3 basic antenas1

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basics of antenna

Transcript of 3 basic antenas1

  • 1.1 Key Points 1. Principals of EM Radiation 2. Introduction to Propagation & Antennas 3. Antenna Characterization

2. 2 1. Principals of Radiated electromagentic (EM) fields two laws (from Maxwell Equation) 1. A Moving Electric Field Creates a Magnetic (H) field 2. A Moving Magnetic Field Creates an Electric (E) field 3. 3 c 3 108 m/s l = /2: wave will complete one cycle from A to B and back to A = distance a wave travels during 1 cycle f = c/ = c/2l l = /2 A B Assume i(t) applied at A with length l = /2 EM wave will travel along the wire until it reaches the B B is a point of high impedence wave reflects toward A and is reflected back again resistance gradually dissipates the energy of the wave wave is reinforced at A results in continuous oscillations of energy along the wire and a high voltage at the A end of the wire. An AC current i(t), flowing in a wire produces an EM field 4. 4 Dipole antenna: 2 wires each with length l = /4 attach ends to terminals of a high frequency AC generator at time t, the generators right side = + and the left side = electrons flow away from the terminal and towards the + terminal most current flows in the center and none flows at the ends i(t) at any point will vary directly with v(t) current distribution at time t + i(t) l = /4 A B + ++++ +++++++ +++++++++++ +++++++++++++++ +++++++++++++++ ----- ----------- ---------------- -------------------- ------------------------ voltage distribution at time t A B cycle after electrons have begun to flow max number of electrons will be at A and min number at B vmax(t) is developed i(t) = 0 5. 5 EM patterns on Dipole Antenna: sinusoidal distribution of charge exists on the antenna that reverses polarity every cycle sinusoidal variation in charge magnitude lags the sinusoidal variation in current by cycle. Electic field E and magnetic field H 90 out of phase with each other fields add and produce a single EM field total energy in the radiated wave is constant, except for some absorption as the wave advances, the energy density decreases Standing Wave center of the antenna is at a low impedance: v(t) 0, imax(t) ends of antenna are at high impedence: i(t) 0, vmax(t) maximum movement of electrons is in the center of the antenna at all times Resonance condition in the antenna waves travel back and forth reinforcin maximum EM waves are transmitted into at maximum radiation 6. 6 POLARIZATION EM field is composed of electric & magnetic lines of force that are orthogonal to each other E determines the direction of polarization of the wave vertical polarization: electric force lines lie in a vertical direction horizontal polarization : electric force lines lie in a horizontal direction circular polarization: electric force lines rotate 360 every cycle An antenna extracts maximumenergy from a passing EM wave when it is oriented in the same direction as E use vertical antenna for the efficient reception of vertically polarized waves use horizontal antenna for the reception of horizontally polarized waves if E rotates as the wave travels through space wave has. horizontal and vertical components 7. 7 Ground wave transmissions missions at lower frequencies use vertical polarization horizontal polarization E force lines are parallel to and touch the earth. earth acts as a fairly good conductor at low frequencies shorts out vertical electric lines of force are bothered very little by the earth. 8. 8 Types of antennas simple antennas: dipole, long wire complex antennas: additional components to shape radiated field provide high gain for long distances or weak signal reception size frequency of operation combinations of identical antennas phased arrays electrically shape and steer antenna 2. Introduction to Antennas and Propagation transmit antenna: radiate maximum energy into surroundings receive antenna: capture maximum energy from surrounding radiating transmission line is technically an antenna good transmission line = poor antenna 9. 9 Major Difference Between Antennas And Transmission Lines transmission line uses conductor to carry voltage & current radio signal travels through air (insulator) antennas are transducers - convert voltage & current into electric & magnetic field - bridges transmission line & air - similar to speaker/microphone with acoustic energy Transmission Line voltage & current variations produce EM field around conductor EM field expands & contracts at same frequency as variations EM field contractions return energy to the source (conductor) Nearly all the energy in the transmission line remains in the system 10. 10 Antenna Designed to Prevent most of the Energy from returning to Conductor Specific Dimensions & EM wavelengths cause field to radiate several before the Cycle Reversal - Cycle Reversal - Field Collapses Energy returns to Conductor - Produces 3-Dimensional EM field - Electric Field Magnetic Field - Wave Energy Propagation Electric Field & Magnetic Field 11. 