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Analysis & Design Software

Centre-Fed Dipole
Radiation from Coaxial Feedline

Author: R.J.Edwards G4FGQ © 25th March 2003

The effects of "unintentional" radiation from an antenna feedline are small changes in the radiating pattern. Signal, noise and interference levels, will be affected in random unpredictable directions. The effects on station operating performance will differ between sites. The most noticeable change will be partial filling-in of a null in the pattern which may be judged either useful or highly undesirable.

If a performance problem is found after erecting an antenna it might be cured by fitting a balun between the coax line and the antenna feedpoint. But there's no guarantee of a cure. This program plus some judgment checks the likelihood of a problem arising from feedline radiation before erecting an antenna.

Reciprocity allows analysis of both receiving and transmitting performance. The easiest quantity to estimate is power radiated from the line as line length and frequency are varied. Uncertainty due to a cluttered environment through which a feedline runs can be considered to be part of the guesswork. In any case it is incorrect to calculate radiated power as if elements behave independently of each other. But in the absence of any other quantitative data any crude estimate is better than none.

This program models the very common and simple radiating system in which an 'unintentional' unbalance current flows down the feedline. It is a symmetrical dipole centre-fed via a coaxial line with the braid grounded at the transmitter end. This ground connection is often of mediocre but satisfactory quality.

With no loss in accuracy, to simplify the model, the transmitter is located at the dipole centre and the coaxial line is replaced by a single conductor of the same diameter as the coaxial braid. There are 3 radiating elements. Radiation resistance, input impedance, and input current of each element is calculated. Finally, the percent of total power radiated by each of the three elements is calculated.

Actual feedline loss due to high SWR does not affect the percentages. However, high line radiation and high line SWR tend to occur at the same frequencies and so an antenna is unlikely to be used at those frequencies in any case. As will be seen, radiation from a coaxial feedline in a particular installation can vary widely with frequency. Random resonances occur. If by chance one occurs in an amateur band it can be shifted by changing the feedline length after the antenna has been erected. In the author's opinion, for simple HF antennas, radiation from coaxial feedlines is small enough to adopt the philosophy "If it works don't fix it". But don't forget TVI, BCI, local noise sources, etc.

Ground connection at the transmitter end of line - from 3 to 50 ohms.

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