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Half-Wave Dipoles - Input Impedance & Feedline VSWR vs. FrequencyAuthor: R.J.Edwards G4FGQ © 16th March 1999 This program analyses both simple and folded dipoles. Folded dipoles allow a higher impedance and hence lower loss feed line to be used. The operating bandwidth is also marginally higher for a given acceptable feedline VSWR. It is assumed antennas are remote from the ground - heights greater than 0.75 lambda.
The program tabulates feedpoint impedance Zin = Rin + jXin, reflection coefficient and VSWR on the feedline over a frequency range +/- 5 percent of resonant frequency. The HF resistance of copper or aluminium cylindrical conductors is taken into account. For a given resonant frequency and conductor dimensions the overall resonant length of the antenna and the 3-dB-point receiving bandwidth are also computed. Computed Q and Rx bandwidth apply to a matched receiver.
Vary wire diameter of a simple dipole; vary both wire diameter and spacing of a folded dipole; and vary feedline Zo to observe the effect on performance and on resonant length.
At low heights, depending on type of ground, behaviour changes somewhat. Input resistance and operating bandwidth will increase when above lossy soil but both will decrease when immediately above sea water, metal decks or ground mats.
The computed resonant antenna length is that at which input impedance is a pure resistance and, as for other computed characteristics, is sufficiently accurate for demonstrating antenna behaviour provided conductor diameter and spacing are both less than about 1/10th of dipole overall length. Such extreme proportions may occur at UHF. At even higher frequencies horns and waveguides are the norm.
Dipole input Rin at resonance, usually referred to as the radiation resistance, is less than the normally assumed value of 73 ohms. This is because the program correctly uses the resonant length of the antenna to calculate radiation resistance - not the somewhat longer exact half-wavelength. By folding the dipole Rin is transformed to a value approximately four times greater.
This program applies also to simple and folded 1/4-WAVE VERTICAL antennas provided a very low resistance ground radial system is used. It is necessary only to halve computed antenna input impedance and the entered value of feedline impedance. Computed values of VSWR bandwidths remain unaffected.
Note: An exact 4-to-1 impedance transformation ratio occurs only when a folded dipole is reconnected as a simple dipole without removing either of the wires. i.e., both wires are cut and connected in parallel at the simple dipole centre.
When the number of wires in a cage is ten or more, just enter the cage diameter.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 swr_freq 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.
Labels: VSWR vs. Frequency