC element acts as the REFLECTOR.
07-10) Impedance Matching (Technician Review)
A FEED LINE is used to deliver the radio signals to the antenna. The connection of the antenna and FEED LINE is called the FEED POINT. Just like OHM's law in Chapter 2, the ratio of frequency voltage to current at an antenna's FEED POINT is the antenna's FEED POINT IMPEDANCE. Resistance and impedance have different fundamental origins even though the calculation for their value is the same: R=V/I An antenna is resonant when its FEED POINT IMPEDANCE is all RESISTANCE with NO REACTANCE. An ideal resistor has zero reactance, whereas ideal inductors and capacitors have zero resistance - that is, respond to current only by reactance. An antenna's FEED POINT IMPEDANCE at a specific frequency depends on how its physical dimensions compare to the wavelength at that frequency. FEED POINT IMPEDANCE changes with frequency because wavelength changes but the physical dimensions do not. An antenna's FEED POINT IMPEDANCE is also affected by nearby conductors and it's height above ground.
07-18) Forward and Reflected Power and SWR
A FEED LINE transfers all of its power to an antenna when the antenna and the FEED LINE impedances are MATCHED. If the FEED LINE and antenna impedances DO NOT MATCH, some of the power is reflected by the antenna. a) Forward Power - is power traveling toward the antenna b) Reflected Power - is power reflected by the antenna. Power in a FEED LINE is reflected at any point at which the impedance of the FEED LINE changes. This can be at the antenna, at a connector, or from a different type of feed line.
G9B06) ? A. 8 feet B. 16 feet C. 24 feet D. 32 feet
ANSWER
G9C02) ? A. The magnitude of the maximum vertical angle of radiation. B. The point of maximum current in a radiating antenna element. C. The maximum voltage standing wave point on a radiating element. D. The direction of maximum radiated field strength from the antenna.
ANSWER
G9C16) ? A. It does require that the elements be insulated from the boom. B. It does not require any inductors or capacitors. C. It is useful for matching multiband antennas D. All of these choices are correct.
ANSWER
G9C17) ? A. It does require that the elements be insulated from the boom. B. It does not require any inductors or capacitors. C. It is useful for matching multiband antennas D. All of these choices are correct.
ANSWER
G9C13) Approximately how long is each side of the driven element of a quad antenna? A. 1/4 wavelength B. 1/2 wavelength. C. 3/4 wavelength D. 1 wavelength
ANSWER A. 1/4 wavelength
G9B12) What is the approximate length in feet of a 1/4 wave ground-plane antenna resonant at 28.5 MHz (10-meter band)? A. 8 feet B. 11 feet C. 16 feet D. 21 feet
ANSWER A. 8 feet
G9C05) How does increasing boom length and adding directors affect a Yagi antenna? A. Gain increases. B. Beamwidth increases. C. Front to back ratio deceases D. Front to side ratio deceases
ANSWER A. Gain increases. b) - A longer boom with a fixed number of directors increases gain up to a maximum length beyond which GAIN is reduced.
G9C12) Which of the following is an advantage of using a gamma match for impedance matching of Yagi antenna to 50 ohm coax feed line? A. It does require that the elements be insulated from the boom. B. It does not require any inductors or capacitors. C. It is useful for matching multiband antennas D. All of these choices are correct.
ANSWER A. It does require that the elements be insulated from the boom.
G9B04) What is the radiation pattern of a dipole antenna in free space in the plane of the conductor? A. It is a figure-eight at right angles to the antenna. B. It is a figure-eight off both ends of the antenna. C. It is a circle (equal radiation in all directions). D. It is a pair of lobes on one side of the antenna and a single lobe on the other side.
ANSWER A. It is a figure-eight at right angles to the antenna.
G9B08) How does the feed point impedance of a 1/2 wave DIPOLE change as the feed point is moved from the center towards the end ? A. It steadily increases B. IA. It steadily decreases C. It peaks at about 1/8 wavelength from the end. D. It is unaffected by the location of the feed point.
ANSWER A. It steadily increases
G9C01) Which of the following would increase the bandwidth of a Yagi antenna? A. Larger diameter elements B. Closer element spacing. C. Loading coils in series with the element. D. Tapered-diameter elements.
ANSWER A. Larger diameter elements. c) Larger diameter elements reduce SWR variation with frequency increase (increases SWR bandwidth).
G9B09) Which of the following is an advantage of horizontally polarized as compared to a vertically polarized antenna? A. Lower ground reflection losses. B. Lower feed point impedance. C. Shorter radials. D. Lower radiation resistance.
ANSWER A. Lower ground reflection losses.
G9A01) Which of the following factors determine the characteristic impedance of a parallel conductor feed line? A. The distance between the centers of the conductors and the radius of the conductors. B. The distance between the centers of the conductors and the length of the line. C. The radius of the conductors and the frequency of the signal. D. The frequency and the signal and the length of the line.
ANSWER A. The distance between the centers of the conductors and the radius of the conductors.
G9C04) Which statement about a three-element, single-band Yagi antenna is true?? A. The reflector is normally the longest element. B. The director is normally the longest element. C. The reflector is normally the shortest element. D. All of the elements must be the same length
ANSWER A. The reflector is normally the longest element.
