Topband: Small loop info

[K9AY]

Topbanders,

In the recent "ARRL Contest Rate Sheet," N0AX mentions a paper on small 
loops that appeared in a recent issue of IEEE Antennas & Propagation 
Magazine. I also read this paper with interest.....

A group of engineers at BAE Systems in the UK ran through the established 
theory of small loops and performed measurements to validate the theory. 
Their stated objective was to fill in some gaps in measured-vs-theoretical 
data available in the literature. The subject loop was 1-meter diameter, 
made of 22 mm dia. copper tubing, evaluated at 3.6, 5.1, 7.2 and 10.1 MHz --  
obviously, there were hams in the group!

Within the limits of measurement technique -- given the very low radiation 
resistance of these antennas and the variability of ground wave measurements 
at HF -- their results validate the currently-accepted design equations 
within 10% to 20% depending on frequency. Here's a brief summary of their 
results for radiation resistance and radiation efficiency --

3.6 MHz:   R= 0.42 milliohms; (0.36 theory); Eff = 0.25%
5.1 MHz:   R = 2.4 milliohms; (1.6 theory); Eff = 0.84%
7.0 MHz:   R = 6.0 milliohms (5.7 theory); Eff = 2.3%
10.1 MHz:   R = 40 milliohms (25 theory); Eff = 18%

For reference, 0.25% efficiency is 26 dB down from 100% efficiency. 
Extrapolating this data to 1.8 MHz gives R ~ 0.03 milliohm, Eff ~ 0.03% and 
signal levels 35 dB down from 100% efficiency. Your TX antenna is probably 
90-100% efficient.

Note that by 10.1 MHz, the efficiency of the 1-meter loop is rising rapidly, 
as the loop begins to approach the limits of "electrically small." At this 
frequency, the loop has "folded dipole" mode radiation that is just 12 dB 
down from the "magnetic dipole" or loop mode.

The paper may have been motivated by recent discussions (in amateur circles, 
not professional) that other modes of radiation are actually larger than 
predicted -- which is why small loops work better than expected.  The 
authors' verification of EM theory reminds us once again that inefficient 
antennas may SEEM to work better than they should, but they are actually 
performing normally. To put it another way, "If signals are 30-over-9 on a 
good antenna, a really lousy antenna may still be S-9."

Other details in the paper may be of practical interest to topbanders, as 
well -- such as determining Q, matching, etc.

73, Gary
K9AY

REF:  Alan Boswell, Andrew J. Tyler and Adam White, "Performance of a Small 
Loop Antenna in the 3-10 MHz Band," IEEE Antennas and Propagation Magazine, 
Vol. 47, No. 2., April 2005.