Antennas 101 Don t Be a 0.97 db Weakling! Ward Silver NØAX

Antennas 101 Don t Be a 0.97 db Weakling! Ward Silver NØAX

Overview Antennas 101 2

Overview Basic Antennas: Ground Plane / Dipole How Gain and Nulls are Formed How Phased Arrays Work How Yagis Work (simplified) Antennas 101 3

The Basics - 1 Antennas radiate (or receive) because electrons are accelerated (or are caused to accelerate) in the antenna s elements Radio or electromagnetic waves are both an E- (electric) and H- (magnetic) field Electrons move parallel to E-fields Strongest radiation from accelerating electrons linearly (back and forth) Antennas 101 4

The Basics - 2 The orientation of the E-field determines the polarization of the wave because that s what makes the electrons move (current) Antennas transmit & receive radio waves in the same way The radiation pattern shows how antennas distribute energy in space Antennas 101 5

The Basics - 3 decibels (db) = 10 log (power ratio) Impedance = ratio of Voltage to Current Feed point - place where power is applied Antennas 101 6

The Basics - 4-1 db from the maximum gain reference value Azimuthal Pattern Antennas 101 7

The Basics - 5 Elevation Pattern Antennas 101 8

The Basics - 6 Front-to-Back, Front-to-Side, and Frontto-Rear ratios Front-to-Rear ratio based on an average across 90 or 180 degrees behind the antenna in the pattern s rear quadrant(s) Antennas 101 9

The Basics - 6 Rear Back Front Side Antennas 101 10

The Basics - 6 Beamwidth - angular width of main lobe Angle between the two points at which power is ½ that at the peak (-3 db points) Antennas 101 11

The Basics - 6-3 db Beamwidth 68 Antennas 101 12

The Basics - 7 Gain measures re-distribution of energy Gain is a comparison of antennas Gain is always with respect to a reference dbi (isotropic), dbd (free-space /2 dipole) /2 dipole has 2.15 dbi gain Ground-plane gain equivalent to /2 dipole 3-element Yagi may have up to 5 dbd gain Antennas 101 13

QUESTIONS?? Antennas 101 14

The Dipole - 1 Oldest and simplest form of antenna Di (two) Pole (voltage polarity) Usually 1/2-wavelength long Similar to a vibrating string s fundamental Current maximum in the middle Voltage maximum at the ends Pattern repeats every 1/2-wavelength Direction or amplitude is reversed Antennas 101 15

The Dipole - 2 Current maximum (I) feed point Voltage minimum (V) 1/2 wavelength IMPORTANT!!! V I A /2 B C /2 /2 Antennas 101 16

The Dipole - 3 Free-space wavelength ( ) = c / f or 300 / f in MHz (in meters) /2 = 492 / f in MHz (in feet) Length-to-diameter effect Makes the antenna a little longer electrically Thicker conductors are longer electrically Effect of height on electrical length 460/f to 490/f (rarely 468/f) Antennas 101 17

The Dipole - 4 Radiated energy is strongest perpendicular to an electron s motion electrons move along the length of a dipole radiation strongest broadside to the dipole Antennas 101 18

The Dipole - 4 Directions of Minimum Radiation Directions of Maximum Radiation Dipole Axis Radiation Pattern Antennas 101 19

The Dipole - 5 Feed point impedance varies with position High at the ends and low in the middle Resonance feed point impedance all resistive, no reactance Z = R + j 0 ohms Doesn t matter what R is (any value, not just 50 Ω) Antennas 101 20

The Dipole - 5 Current Maximum (I) feed point Voltage Minimum (V) 1/2 wavelength High Z Low Z H L H V L I H A /2 B C /2 /2 Antennas 101 21

The Dipole - 6 Inverted-Vee is a bent dipole Radiation pattern adds the effect of ground gain from reflections Can add as much as 6 db over free space Free-space gain best comparative measure Ground gain varies with height and with ground conductivity Antennas 101 22

QUESTIONS?? Antennas 101 23

The Ground Plane - 1 Start with a vertical dipole in free-space Cut off one half of the dipole Replace the missing half with a ground plane or counterpoise Omnidirectional if oriented vertically Also called a monopole Antennas 101 24

The Ground Plane - 2 Antennas 101 25

The Ground Plane - 3 Antennas 101 26

The Ground Plane - 4 Equal radiation broadside Nulls along the axis Antennas 101 27

