Offset Fed Dipoles: The Quest for a Simple Multiband Wire Antenna Hap Griffin WZ4O July 2013

The Conventional Half-Wavelength Dipole ½ λ

Resonance and Standing Waves on an Antenna

Standing Waves on a Transmission Line

Current, Voltage and Impedance on a Dipole Standing Wave Current

Current, Voltage and Impedance on a Dipole

Current, Voltage and Impedance on a Dipole Ohm’s Law: R = E/I Impedance: Z = R ± X

Dipole Operated on 2nd Harmonic Frequency Impedance Voltage Current Ohm’s Law: R = E/I Impedance: Z = R ± X

Dipole Operated on 3rd Harmonic Frequency Impedance Voltage Current Ohm’s Law: R = E/I Impedance: Z = R ± X

Dipole Operated on 4th Harmonic Frequency Impedance Voltage Current Ohm’s Law: R = E/I Impedance: Z = R ± X

Dipole Operated on Fundamental and 2nd Harmonic Frequencies Fundamental Impedance 2nd Harmonic Impedance Feedpoint Impedance is vastly different on fundamental frequency vs 2nd harmonic

Fan Dipole for Two Frequency Bands Fundamental Impedance 2nd Harmonic Impedance Feedpoint Impedance is low on frequencies at which each dipole is ½ wavelength and very high on others

Dipole Operated on Fundamental and 2nd Harmonic Frequencies Fundamental Impedance 2nd Harmonic Impedance Feedpoint Impedance is vastly different on fundamental frequency vs 2nd harmonic Result is very high transmission line VSWR on even harmonics, relatively low on odd harmonics

VSWR vs Matching Antenna Impedance to Transmission Line VSWR = Antenna Feed Impedance Transmission Line Impedance Example: Transmission Line Impedance = 50 ohms Antenna Feedpoint Impedance = 75 ohms VSWR = 75/50 = 1.5 or 1.5:1

Dipole Operated on Fundamental and 2nd Harmonic Frequencies Fundamental Impedance 2nd Harmonic Impedance Feedpoint Impedance is vastly different on fundamental frequency vs 2nd harmonic

Dipole Operated on Fundamental and 2nd Harmonic Frequencies Fundamental Impedance 2nd Harmonic Impedance Feedpoint Impedance is vastly different on fundamental frequency vs 2nd harmonic

Dipole Operated on Fundamental and 2nd Harmonic Frequencies Fundamental Impedance 2nd Harmonic Impedance Feedpoint Impedance is same on fundamental frequency and 2nd harmonic, but is higher than at a center feedpoint.

Impedance Transformation With a Balun

Dipole Operated on Fundamental and 2nd Harmonic Frequencies w/ Offset Feedpoint Fundamental Impedance 2nd Harmonic Impedance Balun

Dipole Operated on Fundamental and 2nd Harmonic Frequencies

Q of Offset Fed Dipole Decreases (Bandwidth Increases)

The Original Windom Antenna

Modern Version – The Offset Fed Dipole

Array Solutions 7-Band 2KW- $259

Buckmaster 7-Band 300 W - $209 7-Band 3 KW - $299

Radioworks 7-Band 1.5 KW - $139