The decibel. Decibel calculation. Decibel scale. Rules of thumb. Decibel units of measure

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1 Tiiti Kellomäki, OH3HNY 1.Decibel 2. Transmission lines, SWR 3. Antenna properties 4. Some common antennas The decibel Decibel scale All circuits either amplify or attenuate power. A = P o /P i = B = P o /P i = As the power ratios can be very large or very small, it is handy to use a logarithmic scale, the decibel scale. A(dB) = 10 log P o /P i = 10 log = 50 db B(dB) = 10 log P o /P i = 10 log = 37 db Positive decibel readings indicate gain and negative values attenuation. Decibel calculation Multiplication of power ratios corresponds to addition of decibel values. If the first block amplifies by 3 db and the second one attenuates by 10 db, the whole circuit has an effect of 7 db, or the output is one fifth of the input. P o /P i = P o /P a P a /P i = = 0.2, or 7 db. Decibel units of measure Decibel readings always mean power related to some known level, e.g. input power, carrier power, or noise level. A commonly used db unit is dbm "decibels over one milliwatt". One watt in dbm is 10 log 1 W / 1 mw = 10 log 1000 = 30 dbm. One milliwatt is no more than one milliwatt, hence the ratio is one and 1 mw in dbm is 10 log 1 = 0 dbm. Rules of thumb Negative decibels are for attenuation, positive for amplification. Adding 0 db is the same as multiplying by one. Adding 10 db is multiplication by 10. Adding 3 db is multiplication by 2. 7 db = 10 db 3 db, or 10 ½ = db = 10 db + 10 db + 10 db 3 db 3 db, or /2/2 =

2 Transmission lines Transmission lines Because the wavelength of a RF signal is short (say, 80 m to 23 cm in your normal frequency range, or millimeters), cable lengths are large in terms of wavelengths. Signal voltage level varies rapidly in time and space. Wires must be thought of as transmission lines. Characteristic impedance Characteristic impedance tells the ratio of voltage to current (or electric to magnetic field) on the line. On a 50-ohm line, a 1-volt signal will be a 20-milliampere signal. This impedance is not related to loss. 50 ohms is most often used in amateur radio. 75-ohm coaxial cable is used in tv networks. Mismatch If the characteristic impedance of the transmitter, the cable, and the antenna are not the same, power is not delivered properly. Power is reflected in every impedance discontinuity. At some points, the forward and reflected voltages will add, and at some points cancel each other. Hot and cold spots are formed on the cable. Standing wave ratio Standing wave ratio is the radio of the maximum and minumum voltage on the line. SWR = 1 means that there is no standing wave, thus no reflections, and all the power is delivered properly. SWR = 2 means that 10 % of the power is reflected at the end. SWR = 3 means that 25 % of the power is reflected at the end. SWR = means that no power is delivered to the load. To avoid reflections, impedance can be matched so that the transmitter sees a 50-ohm load. Coaxial cable Coaxial cable is formed of two coaxial tubes. Between the outer and the inner conductor there is some dielectric material. All electric and magnetic fields are inside the cable. Coaxial cable is not affected by nearby metallic objects or slight bending. 50-ohm coax is the most common transmission line used by radio amateurs. 2

3 Two-wire lines The electric and magnetic fields of a two-wire transmission line spread into the surroundings of the line. Two-wire lines radiate and cannot be used above HF. Nearby metallic objects affect the performance of twowire lines. In twinlead, there is insulating material between the wires. Its characteristic impedance is often 240 or 300 ohms. Open wire is essentially air-insulated and of 450 ohms. Symmetry in transmission lines If one of the conductors on a line can be thought of as a ground, the line is unbalanced. Coaxial cable is unbalanced. If the lines are identical, the line is balanced. Two-wire lines are balanced. Symmetrical antennas need to be connected to symmetrical lines or else a balun (balanced-tounbalanced transformer). Antennas Antennas Antenna is the part of an electromagnetic system that either transforms energy from current and voltage into electromagnetic radiation or vice versa. An antenna has the same properties regardless of whether it is used as a transmitting or a receiving antenna. Antenna as a load When an antenna is connected to a transmitter, the radio sees it as a load, an impedance. The antenna input impedance is strongly dependent on frequency. At resonance the impedance is real (resistance). Antenna bandwidth may be defined as the bandwidth where the reflected power due to impedance discontinuities is sufficiently small, or in other words, the SWR is small enough (say, 2 or 3). Some useful antenna properties Radiation pattern is a plot of radiated power in different directions. An isotropic antenna radiates equally in all directions. Directivity tells how efficiently the antenna radiates in the maximum direction, compared to the isotropic antenna (dbi). Directivity can also be expressed in db compared to a half-wave dipole (0 dbd = 2 dbi). 3

