Something about Antenna design


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Something about Antenna design

  1. 1. Mac 2013 ANTENNA DESIGN
  2. 2. INTRODUCTION A rough outline of some major antennas and their discovery / fabrication dates are listed:  Yagi-Uda Antenna, 1920s  Horn antennas, 1939. Interesting, the early antenna literature discussed waveguides as "hollow metal pipes".  Antenna Arrays, 1940s  Parabolic Reflectors, late 1940s, early 1950s? Just a guess.  Patch Antennas, 1970s.  PIFA, 1980s.Current research on antennas involves metamaterials(materials that have engineered dielectric and magneticconstants that can be simultaneously negative, allowing forinteresting properties like a negative index of refraction). Otherresearch focuses on making antennas smaller. Rectangular patch antenna Array
  3. 3. INTRODUCTIONIn 1913, the Eiffel Tower was used an antenna. Back when communication wasat very low frequencies, the antennas had to be very large to get any sort ofradiation. The Eiffel Tower fit this bill well, and was used to communicate withthe United States Naval Observatory in Arlington, Virginia.
  4. 4. How wave propagates ? Using Dipole antenna as an example Understand this : Electric field will produce magnetic field Changing in magnetic field will produce electric fieldThis process rotate continuously , thus creating waves.
  5. 5. Frequency bandSeveral standard for radio spectrum naming such as : ITU radio bands IEEE bands EU, NATO, US ECM waveguide frequency band Frequency Band Name Frequency Range Wavelength (Meters) Application Extremely Low Frequency 3-30 Hz 10,000-100,000 km Underwater Communication (ELF) AC Power (though not a Super Low Frequency (SLF) 30-300 Hz 1,000-10,000 km transmitted wave) Ultra Low Frequency (ULF) 300-3000 Hz 100-1,000 km Very Low Frequency (VLF) 3-30 kHz 10-100 km Navigational Beacons Low Frequency (LF) 30-300 kHz 1-10 km AM Radio Medium Frequency (MF) 300-3000 kHz 100-1,000 m Aviation and AM Radio High Frequency (HF) 3-30 MHz 10-100 m Shortwave Radio Very High Frequency (VHF) 30-300 MHz 1-10 m FM Radio Ultra High Frequency (UHF) 300-3000 MHz 10-100 cm Television, Mobile Phones, GPS Satellite Links, Wireless Super High Frequency (SHF) 3-30 GHz 1-10 cm Communication Extremely High Frequency 30-300 GHz 1-10 mm Astronomy, Remote Sensing (EHF) 400-790 THz 380-750 nm Visible Spectrum Human Eye (4*10^14-7.9*10^14) (nanometers) ITU radio bands
  6. 6. Field surrounding an antennaDivided into 3 principle region : 1. Reactive Near Field - E- and H fields are out of phase by 90 degrees to each other (recall that for propagating or radiating fields, the fields are orthogonal (perpendicular) but are in phase). 2. Radiating Near Field or Fresnel Region reactive fields are not dominate; the radiating fields begin to emerge. However, unlike the Far Field region, here the shape of the radiation pattern may vary appreciably with distance. Note that depending on the values of R and the wavelength, this field may or may not exist.
  7. 7. Field surrounding an antenna3. Far Field or Fraunhofer Regionthe most important region, determines the antennas radiation pattern, sothis is the region of operation for most antennasMust satisfied all these equation : R = distance D= antenna dimension /diameter λ = wavelength frequency λ= c /f = [speed of light]/[propagating frequency]
  8. 8. General effect of antenna size Small antenna will produce low directivity. Big antenna will produce high directivity if you use an antenna with a total size of 0.25 - 0.5 λ (a quarter- to a half-wavelengthin size), then you will minimize directivity. That is, half-wave dipole antennas or half-wavelength slot antennas typically have directivities less than 3 dB, which is about aslow of a directivity as you can obtain in practice. we cant make antennas much smaller than a quarter-wavelength withoutsacrificing antenna efficiency. for high directivity, well need antennas that are many wavelengths in size. That is,antennas such as dish (or satellite) antennas and horn antennas have high directivity, inpart because they are many wavelengths long.
  9. 9. Understand Efficiency, Directivity, Gainefficiency is defined as the ratio between the input and the output of such system. You could have an antenna that has high directivity, but, due to losses (conductor losses, dielectric losses, measured by the antenna efficiency or deficiency) your antenna sucks and the overall antenna radiation is not the one desired. Thats why we introduce the Gain
  10. 10. Understand Efficiency, Directivity, Gain
  11. 11. Understand Efficiency, Directivity, Gain The Gain is defined as the directivity of antenna after being affected by suchlosses. The gain is always related to the main lobe and is specified in the directionof maximum radiation unless indicated. An antenna with a gain of 3 dB means that the power received far from theantenna will be 3 dB higher (twice as much) than what would be received from alossless isotropic antenna with the same input power. In general gain is measured and directivity is calculated. To be more precise, the term ‘realized gain’ is sometimes used to differentiate the gain defines by IEEE.
