Input Impedance and SWR of an Antenna in the Vicinity of ResonanceAuthor: R.J.Edwards G4FGQ © 31st March 04
This program models an 80m band vertical and a 1/2-wave dipole. It demonstrates the effects on SWR of changing frequency in the vicinity of antenna resonance. Antenna lengths are approximately 20 and 40 metres respectively.
With 14awg wire the resonant frequency of both antennas is 3.637 MHz. Resonance occurs when antenna feedpoint reactance is zero.
SWR's are calculated on 50-ohm and 75-ohm feedlines and also on a matched line. A matched line is one which exactly matches input impedance at the resonant frequency or is connected to the antenna via an impedance matching transformer.
To match the line to the antenna first vary frequency using keys 1,2 until antenna input reactance is zero. Then vary line Zo using keys 5,6 until SWR = 1:1. Line Zo then equals the resistive component of antenna input impedance. Antenna bandwidth is found by varying frequency over the band and observing SWR.
SWR behaves in exactly the same way on all HF wavebands.
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