The Ground Plane - 5 Length (ft) 234 / f (MHz) 231 / f if #14 wire used, - 221 if 5/8 tubing Feed point impedance 35 Ω if radials used, sloping increases feed point impedance approximately 45 of droop gives best match halfway between dipole (72 Ω) and ground plane (35 Ω) Antennas 101 28

QUESTIONS?? Antennas 101 29

Antennas with Gain - 1 Gain is obtained by directing more energy in some directions Gain is created by arranging currents so that their radiated fields add together Current can be added in separate elements, by folding or bending, or by simply lengthening a single element Antennas 101 30

Gain in dbi Antennas with Gain - 2 Dipole Gain vs. Length 4 3 2 1 0 1/2 3/4 1 1-1/2 Length in Wavelengths Dipole axis Lengthening the Dipole Antennas 101 31

Antennas with Gain - 3 Fields from currents along the antenna can reinforce each other reinforcement at large distances (>> ) this is the far field where the pattern is independent of distance (>>10 ) Reinforcement may occur at angles far from broadside Reinforcement: constructive interference Antennas 101 32

Antennas with Gain - 4 Signals add Signals cancel i i Signals add +peak -peak null i Signals cancel Signals cancel Antennas 101 33

Antennas with Gain - 5 Reinforcement for gain also causes destructive interference, creating nulls Nulls are good, too! Avoid interference (both to and from) Improve signal-to-noise ratio in desired directions Adding elements also creates gain E-fields can add or cancel Antennas 101 34

QUESTIONS?? Antennas 101 35

Phased Arrays - 1 Array - multiple antenna elements Phased - pattern created by phase differences in each element (all driven) Broadside - maximum radiation perpendicular to the plane of the array End-Fire - maximum radiation in the plane of the array Antennas 101 36

Phased Arrays - 2 The original beam antennas Most constructed from /2 dipoles or /4 ground-planes Array plane can be vertical or horizontal Symmetrical arrays (including feeds) have symmetric patterns Antennas 101 37

Phased Arrays - 3 Common End-Fire Phased Arrays W8JK two dipoles fed out-of-phase horizontal plane Antennas 101 38

Phased Arrays - 3 /2 1/2-twist open-wire line to tuner /8 - /4 i i Looking at the antenna end-on 0 180 W8JK Array Antennas 101 39

Phased Arrays - 3 Phase matters! Phase is just time Controlled by orientation of current physical orientation Change W8JK pattern by moving elements and altering currents Antennas 101 40

Phased Arrays - 3 i i /2 0 0 i i /2 0 90 i i /2 0 180 Antennas 101 41

QUESTIONS?? Antennas 101 42

Yagi Antennas - 1 Properly known asyagi-uda arrays Yagis are parasitic arrays non-driven elements that re-radiate energy to form the pattern are parasitic elements Length and spacing of the elements determines the phase of the re-radiated signals Antennas 101 43

Reflector Driven Element Yagi Antennas - 2 Directors Spacing usually about 1/8-wavelength Boom Maximum Signal longer than DE shorter than DE Ele #1 Ele #2 Ele #3 Induced Currents from other elements Antennas 101 44

Yagi Antennas - 3 Driven element (resonant) Current & voltage in-phase Reflector ( 5% longer than resonant) Inductive, current lags 0-90 Director ( 5% shorter than resonant) Capacitive, current leads 0-90 Reradiation 180 out of phase with inducing field Antennas 101 45

To the front: Yagi Antennas - 4 0 at DE + travel delay (t del ) + director lead + re-radiation (180 ) results in reinforcement To the rear: 0 at DE + travel delay - reflector lag + reradiation (180 ) results in cancellation fields cancel rear refl t del DE t del dir front fields add Antennas 101 46

QUESTIONS?? Antennas 101 47

Useful References and Books ARRL Publications Antenna Book, Basic Antennas Compendium and Classics series RSGB Publications Practical Wire Antennas and HF Antennas for all Locations CQ Communications Sevick s and Maxwell s books on xmsn lines, baluns Vertical Handbook, All About Quads Antennas 101 48

Useful On-Line References ARRL Technical Information Service AC6V.com and Dxzone.com L.B. Cebik s web site - www.cebik.com Part of the Antennex.com site (subscription) Antennas and TowerTalk reflectors at www.contesting.com Antennas 101 49

Thank You! Antennas 101 50