  12. 12. Why we need matching circuit ?Matching is the process of removing mismatch loss due to wave travellingthrough different impedance.Reduce the power reflected from the load (the antenna) maximize the power delivered to the antenna.The reflection coefficient given by:
  13. 13. Why we need matching circuit ? Simulation shows a standing wave when 100% reflection. When reflection coefficient equal to 1
  14. 14. Why we need matching circuit ?Example : A 50 Ω transmission line is connected to a 30 Ω antenna . Calculatethe reflection coefficient. ZL Γ = (ZL-ZA) / (ZL+ZA) = (50-30)/(50+30) = 0.25 Picture above shows circuit model of an antenna connected to signal source, V. ZL = line characteristic impedance ZA = antenna impedance Notes: Uniform transmission line impedance do not dependent on its length, the value is the same no matter how long the transmission is. Therefore, it is known as characteristic impedance. However, the characteristic impedance depends on the line width, dielectric and propagating frequency. Transmission line normally affects the higher frequency which the transmission line is longer than the wavelength.
  15. 15. Why we need matching circuit ?Impedance normally consists of real and imaginary part.The real part of the antenna impedance represents power that is either radiatedaway or absorbed within the antenna.The imaginary part of the impedance represents power that is stored in the near fieldof the antenna.Both are non-radiated powers.Maximum power transfer occur when: Zs* (conjugate) If ZA=50-j20 , then Zs = 50 + j20
  16. 16. Bandwidth Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. Described in Return loss S11=20 log (0.2)= -13.98 dB.Take note :S11 is a measure of the reflection from an antenna. 0dB means that all the power is reflected, hence the matching is notgood. -10dB means that 10% incident power is reflected, meaning 90% power is accepted by the antenna. However, a goodS11 response does not necessarily mean the antenna is radiating. S11 is still typically used though to show an antennasresponse; the underlying assumption is that the losses are not so great. There are also other criteria which may be used to characterize bandwidth :  polarization over a certain range, for instance, an antenna may be described as having circular polarization with an axial ratio < 3dB (less than 3 dB) from 1.4-1.6 GHz. This polarization bandwidth sets the range over which the antennas operation is approximately circularly polarized.  Fractional Bandwidth (FBW). The FBW is the ratio of the frequecny range (highest frequency minus lowest frequency) divided by the center frequency. The antenna Q also relates to bandwidth (higher Q is lower bandwidth, and vice versa).
  17. 17. Several common antennas bandwidth
  18. 18. Beamwidth and sidelobes Figure shows the radiation pattern of an antenna main beam is the region around the direction of maximum radiation (usually theregion that is within 3 dB of the peak of the main beam). The beamwidth of the mainbeam is sometimes called Half Power Beamwidth (HPBW) just beamwidth. sidelobes are smaller beams that are away from the main beam. These sidelobes areusually radiation in undesired directions which can never be completely eliminated. Null to Null Beamwidth. This is the angular separation from which the magnitude ofthe radiation pattern decreases to zero (negative infinity dB) away from the mainbeam.
  19. 19. Polarization
  20. 20. PolarizationCircular polarization is a desirable characteristic for many antennas. Two antennas that are bothcircularly polarized do not suffer signal loss due to polarization mismatch. Antennas used in GPS systemsare Right Hand Circularly Polarized.Suppose now that a linearly polarized antenna is trying to receive a circularly polarized wave.Equivalently, suppose a circularly polarized antenna is trying to receive a linearly polarized wave.What is the resulting Polarization Loss Factor?Recall that circular polarization is really two orthongal linear polarized waves 90 degrees out of phase.Hence, a linearly polarized (LP) antenna will simply pick up the in-phase component of the circularlypolarized (CP) wave. As a result, the LP antenna will have a polarization mismatch loss of 0.5 (-3dB), nomatter what the angle the LP antenna is rotated to. Circular polarization
  21. 21. Friss Transmission EquationFriis Transmission Equation is used to calculate the power received from one antenna(with gain G1), when transmitted from another antenna (with gain G2), separated by adistance R, and operating at frequency, f or wavelength, λ. Received power, R Gain 1 Gain 2In general, for two linearly polarized antennas that are rotated from each other by an angle ø, the powerloss due to this polarization mismatch will be described by the Polarization Loss Factor (PLF). Friis Transmission Equation says that the path loss is higher for higher frequencies. The importance of this result from the Friis Transmission Formula cannot be overstated. This is why mobile phones generally operate at less than 2 GHz. There may be more frequency spectrum available at higher frequencies, but the associated path loss will not enable quality reception.However, lower frequency making the antenna bigger. The challenge for antenna designer is to build antenna for lower frequency with smaller size.