G9C11) What is the purpose of a gamma match used with Yagi antennas? A. To match the relatively low feed point impedance to 50 ohms. B. To match the relatively high feed point impedance to 50 ohms. C. To increase the front-to-back ratio. D. To increase the main lobe gain.
ANSWER A. To match the relatively low feed point impedance to 50 ohms. .... transform the low impedance of the FEED POINT (typically 20 to 25Ω ) up to a higher value (50Ω in our case).
G9C20) What is meant by the terms dBi and dBd when referring to antenna gain? A. dBi refers to an isotropic antenna, dBd refers to a dipole antenna. B. dBi refers to an ionospheric reflecting antenna, dBd refers to a dissipative antenna C. dBi refers to an inverted-vee antenna, dBd refers to a downward reflecting antenna. D. dBi refers to an isomeric antenna, dBd refers to a discone antenna.
ANSWER A. dBi refers to an isotropic antenna, dBd refers to a dipole antenna.
G9C02) What is the approximate length of the driven element of a Yagi antenna? A. 1/4 wavelength B. 1/2 wavelength C. 3/4 wavelength D. 1 wavelength
ANSWER B. 1/2 wavelength
G9A02) What are the typical characteristic impedances of coaxial cables used for antenna feed lines.? A. 25 and 30 ohms B. 50 and 75 ohms C. 80 and 100 ohms D. 500 and 750 ohms
ANSWER B. 50 and 75 ohms
G2D04) Which of the following describes an azimuthal projection map? A. A map that shows accurate land masses. B. A map that shows true bearings and distances from a particular location. C. A map that shows the angle at which an amateur satellite crosses the equator. D. A map that shows the number of degrees longitude that an amateur satellite appear to move westward at the equator with each orbit.
ANSWER B. A map that shows true bearings and distances from a particular location.
G9C14) How does the forward gain of a two-element quad antenna compare to the forward gain of a three element Yagi antenna? A. About 2/3 as much B. About the same C. About 1.5 times as much D. About twice as much
ANSWER B. About the same
G9B03) What happens to the feed point impedance of a ground-plane antenna when its radials are changed from horizontal to sloping downward? A. It decreases B. It increases C. It stays the same D. It reaches a maximum angle of 45 degrees
ANSWER B. It increases
G9B07) How does the feed point impedance of a 1/2 wave dipole change as the antenna is lowered below 1/4 wave above the ground? A. It steadily increases B. It steadily decreases. C. It peaks out at 1/8 wavelength above ground D. It is unaffected by the height above the ground
ANSWER B. It steadily decreases.
G9C15) Approximately how long is each side of the reflector element of a quad antenna? A. Slightly less than 1/4 wavelength B. Slightly more than 1/4 wavelength. C. Slightly less than 1/2 wavelength D. Slightly more than 1/4 wavelength
ANSWER B. Slightly more than 1/4 wavelength. (bad question: text specifies "about" in length)
G9B02) Which of the following is a common way to adjust the feed point impedance of a quarter wave ground plane vertical antenna to be approximately 50 ohms? A.Slope the radials upward B. Slope the radials downward C. Lengthen the radials. D. Shorten the radials.
ANSWER B. Slope the radials downward
G9C03) Which statement about a three-element, single-band Yagi antenna is true? A. The reflector is normally the shortest element. B. The director is normally the shortest element. C. The driven element is the longest element. D. Low feed point impedance increases bandwidth
ANSWER B. The director is normally the shortest element.
G9B01) What is the disadvantage of a directly fed random-wire HF antenna? A. It must be longer than 1 wavelength. B. You may experience RF burns when touching metal objects in your station. C. It produces only vertically polarized radiation. D. It is more effective on the lower HF bands than on the higher bands.
ANSWER B. You may experience RF burns when touching metal objects in your station.
G9C19) How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna? A. dBi gain figures are 2.15 dB lower than dBd gain figures B. dBi gain figures are 2.15 dB higher than dBd gain figures C. dBi gain figures are the same as the square root of dBd gain figures multiplied by 2.15 D. dBi gain figures are the reciprocal of dBd gain figures + 2.15 dB.
ANSWER B. dBi gain figures are 2.15 dB higher than dBd gain figures.
G9B11) What is the approximate length in feet of a 1/2 wave dipole resonant at 3.550 MHz (80-meter band)? A. 42 feet B. 84 feet C. 131 feet D. 263 feet
ANSWER C. 131 feet
G9A04) What might cause reflected power at the point where a feed line connects to an antenna? A. Operating an antenna at its resonant frequency. B. Using more transmitter power than the antenna can handle. C. A difference between feed line impedance and antenna feed point impedance. D. Feeding the antenna with an unbalanced feed line.
ANSWER C. A difference between feed line impedance and antenna feed point impedance.
G2D11) Which HF antenna would be the best to use for minimizing interference? A. A quarter-wave vertical antenna. B. An isotropic antenna. C. A directional antenna. D. An omnidirectional antenna.
ANSWER C. A directional antenna.
G9B05) How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal DIPOLE HF antenna? A. if the antenna height is too high, the pattern becomes unpredictable. B. Antenna height has no effect on the pattern C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional D. If the antenna is less than 1/2 wavelength high, radiation of the ends of the wire is eliminated.
