Chapter 19: Antennas By: James VE3BUX

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. 2

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 3

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 4

Antennas: Major design types 5

Antennas: Vertical “Whip” 6 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

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

Antennas: Yagi-Uda 8 Directional antenna – Design parameters determine F/B ratio Excellent for basic radio direction finding Three basic elements: 1.Reflector(s) 2.Driven element 3.Director(s) 12 3

Antennas: Cubic Quad 9 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)

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

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

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

Properties: Resistive at F r 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 R load = R internal the maximum power transfer occurs – (Table 7-1, p65) 13

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 14

Polarization: Importance Above HF frequencies, matching polarization becomes extremely important – Loss can be as much as -40dB – Loss = 10log 10 (cos Θ) 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 15

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… 16

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 17

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 20log 10 (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 18

Radiation Pattern: Basics To discuss radiation pattern, we need to establish how to present field strength data 19 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

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 20

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

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 22

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

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) 24

Impedance: Design Antenna TypeFree-space impedance (Ω) Yagi (direct feed) /4λ vertical (whip)37 Dipole (Inverted V)~50 Dipole (Basic Design)73 Cubic Quad115 Dipole (Folded Design)300 Windom (Off-center fed dipole) Plenty of ways to match the radio to the antenna Matching networks (“tuners”) are often the solution

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

Properties: Bandwidth Some parameters which affect bandwidth are: – Physical design of antenna (dipole vs Yagi, 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) 27

Questions? 28