  22. 22. Antenna material (Radiator)High relative permittivity means that the material is magnetic which mean it attracts tomagnet. A positive relative permeability greater than 1 implies that the materialmagnetizes in response to the applied magnetic field. Generally these elements are notsuitable for making antenna/radiator parts.I can say that carbon (graphite) is also a good material of making antenna
  23. 23. Some of antenna design1. Dipole antenna Antenna length= 0.48λ Normally using 70Ω transmission line Can get higher gain with L= 3λ/2 Increase the BW by using thicker wire L
  24. 24. Some of antenna design2. Monopole antenna Monopole antenna is twice the directivity of dipole antenna
  25. 25. Some of antenna design3. Helical/ Helix antenna A wide bandwidth, is easily constructed, has a real input impedance, and can produce circularly polarized fields. Helix antennas of at least 3 turns will have close to circular polarization in the +z direction C=πD Typically, the pitch angle is taken as 13 degrees pitch angle,
  26. 26. Some of antenna design4. Yagi-Uda antenna Driven element normally used dipole antenna
  27. 27. Some of antenna design5. Planar Inverted-F Antenna (PIFA) Side view Top view λ/4 = L+W1-W2 Making shorter W2 will get lower bandwidth… having higher the value have higher bandwidth Create dual band antenna
  28. 28. Some of antenna design 6. Folded Inverted Conformal Antenna (FICA)PIFAs exhibit two resonant modes, which operate by sharing the same availableantenna volume, the FICA structure is synthesized in order to sustain three resonantmodes that better reuse the volume.
  29. 29. Some of antenna design 7. Parabolic Dish antennaDish diameter = 10 λ  50 λ (larger is better)G = ε(πD)2/ λ , ε = Aperture area efficiencyFocal point, F = 0.35D 0.7DParabolic formulax2= 4F (F-y), x<=D/2
  30. 30. Various antenna
  31. 31. Various antenna
  32. 32. Various antenna
  33. 33. Various antenna
  34. 34. Various antenna
  35. 35. Various antenna
  36. 36. Various antenna
  37. 37. Various antenna
  38. 38. Various antenna
  39. 39. Some research antenna Nuclear fusion antenna Metamaterial antenna
  40. 40. Measurements 1. Radiation pattern and gainWe are measuring the gain of antenna not the directivity.The gain, G of an antenna is an actual or realized quantity which is less than the directivity, D due toohmic losses in the antenna or its radome (if it is enclosed). In transmitting, these losses involvepower fed to the antenna which is not radiated but heats the antenna structure Measurement is done inside Anechoic chamber Equipment setting
  41. 41. Measurements 2. Polarization measurementTo perform the measurement, we will use our test antenna as the source. Then we will use a linearlypolarized antenna (typically a half-wave dipole antenna) as the receive antenna. The linearly polarizedreceive antenna will be rotated, and the received power recorded as a function of the angle of thereceive antenna. In this manner, we can gain information on the polarization of the test antenna. Thisreceived information only applies to the polarization of the test antenna for the direction in which thepower is received. For a complete description of the polarization of the test antenna, the test antennamust be rotated so that the polarization can be determined for each direction of interest.
  42. 42. Measurements 2. PolarizationFigure shows the example result ofpolarization measurement Horizontal Vertical Elliptical Circular
  43. 43. Measurements 3. ImpedanceImpedance measurements are pretty easy if you have the right equipment. In this case,the right equipment is a Vector Network Analyzer (VNA). This is a measuring tool thatcan be used to measure the input impedance as a function of frequency. Firstcalibration is needed. Typically VNA will be supplied with a "cal kit" which contains a matched load (50 Ohms), an open circuit load and a short circuit load. If the circuit matched to 50 Ohm then we will get low value of S11(Return loss).
  44. 44. Measurements3. Impedance Figure shows S11 measurement result using VNA So this measurement typically measures how close to 50 Ohms the antenna impedance is.
  45. 45. Measurements 4. Specific Absorption Rate (SAR)Specific Absorption Rate (SAR) is a measure of how transmitted RF energy is absorbedby human tissue. SAR is a function of conductivity (ς), induced E-field (E) and the mass density of the tissue (ρ) SAR measure in W/kg = mW/gThe SAR limit in the US for mobile phones is 1.6 W/kg, averaged over 1 gram of tissue. InEurope, the SAR limit is 2.0 W/kg averaged over 10 grams of tissue. It is typically harder toachieve the US specification than the Europe spec, so if the phone meets the US spec it willtypically also meet the European spec. The SAR values quoted for a mobile phone are the highest value of SAR measured for any frequency the phone operates in, from both the left and right side of the head.The antennas for mobile phones are typically on the bottom of the phone, to keep the radiating part of the phone as far as possible from the brain region.
  46. 46. Measurements4. Specific Absorption Rate (SAR) Picture shows the SAR Measurement system To simulate the conductivity and density correctly, the tub is filled with a fluid that has similar properties to human tissue.