ANSWER C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
G4E06) What is one of the disadvantages of using a shortened mobile antenna as opposed to a full size antenna? A. Short antennas are more likely to cause distortion of transmitted signals. B. Short antennas can only receive circularly polarized signals. C. Operating bandwidth may be very limited. D. Harmonic radiation may increase
ANSWER C. Operating bandwidth may be very limited
G9C07) What does the front-to-back ratio mean for a Yagi antenna? A. The number of directors versus the number reflectors. B. The relative position of the driven element with respect to the reflectors and directors C. The power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction. D. The ratio of forward gain to dipole gain.
ANSWER C. The power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction.
G4E01) What is the purpose of a capacitance hat on a mobile antenna? A. To increase the power handling capacity of a whip antenna. B. To allow automatic band changing. C. To electrically lengthen a physically short antenna. D. To allow remote tuning
ANSWER C. To electrically lengthen a physically short antenna.
G9A03) What is the characteristic impedance of flat ribbon TV type twin lead? A. 50 ohms B. 75 ohms C. 100 ohms D. 300 ohms
ANSWER D. 300 ohms
G9B10) What is the approximate length in feet of a 1/2 wave dipole resonant at 14.250 MHz (20-meter band)?? A. 8 feet B. 16 feet C. 24 feet D. 32 feet
ANSWER D. 32 feet
G9C10) Which of the following is a Yagi antenna design variable that could be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth.? A. The physical length of the boom. B. The number of elements on the boom. C. The spacing of each element along the boom. D. All of these choices are correct.
ANSWER D. All of these choices are correct. (I disagree: a longer boom WITH a fixed number of directors on it)
G9C08) What is meant by the "main lobe" of a directive antenna? A. The magnitude of the maximum vertical angle of radiation. B. The point of maximum current in a radiating antenna element. C. The maximum voltage standing wave point on a radiating element. D. The direction of maximum radiated field strength from the antenna.
ANSWER D. The direction of maximum radiated field strength from the antenna.
G9C06) What configuration of 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? A. The driven element must be fed with a balun transformer. B. There must be an open circuit in the driven element at the point opposite the feed point. C. The reflector element must be approximately 5% shorter than the driven element. D. The reflector element must be approximately 5% longer than the driven element.
ANSWER D. The reflector element must be approximately 5% longer than the driven element.
G4E02) What is the purpose of a corona ball on a HF mobile antenna? A. To narrow the operating bandwidth of the antenna. B. To increase the "Q" of the antenna. C. To reduce the chance of damage if the antenna should strike an object. D. To reduce high voltage discharge from the tip of the antenna.
ANSWER D. To reduce high voltage discharge from the tip of the antenna.
007-6) Effects of Height Above Ground - Impact Antenna Performance
An antenna's feed point impedance and radiation pattern are both affected by the antenna's physical height above the ground. The effects caused by the presence of the electrical image of the antenna created in the electrically conducing ground below the antenna. Basically, antenna height above the ground affects antenna performance. Why does height matter?
07-11) Loop Antennas
Are usually one wavelength or more in circumference Loop antennas can be oriented vertically or horizontally. Loop antennas can be circular, square, triangular or any simple open shape that is not too narrow. a) A square loop with each leg λ/4 long is called a Quad Loop. b) A Triangular or Delta Loops are usually symmetrical, with each leg λ/3 long. Figure 7.10 shows that the direction of maximum signal is broadside to the plane of the loop. If the Loop antenna is oriented horizontally, most of its signal will go straight up, making it a good antenna for local or regional contacts. If the Loop antenna is oriented vertically, aims the maximum signal toward the horizon, making it a good antenna for making DX contacts.
007-4) Mobile HF Antennas - Loading (sidebar)
As a practical matter, mobile HF antennas must be small to be safe and manageable on a vehicle. Some common loading techniques are used to electrically lengthen the physically short antennas: a) Loading coils - a coil is added to the base or somewhere along the length of the antenna. b) Capacitance hats - spokes or a wheel shaped structure is added near the TOP of the antenna. c) Linear loading - part of the antenna is folded back on itself. Corona Ball - another feature on mobile whips is the CORONA BALL at the tip of the antenna. This adds a small amount of loading capacitance, it's primary function is to eliminate any high-voltage discharges from the sharp tip of the antenna while transmitting.
07-17) Feed Lines - Characteristic Impedance
Balanced FEED LINES consist of TWO parallel conductors separated by insulating material (dielectric) in the form of strips or spaces. See Figure 7.16 FEED LINES have different characteristic impedances, denoted (Z0), that characterise how electromagnetic energy is carried by the FEED LINE. This is not the same as the resistance of the conductors if measured from end-to-end of the FEED LINE. The geometry of the feed line conductors determines the characteristic impedance: a) For PARALLEL FEED LINES, the radius of the conductors and the distance between them determine Z0. ####################################### (GEOMETRY EXPLAINED) Coaxial transmission lines are constructed so that there is a certain PARALLEL CAPACITANCE per unit length as well as a certain SERIES INDUCTANCE per unit length of the line. The CHARACTERISTIC IMPEDANCE is the square root of the ratio of the INDUCTANCE to CAPACITANCE.
07-17) Feed Lines - Technician Class Review - Coaxial Cable
Coaxial cable carries the radio signal on the surface of the center conductor and the inside surface of the shield. That means that it can be placed next to other cables or conducting surfaces such as conduit or antenna support masts without affecting the signal inside.
