Antenna Design
Analysis & Design Software

Input Impedance and SWR of an Antenna in the Vicinity of Resonance

Author: 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.

Run this Program from the Web or Download and Run it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link r_x_swr then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.

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