11 transmit & receive antennas theoretically are the same (e.g. radiation fields, antenna gain) practical implementation issue: transmit antenna handles high power signal (W-MW) - large conductors & high power connectors, receive antenna handles low power signal (mW-uW) Antenna Performance depends heavily on Channel Characteristics: obstacles, distances temperature, Signal Frequency Antenna Dimensions 12. 12 Propagation Modes five types (1) Ground or Surface wave: follow earths contour affected by natural and man-made terrain salt water forms low loss path several hundred mile range 2-3 MHz signal (2) Space Wave Line of Sight (LOS) wave Ground Diffraction allows for greater distance Approximate Maximum Distance, D in miles is (antenna height in ft) No Strict Signal Frequency Limitations rxtx hh 22 +D = hrx htx 13. 13 (3) Sky Waves ionosphere transmitted wave reflected wave refracted wave skip distance reflected off ionosphere (20-250 miles high) large ranges possible with single hop or multi-hop transmit angle affects distance, coverage, refracted energy 14. 14 Ionosphere is a layer of partially ionized gasses below troposphere - ionization caused by ultra-violet radiation from the sun - affected by: available sunlight, season, weather, terrain - free ions & electrons reflect radiated energy consists of several ionized layers with varying ion density - each layer has a central region of dense ionization Layer altitude (miles) Frequency Range Availability D 20-25 several MHz day only E 55-90 20MHz day, partially at night F1 90-140 30MHz 24 hours F2 200-250 30MHz 24 hours F1 & F2 separate during daylight, merge at night 15. 15 Usable Frequency and Angles Critical Frequency: frequency that wont reflect vertical transmission - critical frequency is relative to each layer of ionosphere - as frequency increases eventually signal will not reflect Maximum Usable Frequency (MUF): highest frequency useful for reflected transmissions - absorption by ionosphere decreases at higher frequencies - absorption of signal energy = signal loss - best results when MUF is used Frequency Trade-Off high frequency signals eventually will not reflect back to ground lower frequency signals are attenuated more in the ionosphere 16. 16 angle of radiation: transmitted energy relative to surface tangent - smaller angle requires less ionospheric refraction to return to earth - too large an angle results in no reflection - 3o -60o are common angles critical angle: maximum angle of radiation that will reflect energy to earth Determination of minimum skip distance: - critical angle - small critical angle long skip distance - height of ionosphere - higher layers give longer skip distances for a fixed angle multipath: signal takes different paths to the destination angle of radiation ionosphere Critical Angle 17. 17 (4) Satellite Waves Designed to pass through ionosphere into space uplink (ground to space) down link (space to ground) LOS link Frequencies >> critical frequency penetrates ionosphere without reflection high frequencies provide bandwidth Geosynchronous orbit 23k miles (synchronized with earths orbit) long distances result in high path loss EM energy disperses over distances intensely focused beam improves efficiency 18. 18 total loss = Gt + Gr path loss (dB) Free Space Path Loss equation used to determine signal levels over distance G = antenna gain: projection of energy in specific direction can magnify transmit power increase effective signal level at receiver 2 4 = c fd P P r t c fd4 log20 10 (dB) 19. 19 (5) radar: requires high gain antenna sensitive low noise receiver requires reflected signal from object distances are doubled only small fraction of transmitted signal reflects back 20. 20 3. Antenna Characterization antennas generate EM field pattern not always possible to model mathematically difficult to account for obstacles antennas are studied in EM isolated rooms to extract key performance characteristics absolute value of signal intensity varies for given antenna design - at the transmitter this is related to power applied at transmitter - at the receiver this is related to power in surrounding space antenna design & relative signal intensity determines relative field pattern 21. 21 forward gain = 10dB backward gain = 7dB +10dB +7dB + 4dB 0o 270o 180o 90o Polar Plot of relative signal strength of radiated field shows how field strength is shaped generally 0o aligned with major physical axis of antenna most plots are relative scale (dB) - maximum signal strength location is 0 dB reference - closer to center represents weaker signals 22. 22 radiated field shaping lens & visible light application determines required direction & focus of signal antenna characteristics (i) radiation field pattern (ii) gain (iii) lobes, beamwidth, nulls (iv) dire