007-2) Dipoles cont'd: Current, Voltage & Feed Point
Current in a 1/2 wavelength (λ/2) long DIPOLE is HIGHEST in the MIDDLE and ZERO at the ends. Voltage in a 1/2 wavelength (λ/2) long DIPOLE is HIGHEST at the ENDS and LOWEST in the MIDDLE. The feed point impedance (the ratio of RF voltage to current) of a center-fed DIPOLE in free space is approximately 72 Ω but it varies widely depending on it's height above ground. Impedance increases as the feed point is moved away from center and is several thousand ohms at the ends.
007-8) Yagi Antennas - How they Work
Directional antennas are widely used because they create gain as well as reject interference and noise from other than the desired direction. The Yagi remains the most popular of all directional antennas. Even a simple Yagi can REDUCE interfering signals and noise from unwanted directions to the REAR and SIDES of the antenna -- an important feature in a crowded band like 20 meters.
07-17) Feed Lines - Technician Class Review
FEED LINES: The radio signal is carried on the the conductors and in the space between them. FEED LINES used at radio frequencies use special materials and construction methods to minimize power being dissipated as heat by FEED LINE loss and to avoid signals leaking out (called "Common XXXXXXXX.) FEED LINE loss increases with frequency for all types of FEED LINES.
007-6) Effects of Height Above Ground - Impact on Feed Point Impedance
Feed point impedance is affected by the height of the antenna above the ground because - the electrical image, like a all mirror images, is electrically reversed from the actual antenna. As the image and the antenna get closer together, the actual antenna begins to be "shorted out" by the image. (I guess they cancel each other out?) Below 1/2 wavelength in height, the antenna's feed point impedance steadily decreases until it is close to zero at ground level Above 1/2 wavelength the impedance varies (as suggested by Figure 7.5) eventually reaching a stable value at a height of several wavelengths.
007-6) Effects of Height Above Ground Figure 7.5
Figure 7.5 -- plotting Height of Horizontal Half-Wave in Wavelengths (x-axis) against Impedance in Ohms (y-axis) the feed point impedance of a horizontal dipole over the perfect ground varies dramatically with height. a) At ground level (height = 0.0), the impedance is 0 Ohms, and the antenna is effectively "shorted out" by it's electrical image. b) As the antenna is raised, as measured in Horizontal Half-Wave Wavelengths, not distance, the impedance gradually approaches the 72-Ω, feed point impedance of a DIPOLE in free space when Horizontal Half-Wave Wavelengths equals 1.0 or one complete 1/2 wave wavelength
007-6) Effects of Height Above Ground Figure 7.6
Figure 7.6 Shows what happens when a DIPOLE is raised in steps from a very low height to more than one wavelength (1/2λ, 3/4λ and 1λ above the ground. Figure 7.6 -- As a low DIPOLE starting at 1/8 wavelength above ground is raised, the effects of it's electrical ground image cause the elevation pattern to flatten out. At multiples of 1/2 wavelength in height, the pattern has a null in every direction.
07-17) Feed Lines - Characteristic Impedance Cont'd
Flat Ribbon TV-type Twin Lead has a characteristic impedance of 300Ω Amateurs also use parallel-conductor FEED LINES (also called OPEN-WIRE and LADDER or WINDOW LINE) that have impedances of 300 - 600 ohms. In a coaxial FEED LINE, Z0 is determined by the diameters of the inner and outer conductors and the spacing between them. (Note: another explanation of Z0 is: The CHARACTERISTIC IMPEDANCE is the square root of the ratio of the INDUCTANCE to CAPACITANCE. BOOK: The characteristics of the insulating material has some effect on characteristic impedance, but has a larger affect on FEED LINE LOSS and the VELOCITY of PROPAGATION. The most common characteristic impedance for coaxial FEED LINEs used by amateurs are 50Ω and 75Ω
007-8) Random Wires
For portable operation and in other special circumstances, a RANDOM WIRE antenna can be used. The feed point impedance and radiation pattern are unpredictable. A true RANDOM WIRE is connected directly to the output of the transmitter or antenna tuner WITHOUT a feed line. Using this type of an antenna may result in significant RF current sna voltages on the station equipment that could cause RF burns. Nevertheless, this simple antenna can give excellent results on any band for which the transmitter of tuner can accept the feed point impedance.
