Loss Mechanisms in the Electrically Small Loop Antenna [Antenna Designer's Notebook

Abstract

The characteristics of an electrically small loop antenna are readily calculable. Being an inherently high-Q device, it requires careful tuning. Its performance as a transmitting antenna is sometimes limited by low radiation efficiency because of inherent losses, combined with a radiation resistance that varies as the fourth power of frequency. An apparent anomaly, frequently encountered in practice, is that the measured Q factor differs significantly from that calculated or obtained by computer simulation. This finding has led some to believe that the loop has a higher radiation resistance than conventional analytical techniques would imply. However, this conclusion is unfounded. Experimental evidence presented here indicates that Q factors close to those predicted can be achieved when appropriate consideration is given to reducing the loss associated with the tuning-capacitor mechanism, in particular. The ground loss (another significant factor) is also calculable, but since it decreases rapidly with increasing loop height above ground, its effect can readily be reduced. Vital to the experimental program that accompanied this work was an effective method - using a toroidal transformer - of measuring the total resistance of the loop.