4 Some useful antenna properties Gain is the same thing but multiplied by efficiency. It is the property of a real antenna. Gain is also expressed in dbi or dbd. Efficiency tells the ratio of radiated power to power delivered to the antenna. If the efficiency is 73 %, 27 % of the delivered power is lost as heat because the antenna is made of lossy materials. If the directivity is 10 dbd and efficiency 50 %, the gain is 10 dbd 3 db = 7 dbd (one half of the original). Dipoles A half-wave dipole is the simplest antenna. The dipole is commonly used in HF. You need a balun to feed it properly. A half-wave dipole has an input impedance of 73 ohms, so you can connect it directly to a 50-ohm radio. The bandwidth covers any one HF band. The half-wave dipole gain is 2 dbi = 0 dbd (small). A dipole can be of any length, e.g. λ, 2λ Dipoles are balanced. Monopoles Images of monopoles and dipoles A monopole is half a dipole used with a ground plane. The monopole and its mirror image form a dipole. Common sizes are λ/4 (gain 0 dbd), 5λ/8 (gain 2 dbd), ½λ and one λ. The longer the monopole, the higher the gain. Monopoles are commonly used in HF, VHF, UHF, and in handheld devices. Monopoles are unbalanced. 5λ/8 radiation pattern folded dipole λ/4 radiation pattern dipole radiation pattern Loops Yagis directors boom radiator The most common loop size is one wavelength. The loop radiates in the direction of the hole. Loops are used in HF. The exact shape of the loop is not important, just set up some wire in trees. The loop has a small gain, 1 dbd. Loops are balanced. The Yagi-Uda antenna is a dipole with some additional elements. The (half-wave) dipole part radiates, behind it is a long reflector element, and in front are one or more directors. The yagi is a directive antenna with a gain of 2 to 20 dbd. To achieve more gain, the number of elements must be increased. reflector 4

5 Yagis Yagis are used in HF above 14 MHz, and up to 1200 MHz. Yagis need to be carefully designed and fed. Two or more yagis can be stacked in order to achieve gain: if you place them side by side, the horisontal lobe will narrow, one upon the other narrows the vertical lobe. Quads A quad is like a yagi but formed of loops instead of dipoles. You can place loops of different sizes within each other without problems. Quads are used in HF and VHF. A quad may have a gain of 4 to 10 dbd. Helices A helix can be used in two ways: if the diameter is small compared to a wavelength, the helix will radiate like a dipole, if the diameter is a large fraction of a wavelength, the helix radiates axially and its polarisation is circular. The former type is used in handheld radios. The latter is used in UHF and up. Reflector antennas To achieve large gains, reflector antennas can be used. The diameter of the reflector must be several wavelengths. Reflector antennas are used in microwave frequencies, SHF. A gain of 30 dbi may be achieved. Exam questions You are constructing a 432 MHz transmitting antenna. Which one is correct?? 43 cm half-wave dipole? 34 cm half-wave dipole? 43 cm 5/8-wavelength monopole? 34 cm monopole and ground plane Exam questions You are constructing a 432 MHz transmitting antenna. Which one is correct? 43 cm half-wave dipole + 34 cm half-wave dipole + 43 cm 5/8-wavelength monopole 34 cm monopole and ground plane 5

6 More exam questions Coaxial cable has the property? not to radiate because the electromagnetic field stays inside the cable? its characteristic impedance is affected by the distance between the conductors? it can be mounted on a metallic roof? it can be bent with a sharp radius (minimum radius 5D) More exam questions Coaxial cable has the property + not to radiate because the electromagnetic field stays inside the cable + its characteristic impedance is affected by the distance between the conductors + it can be mounted on a metallic roof it can be bent with a sharp radius (minimum radius 5D) Questions? Feel free to ask further questions. Check the nice antenna books at the club room: ARRL Antenna Book Antennisuunnittelu-kurssin pruju Simple and Fun Antennas for Hams 6