Antennas 101 "Don't Be A 0.97 DB Weakling!" - ARRL

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Antennas 101The BasicsWard Silver NØAX

The Basics - 1 Antennas radiate (or receive) becauseelectrons are accelerated (or are caused toaccelerate) in the antenna’s elements Radio or electromagnetic waves are bothan E- (electric) and H- (magnetic) field Electrons move parallel to E-fields Strongest radiation from acceleratingelectrons linearly (back and forth)2013Antennas 1012

The Basics - 2 The orientation of the E-field determinesthe polarization of the wave because that’swhat makes the electrons move (current) Antennas transmit & receive radio wavesin the same way The radiation pattern shows howantennas distribute energy in space2013Antennas 1013

The Basics - 3 deciBels (dB) 10 log (power ratio) Impedance ratio of Voltage to Current Feed point - place where power is applied2013Antennas 1014

The Basics - 4-1 dB from themaximum gainreference valueAzimuthal Pattern2013Antennas 1015

The Basics - 5Elevation Pattern2013Antennas 1016

The Basics - 6 Front-to-Back, Front-to-Side, and Frontto-Rear ratios– Front-to-Rear ratio based on an averageacross 90 or 180 degrees “behind” the antennain the pattern’s rear quadrant(s)2013Antennas 1017

The Basics - 6RearFrontBackSide2013Antennas 1018

The Basics - 6 Beamwidth - angular width of main lobe– Angle between the two points at whichpower is ½ that at the peak (-3 dB points)2013Antennas 1019

The Basics - 6- 3 dBBeamwidth 68 2013Antennas 10110

The Basics - 7 Gain measures re-distribution of energy Gain is a comparison of antennas Gain is always with respect to a reference––––2013dBi (isotropic), dBd (free-space /2 dipole) /2 dipole has 2.15 dBi gainGround-plane gain equivalent to /2 dipole3-element Yagi may have up to 5 dBd gainAntennas 10111

QUESTIONS?2013Antennas 10112

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 reversed2013Antennas 10113

The Dipole - 2Current maximum (I)feed pointVoltage minimum (V)1/2 wavelengthIMPORTANT!!!A /22013B /2Antennas 101VIC /214

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)2013Antennas 10115

The Dipole - 4 Radiated energy is strongest perpendicularto an electron’s motion– electrons move along the length of a dipole– radiation strongest broadside to the dipole2013Antennas 10116

The Dipole - 4Directions ofMaximumRadiationDirections ofMinimumRadiationDipole AxisRadiation Pattern2013Antennas 10117

The Dipole - 5 Feed point impedance varies with position– High at the ends and low in the middle– Resonance – feed point impedance allresistive, no reactance Z R j 0 ohms Doesn’t matter what R is (any value, not just 50 Ω)2013Antennas 10118

The Dipole - 5Current Maximum (I)feed pointVoltage Minimum (V)1/2 wavelengthHigh ZLow ZHA /22013LB /2Antennas 101HVLIHC /219

The Dipole - 6 Inverted-Vee is a “bent” dipole Radiation pattern adds the effect ofground 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 withground conductivity2013Antennas 10120

The Dipole - 635 /2groundInverted-VeeRadiation Pattern2013 /2Antennas 10121

QUESTIONS?2013Antennas 10122

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 groundplane or counterpoise Omnidirectional if oriented vertically Also called a monopole2013Antennas 10123

The Ground Plane - 22013Antennas 10124

The Ground Plane - 32013Antennas 10125

The Ground Plane - 4 Equal radiationbroadside Nulls along theaxis2013Antennas 10126

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 pointimpedance– approximately 45 of droop gives best match– halfway between dipole (72 Ω) and groundplane (35 Ω)2013Antennas 10127

Useful References and Books ARRL Publications– Antenna Book, Basic Antennas– Compendium and Classics series RSGB Publications– Practical Wire Antennas and HF Antennas for allLocations CQ Communications– Sevick’s and Maxwell’s books on xmsn lines, baluns– Vertical Handbook, All About Quads2013Antennas 10128

Useful On-Line References ARRL Technical Information Service and L.B. Cebik’s web site -– Part of the site (subscription) Antennas and TowerTalk reflectors atwww.contesting.com2013Antennas 10129

Thank You!2013Antennas 10130

Antennas 101 11 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 2013

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