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Chapter 19: Antennas

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  1. Chapter 19: Antennas By: James VE3BUX

  2. Definition The Modern Dictionary of Electronics defines an antenna as: • That portion, usually wires or rods, of a radio transmitter or receiver station used for radiating waves into or receiving them from space. It changes electrical currents into electromagnetic waves, and vice versa.

  3. Basic Electrical Properties Antennas: • Behave differently based on design • Are (ideally) purely resistive at resonance • Exhibit polarization • Radiate in a predictable pattern • Have an input impedance & exhibit inductive or capacitive character out of resonance • Have bandwidth

  4. Antennas: How important? • Antennas are the “portal to the ether” • Design and build of antennas are often one of the most critical aspects of radio communications • A really well designed and built antenna can work wonders whereas a “wet noodle” or “rubber resistor” antenna are almost entirely useless

  5. Antennas: Major design types

  6. Antennas: Vertical “Whip” • Until recently, these were the most prevalent antennas • Generally used in a vertical arrangement • Can be multi-band (ie. Cover multiple frequencies without a tuner) • Omnidirectional

  7. Antennas: Dipole (“Doublet”) • Extremely simple to make & costs very little • Exhibits some directionality • Can be stacked / arrayed to increase “gain” • Very common first project • Effectively omnidirectional

  8. Antennas: Yagi-Uda • Directional antenna • Design parameters determine F/B ratio • Excellent for basic radio direction finding • Three basic elements: • Reflector(s) • Driven element • Director(s) 3 2 1

  9. Antennas: Cubic Quad • Directional antenna • Design parameters determine F/B ratio • Very similar to Yagi-Uda, very slightly better • Same three basic elements: • Reflector(s) • Driven element • Director(s)

  10. Antennas: Magnetic Loop • Omnidirectional antenna • Does have some directionality • Reasonably immune to local noise due to principle of operation • Can generate very significant voltages! • Very small size

  11. Antennas: Dish • Very directional antenna • Design parameters determine F/B ratio • Significant “gain” as frequency increases • Great for E-M-E • Polarized!

  12. Antennas: Horn • Very directional antenna • Generally not used below 10GHz • Polarized! • Extremely efficient (low radiation resistance) • Can achieve incredible gain figures Holmdel Horn

  13. Properties: Resistive at Fr • Antennas are said to behave in a purely resistive manner when they are driven with energy near their resonant point • This resistive state indicates a good “match” of impedances between the radio and the antenna system • Recall that when Rload = Rinternal the maximum power transfer occurs • (Table 7-1, p65)

  14. Properties: Polarization • Electromagnetic waves are orthogonal fields which are composed of lines of force which are: • Electric • Magnetic • Direction of electric field determines polarization • Generally the same direction as the most significant radiating element

  15. Polarization: Importance • Above HF frequencies, matching polarization becomes extremely important • Loss can be as much as -40dB • Loss = 10log10(cosΘ)2 where Θ = difference angle • HF propagation makes polarization far less important • Special types of polarization exist to solve particular challenges • LH or RH Circular

  16. Properties: Have radiation pattern • Antennas radiate their energy differently based on their design • Radiation pattern is effectively the “shape” of an RF field being generated by a transmitting antenna • Need a bit more information to discuss patterns…

  17. Field Strength • The strength of a radio signal is defined by the amount of voltage induced onto a second antenna at some distance • Due to varying types of antennas and their differences in reception, a standard antenna with a length of 1m is used • Field strength is thus defined to have the unit volts per meter, or v/m • Generally μv/m due to the weak nature of radio signals

  18. S-units: An aside • S-units on a radio are based on 6dB difference • Recall field strength is in volts and therefore the log math requires 20log10(V1/V2) so 6dB = 2x • A signal which is said to be “S9” should mean that the receiving antenna would measure 50μv across a 50Ω load (antenna) • Also defined as: • -73dBm for HF • -93dBm for VHF

  19. Radiation Pattern: Basics • To discuss radiation pattern, we need to establish how to present field strength data • Antenna is placed at the center and where necessary, “elements” are shown on the graph • Concentric rings indicate field strength, usually in a relative sense

  20. Radiation Pattern • The most basic pattern is a so-called isotropic radiator • The antenna radiates energy in all directions “equally poorly” • Imagine a small sphere at the center of a beach ball • Small sphere at the center is the antenna • Beach ball is the shape that the radiated energy will take in ideal conditions • The surface of the ball is EM energy - as the radius increases, the “thickness” of the ball decreases; think of the EM field magnitude as the thickness • Inverse square law

  21. Radiation Patterns: Dipole • Dipole antennas are the first to exhibit directivity • Imagine a wire antenna which is vertical, the energy radiates as shown:

  22. Radiation Patterns: Vertical • Simple ground mounted 1/4λ monopole (vertical) antenna • Good “take-off” angle provides DX opportunities • Ground conductance becomes important and will affect pattern • Antennas of this nature generally rely on (or are significantly enhanced with) the presence of “ground radials” which serves as a distributed ground plane

  23. Radiation Pattern: Yagi-Uda • Great “directional” antenna • Focuses energy well and is thus said to have “gain” • Antennas on towers are very often of the Yagi-Uda style

  24. Input Impedance: Design & Location • Antenna impedance depends on a few factors: • Design • Height above ground • Resonant frequency • Matching 50Ω output of radio to the antenna’s impedance is important • Ideal power transfer • Reduce wasted energy coming back as reflected power (SWR)

  25. Impedance: Design • Plenty of ways to match the radio to the antenna • Matching networks (“tuners”) are often the solution

  26. Properties: Bandwidth • Antennas have a resonant point due to electrical length of elements • As you move away from the resonant point, the impedance changes

  27. Properties: Bandwidth • Some parameters which affect bandwidth are: • Physical design of antenna (dipole vsYagi, etc) • Antenna element diameter • Environment around antenna • Feedline losses • Shortening / “Loading” of antenna • Bandwidth edges are generally defined by 2:1 SWR points (similar to -3dB concept)

  28. Questions?