YouTube Antenna front to back ratio using a Network Analyzer
Front-to-back ratio is a comparison of the signal that comes off the front of the antenna to that of the back of the antenna. You are determining INSERTION LOSS. You set the test exactly the same way as if you were measuring Beam Width. Point the Yagi directly at a second antenna that will be receiving your test signals. The test signal travels from the output port on the NA to the Yagi, goes through space to the receive antenna and back to the input port of the NA. In this way you're comparing the signal that comes out of the Yagi to that received by the RX antenna. To establish our reference point, calibrate the NA to 0 dB so that the strength of the output signal out equals the strength of the input signal. In this way signal strength off the FRONT of the antenna is 0 dB. Rotate the Yagi antenna 180 degrees and retest. In this way the receive antenna is now receiving a signal from the back of the Yagi antenna The INSERTION loss will be NEGATIVE because the greatest signal you're going to get is when the Yagi is pointed directly at the receive antenna, not the other way around. Lets pretend for this Yagi antenna the INSERTION loss in db is -17 dB. In this case the front-to-back ration is 17 dB - meaning the signal off the front of the antenna is 17 dB stronger than the back of the antenna. ####################################### Source: https://youtu.be/dmPGLeB1vCU
007-4) Ground-planes Cont'd
GROUND PLANES are often called simply "verticals" because that is the usual way of constructing and installing them. If installed vertically, this means that the GROUND PLANE antenna is OMNIDIRECTIONAL. This is a very useful characteristic for VHF and UHF communications while MOBILE or PORTABLE and for an HF antenna where signals may come from all directions. Feed Point impedance at the base of the ideal GROUND PLANE is 35Ω, half of the complete DIPOLE'S impedance, because only half of the antenna is physically there and able to radiate energy. Sloping the radials of an elevated GROUND PLANE antenna DOWNWARD raises the feed point impedance. A droop angle of between 30 and 45 degrees as shown in FIGURE 7.4 results in the feed point impedance increasing to approximately 50Ω which matches coaxial cable nicely.
007-4) Ground-planes Sloping Angles Cont'd
If the droop angle continues to increase, (beyond 35 and 40 degrees) the antenna functions more and more like a physical DIPOLE and the feed point impedance eventually reaches 72Ω of the DIPOLE. Also, as with a DIPOLE, moving the feed-point away from the base - the midpoint of the combined physical antenna and its mirror image -- raises the impedance. As for the DIPOLE antenna, it is not useful to provide a one-size-fits all formula for the length of a GROUND PLANE antenna. Since the GROUND PLANE is one-half the size of a DIPOLE, start with one-half the FREE SPACE length. (246/f MHz)
007-8) Yagi Antennas Figure 7.7
If two antenna elements are separated by more than a small fraction of a second of a wavelength, the difference in travel time to a distant antenna from each are enough to result in cancelation that varies with the position of the distant antenna. Figure 7.7 shows an example of cancellation for a pair of dipole antennas. In a DRIVEN array power is applied to all of the elements, such as in Figure 7.7. The radiated fields from each antenna add and subtract at different angles around the antennas so that lobes and nulls are formed. In a PARASITIC array, the antenna elements are close together that the energy form the DRIVEN element induces a current to flow in the PARASITIC element. That current radiates a field -- called RE-RADIATION -- just as if it had been supplied by the FEED LINE. By careful placement and tuning of the PARASITIC and DRIVEN elements, a DIRECTIONAL antenna pattern can be created.
07-17) Feed Lines - Technician Class Review - Characteristic Impedance
Impedance with regard to transmission lines and electromagnetic fields is defined in terms of where it is applied. On a transmission line, such as coaxial cable, twin lead, or open-wire, three definitions of impedance are: a) Characteristic impedance (Tech and General) b) Intrinsic impedance c) Wave impedance Characteristic impedance (Z0) is defined as the ratio of voltage to current on a transmission line. It is a property related to the construction and dimensions of the transmission line. ######################################## FEED LINES have a Characteristic impedance, denoted (Z0), a measurement of how [electromagnetic] energy is carried by the FEED LINE. This is not the same as the resistance of the conductors if measured from end-to-end of the FEED LINE. It is the dimensions of FEED LINE conductors, the spacing between them, and the insulating material that determines Characteristic impedance. a) Most coaxial cables used in ham radio has a Characteristic impedance of 50 ohms. b) Most coaxial cables used for video and television has a Characteristic impedance of 75 ohms. c) Open wire feed lines have a Z0 of 300 to 600 ohms.
007-2) Dipoles cont'd: Resonant or Not
In FREE SPACE, 1/2 wavelength in feet equals 492/f (MHz). However if you cut a piece of wire to that length you will generally find it too long to RESONATE at that frequency. At RESONANCE, a 1/2 wave dipole made of ordinary wire will be SHORTER that the FREE SPACE 1/2 wavelength for several reasons. a) The physical thickness of the wire makes it look a bit longer electrically than it is physically. The lower the LENGTH-TO-DIAMETER (l/d) ratio of the wire, the shorter it will be when it is resonant. b) The DIPOLES height above the ground also affects the antennas resonant frequency. c) Nearby conductors, insulators on the wire, means by which the wire is secured to the insulators, and the feed line also affect RESONANT LENGTH. For these reasons, a single universal formula for DIPOLE length, such as the common 468/f (MHz) IS NOT VERY USEFUL!!! SOLUTION - You should start with the length near the FREE-SPACE length and be prepared to trim the DIPOLE to RESONANCE using an SWR meter or antenna analyzer. ####################################### REMEMBER: c) Feed Point Impedance - Is the ratio of RF VOLTAGE to CURRENT at the antenna's feed point. d) Resonant - an antenna is resonant when it's feed point impedance is completely resistive with no reactance. [note different definitions of impedance]
07-12) Loop Antennas Cont'd
Loops can be used in array's (an antenna with more than one element is called an array), in fact a popular variation of the Yagi beam uses quad loops for elements. This type beam antenna is called a QUAD. The QUAD or DELTA LOOP beam driven elements are approximately 1 λ in circumference and operate on the same principle of re-radiation as phase shirt as does the Yagi. The DRIVEN ELEMENT of a: a) Quad loop is about 1/4 wavelength per side b) Symmetrical Delta loop is about 1/3 wavelength per side. QUAD and DELTA LOOP reflectors are about 5% longer in circumference than the DRIVEN ELEMENT and the DIRECTORS are about 5% shorter than the DRIVEN ELEMENT. Quad antenna is mechanically more complex that a Yagi, requiring cross-arms to hold the loop. It has more surface area and wind loading than a Yagi. Quad and Delta Loop beams with the SAME NUMBER OF ELEMENTS HAVE ABOUT THE SAME GAIN.
