Measurements of the radiation and loss resistances of small HF multi-turn multi-resonant tuned loops

We use our 'wideband-Q' method of finding and measuring the various resistance components of a small tuned loop (Underhill and Harper, IEE Electronics Letters, vol.38, no.18, p.1006-8) to measure and assess a variety of small HF tuned loops. The method relies on bandwidth, and hence Q, measurements being made over the two to three octave tuning range of such tuned loop antennas. Because the measured loop inductance remains constant, until the self resonance of the loop is approached, the Q values also deliver the total combined loss and radiation resistance as measured at the antenna terminals. We find from theory, and confirm by measurement, that the several radiation modes of a loop and the identifiable loss mechanisms in general vary with different powers of the frequency. Where two components vary with the same power of frequency, we can separate these by varying the loop or loop conductor sizes, or the placement and orientation of the loop in the environment, and then by applying simple multi-factor analysis to the measurements. The results are found to be highly repeatable and have error limits of a few percent only. We also observe that the dipole radiation mode is much reduced for the multi turn loop, typically to the point of insignificance, particularly when the loop is well balanced (Underhill and Blewett, M.J., 1997).