Antennas 101 barrowhamradio org

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Antennas 101

Part 1 Dipoles, Doublets and Verticals

Rob May NV5E

Including:

Dipoles Doublets Loops Windoms G5RV End Fed Half Wave (EFHW) Beverage

Definitions

Isotropic Radiator ? A theoretical antenna in free space that radiates equally well in all directions.

Gain ? Increase in amplitude of a signal, measured in dB

Decibel (dB) ? Logarithmic measurement of gain.

dBi ? Gain measured in relation to an isotropic radiator

Definitions

dBd ? Gain measured in relation to a dipole, dBd = dBi + 2.15.

Azimuth Pattern ? Radiation pattern of the antenna when viewed from above. Directional or omni directional.

Elevation Pattern ? Angle of maximum radiation in relation to the ground. Lower is better for DX.

Balun ? Short for BALanced/UNbalanced. A device to force equal currents in coax.

How Much Gain is a dB?

0 dB = 1 1 dB = 1.26 2 dB = 1.58 3 dB = 1.99 4 dB = 2.51 5 dB = 3.16

6 dB = 3.98 = 1 S unit

7 dB = 5.01 8 dB = 6.31 9 dB = 7.94 10 dB = 10

Dipole

2.15 dB gain over an Isotropic Radiator Balanced design Resonant on one band Traps can be added to make it multiband Fan Dipole Nominal 50 ohm impedance Gain increases with height (true for all antennas) Formula for calculating dipole length 468/F

(MHz)