007-4) Mobile HF Antennas
Mobile HF antennas are often some form of GROUND PLANE antenna. The most popular mobile antenna by far is the vertically - oriented WHIP - a thin steel rod mounted over the conducting surface of the vehicle, giving OMNIDIRECTIONAL coverage. A full-size 4/λ mobile WHIP antenna is not feasible on the HF bands below 10 meters. (recall from our example the length of an antenna in the 10 meter band is about 9 feet) - so loading techniques are used to increase a physically small antennas electrical size. While loading can cause an antenna to present reasonable feed point impedances, a LOADED antenna is not as efficient as a full-sized straight WHIP and will have a small operating bandwidth without RETUNING. RETUNING - The "screwdriver" antenna, a whip with an adjustable loading coil in the base, has gained popularity for HF mobile operations.
07-10) Impedance Matching - Gamma Match
Most Yagi antenna designs that have desirable radiation patterns have a FEED POINT IMPEDANCE below the 50 Ω of regular coaxial cable; typically the FEED POINT IMPEDANCE is 20 to 25 Ω. This results in an undesirable SWR of greater than 2:1. To change the FEED POINT IMPEDANCE back to 50 Ω, various impedance matching techniques are used. The most common technique is the GAMMA MATCH shown in Figure 7.9. The GAMMA MATCH is actually a short section of parallel-conductor transmission line that uses the driven element as one of its conductors. The purpose of the transmission line it to transform the low impedance of the FEED POINT (typically 20 to 25Ω ) up to a higher value (50Ω in our case). You'll need an adjustable capacitor -- either an actual variable capacitor or a short piece of INSULATED wire inside a hollow gamma rod -- is used to adjust the GAMMA MATCH for an SWR of 1:1.
Angle of Radiation - What is that?
One of the more important performance characteristics of an antenna system is its angle of radiation. Angle of radiation is not built into an antenna, you the amateur make that happen by the placing your antenna at the proper height Angle of radiation when referring to antennas, is simply the take off angle of the RF field when launched from your antenna in relation to the ground (earth). That is, if your dipole antenna is low to the ground (< 1/2 wavelength), in relation to its frequency of operation, the angle of radiation from the dipole will be at or close to 90 degrees - straight up and the dipole will behave as an omni-directional antenna. The higher you place your antenna above ground, the lower the radiation angle. A height is finally reached (depending on installation), when the "Take off" angle (Magic Height) is in the 20 degree range or lower. A radiation angle of 20 degrees or lower is an ideal angle for working long range DX. This means, the major lobe of your RF energy is radiated at an angle of 20 degrees in relation to the horizon. The horizon being zero. The "Magic" antenna height is generally achieved when your antenna is 1/2 wavelength above ground assuming a perfectly conducting ground (earth) How do you know when you are about 1/2 wavelength above your ground? Simple math. Take 468 and divide it by the frequency you intend to operate. Again assuming perfect ground, this number just happens to be the same number you would use to cut a resonant dipole to length. This begs the question, "Why should I bother to achieve a low angle of radiation". If you want the strongest signal possible at a distant point, a low angle of radiation is essential https://www.eham.net/articles/7684
007-2) See Figure 7.1
Part A. shows the radiation pattern in the plane of a DIPOLE located in free space. The DIPOLE element is located on the line from 270 to 90 degrees in this figure. Part B. shows the three-dimensional radiation pattern in all directions around the pole.
007-7) Effects of Polarization
Polarization also affects the amount of SIGNAL LOST from the resistance of the ground. Radio waves reflecting from the ground have lower losses when the polarization of the wave is parallel to the ground. That is, when the waves are horizontally polarized. Because the antenna's radiation pattern is made up of the reflected waves combing with the direct waves that are not reflected, LOWER REFLECTION LOSS results in stronger maximum signal strength. Ground mounted vertical antennas, however, are able to generate stronger signals at low angles of radiation that horizontally polarized antenna at low heights. [think about this statement] ######################################## Antenna height is generally achieved when your antenna is 1/2 wavelength above ground assuming a perfectly conducting ground (earth) How do you know when you are about 1/2 wavelength above your ground? Simple math. Take 468 and divide it by the frequency you intend to operate. [A 80 meter 1/2 wave distance would be 33 feet off the ground] ######################################## This means that they are often preferred for DX contacts on the lower HF bands where it is impractical to raise horizontally polarized antenna to the height necessary for strong low-angle signals.
07-13) Loop Antennas Polarization
Polarization of a Loop that is oriented horizontally is always horizontal, no matter where the FEED POINT is located. Polarization of a Loop that is oriented vertically depends on where the FEED POINT is attached.
007-6) Effects of Height Above Ground - Impact on Radiation Patterns
Radiation patterns are affected by the height of the antenna above the ground because -- of reflection of the antenna's radiated energy by the ground. a) The actual radiation pattern is composed of energy received directly from the antenna and energy that has been reflected by the ground. b) The direct and reflected signals take different amounts of time to travel to the receiving antenna so they can add together, cancel each other out, or any combination. This creates a new pattern of lobes and nulls not present for an antenna in free space. c) Figure 7.6 Shows what happens when a DIPOLE is raised in steps from a very low height to more than one wavelength (1/2λ, 3/4λ and 1λ above the ground. 1) At heights below 1/2 wavelength, the DIPOLES pattern is almost omnidirectional and is maximum straight up. 2) As a height of 1/2 wavelength is reached, the reflected and directed energy cancel in the vertical direction and add together at intermediary angle, creating a pattern of peaks and nulls in the radiation pattern for the antenna. Thus selecting the proper antenna height is important in achieving the desired goals for the antenna.
007-8) Yagi Antennas - How they Work Cont'd Killer Video: https://youtu.be/IKK17l2S2lU
Single Radiating Element - While the DIPOLE, Ground-Plane, and RANDOM wire use a single radiating element. Yagi uses more than one radiating element. The Yagi is an example of an ARRAY antenna, in which one or two or more elements are used to create maximum field strength in a specific direction., called the MAIN LOBE or MAJOR LOBE of the radiation pattern. There are two types of arrays: a) Driven - In a DRIVEN array, all of the antenna elements are connected to the transmitter and are called DRIVEN ELEMENTS. b) Parasitic - In a PARASITIC array, one or more of the elements are not connected to the feed line but influence the antenna's pattern by interacting with the radiated energy from the driven element. Whether an array is a DRIVEN or PARASITIC array, its radiation pattern is determined by CONSTRUCTIVE and DESTRUCTIVE interference. When two waves interfere with each other, they can: a) Reinforce each other if they are in phase and b) Cancel if they are out of phase. Partial cancellation occurs otherwise.
07-17) Figure 7.16
Some common types of PARALLEL conductor and coaxial cables used by amateurs. PARALLEL conductor line (A, B, C) has TWO PARALLEL conductors separated by insulation (dielectric). Coaxial cable (D, E, F, G) has a center conductor surrounded by insulation. The second conductor, called the SHIELD, covers the insulation and is in turn covered by the plastic outer jacket. ######################################## Source: Ask Dave - All About Baluns https://youtu.be/GMeOMwf2DJU
007-4) Ground-planes
The GROUND PLANE antenna is one-half of a DIPOLE with the missing portion made up by an electrical mirror, called the GROUND PLANE. The GROUND PLANE can be made from a sheet of metal or screen of radial wires. The basic GROUND PLANE is 1/4-wavelength (λ/4 ) long with the feed point at the junction of the antenna and the GROUND PLANE. Currents in the GROUND PLANE create the effect of an electrical image of the physical portion of the antenna as shown in Figure 7.3 For HF GROUND PLANE antennas mounted at ground level, the radial wires are laid on the surface of the ground or buried within a few inches of the surface.
007-2) Dipoles
The most fundamental antenna is a DIPOLE (from "two electrical polarities") -- a straight conductor the is 1/2 wavelength (λ/2) long with its feed point in the middle. (do we assume the antenna is resonant?) [Notice that no orientation is mentioned] A dipole radiates its strongest signal broadside to its axis and weakest off the ends. See Figure 7.1 This "Figure-eight" is the shape of the azimuthal pattern for a dipole in free space.
007-9) Yagi Structure and Function cont'd
The simplest two-element Yagi consists of a driven element and a reflector The DRIVEN element (DE) is a resonant DIPOLE, approximately 1/2 wavelength long. The reflector is slightly longer that the DE by about 5% and placed about 0.15 to 0.2 wavelengths behind the DE, opporist the direction of maximum signal HOW IT WORKS - The original signal from the DE travels to the reflector where it causes current to flow, re-radiating a signal. Re-radiated signals are 180 degrees out of phase with the original signal, so the re-radiated and DE signals cancel in the direction of the reflector (to the back of the antenna). To the front of the antenna the extra travel time for the re-radiated signal from the reflector causes it to reinforce the DE signal.
07-10) Design Tradeoffs
There are many ways to optimize the design of an actual antenna to fit a specific need. For example: a) Is it more important to have maximum gain or the best front-to-back ratio b) How much SWR is allowed across the entire band...... Making a "one-size-fits-all" antenna is quite difficult. The primary variables for Yagi antennas are: 1) Length and diameter of each element 2) Placement of the element(s) along the boom of the antenna. These variables..... 1. Gain 2. SWR - 3. Front-to-back ratio Affect the following variables in different ways: a) - More directors increase gain. b) - A longer boom with a fixed number of directors increases gain up to a maximum length beyond which GAIN is reduced. c) Larger diameter elements reduce SWR variation with frequency increase (increases SWR bandwidth). d) Placement and tuning of elements affects GAIN and feed point impedance (and SWR). To be sure, there are other general rules of cause-and-affect, but these are typical of decisions that antenna designers (and purchasers) should consider.
07-11) Impedance Matching - Other Methods of Impedance Matching.
There are other techniques of impedance matching Yagi antenna, such as: the BETA MATCH (or "hair pin"), the OMEGA MATCH, Impedance Transformers, and Transmission Line Stubs. You could also use relatively large diameter elements so that the FEED POINT IMPEDANCE is close to 50Ω without ANY external matching devices.
007-7) Aiming Antennas (sidebar)
There are two reasons you want to be able to aim an antenna; a) To be heard better by a desired station. b) To hear the desired station better. The radiation pattern of a unidirectional antenna like a Yagi usually has one main lobe and at least three nulls -- two side nulls and one rear null. The device that does the actual mechanical moving is called a ROTATOR (not rotor). Main lobe is along the boom of the antenna. The side nulls are will be 90 degrees to either side where the ends of the elements face the signal. Rear lobe is usually aligned directly opposite the main lobe. To be better heard by the station you're contacting means that you need to point the main lobe at the station. To aim the antenna accurately if that station is BEYOND YOUR LINE OF SIGHT you'll need a special kind of map called an AZIMUTHAL PROJECTION map. An AZIMUTHAL PROJECTION map shows the world squashed into a circle centered on a particular location (such as your station) so that the paths to all other locations are shown as great circle paths, given the TRUE BEARING AND DISTANCE to any other point. ######################################## You can request an Azimuthal map for any location here: https://ns6t.net/azimuth/azimuth.html
07-10) Design Tradeoffs Cont'd
To be sure there are other general rules of cause-and-effect, but these (those mentioned above) are typical of the decisions that antenna designers (and purchasers) should consider
007-3) Dipoles cont'd: Center Fed
a) Center fed DIPOLES are easiest to use on the band of which they are resonant. b) The feed point impedance of such an antenna is a good match for the 50 or 74-Ω coaxial cable that most hams use. c) The feed point impedance of a half-wave DIPOLE is also a good match for coax on odd multiples of the fundamental frequency. For example, a DIPOLE for the 40 meter band (7 MHz) can also be used on 15 meters (21 MHz). On its third harmonic the DIPOLE "looks like" the three half wave DIPOLES in Figure 7.2 connected end-to-end. d) On even numbered harmonics and non-resonant bands, the feed point impedance of the DIPOLE can be high, just as it is near the antenna's end.
007-1) Antenna Basics - Technician Class Review
a) Elements - are the conducting portions of an antenna that radiate or receive a signal. b) Polarization - refers to the orientation of the electric field radiated by the antenna and is determined by the physical orientation of the elements with respect to the earth's surface: 1) If an element is horizontal, then the signal it radiates is HORIZONTALLY POLARIZED. c) Feed Point Impedance - Is the ratio of RF VOLTAGE to CURRENT at the antenna's feed point. d) Resonant - an antenna is resonant when it's feed point impedance is completely resistive with no reactance. e) Radiation Pattern -.is a graph of signal strength in EVERY direction or at EVERY vertical angle f) Azimuthal Pattern - shows signal strength in horizontal directions g) Elevation - shows signal strength in vertical directions An antenna transmits and receives with the same pattern. h) Lobes - are regions in the radiation pattern where the antenna is radiating a signal I) Nulls - are the points at which radiation is at a minimum between lobes. J) Isotropic - antenna radiates equally in every possible direction, horizontal and vertical Isotropic antennas do not exist in practice and are only used as a reference. K) Omnidirectional - antenna radiates a signal of equal strength in every HORIZONTAL direction. L) Directional - antenna radiates preferentially in one or more direction. M) Gain - concentrating transmitted or received signals in a specific direction. 1) Signal strength is increased in that direction for both receiving and transmitting. 2) Antenna GAIN is specified in decibels (dB) with respect to an identified reference antenna. 3) Gain with respect to an isotropic antenna is called dBi 4) Gain with respect to a dipole antenna's maximum radiation is called dBd You can convert dBd to dBi by adding 2.15 dB and from dBi to dBd by subtracting 2.15 dB. N) Front-to-Back Ratio (F/B) - is the gain in the preferred or forward direction to the opposite direction. All ratios are measured in dB. O) Front-to-Side Ratio (F/S) - is the gain in the preferred or forward direction to directions at right angles. All ratios are measured in dB.
007-9) Yagi Structure and Function
a) The Yagi is a PARASITIC array with a single DRIVEN element and at least one PARASITIC element as shown in Figure 7.8 (there is always a DRIVEN element with the PARASITIC element being either a DIRECTOR, or a REFLECTOR.) b) The elements are physically arranged to create gain in a single MAJOR or MAIN LOBE and cancel signals in opposite directions. c) The PARASITIC elements placed in the direction of maximum gain are called DIRECTORS and are slightly shorter than the driven elements. d) The PARASITIC elements placed in the direction of minimum gain are called REFLECTORS and are slightly longer than the driven elements. (remember: long reflectors and short directors) e) For a Yagi antenna, the front-to-back ratio is the ratio of signal strength at the peak of radiation pattern's major lobe to that in exactly the opposite direction. ######################################## This is parameter is important in circumstances where interference or coverage in the reverse direction needs to be minimized - YOU WANT to eliminate noise from side and back of the antenna and maximize gain along the forward direction of the boom.
