Dorking & District Radio Society

Small Transmitting Loops - 25 September 2018

Mike Underhill, G3LHZ

Electrically small transmitting loops typically consist of a high Q inductive reactance antenna which can be tuned by means of a capacitor. Power can be coupled in via a small auxiliary loop, or a small gamma-match, the dimensions of which can be adjusted to provide the required input impedance. The comparatively small physical size of these antennas is attractive in situations where space is limited. Conventional antenna theory suggests that compact antennas of this form should not work effectively. However, measurements indicate that they do indeed work well and can attain efficiencies of between 80 to 95%. Professor Mike Underhill's talk will cover some examples and applications of these novel antennas. Of particular note is a new two-parallel turn loop design that has a lower Q, wider bandwidth, higher power handling, and the same high efficiency.

Mike passed his Radio Amateur's Exam at the age of 14 in 1953 and the Morse Test in 1955: obtaining his full licence G3LHZ in 1956, and joining the RSGB in 1957. He has been reasonably active on the air ever since, continuously on the BAD-net (Billingshurst and District net) at 10:00 local-time on 3722 kHz. Mike is a founding member of the Crawley Amateur Radio Club (CARC) and was the winning NFD operator on 23 cm for CARCRATS in 2012 with G3GRO, and single operator in 2014. He has given many talks to clubs mainly but not exclusively on small loops; this is the 55th since 1998. Currently Mike is very busy on some interesting basic physics and electro-magnetic research on Antennas and Propagation, a small part of which relates to and springs from Small Transmitting Loops! You may hear a little bit about this. Mike has been to the annual Hamvention in Dayton, Ohio about four times and is a "regular" (seven or eight times) at the International Amateur Radio Exhibition held annually in Friedrichshafen, Germany.

Mike's career included 25 years at Philips Research Labs, rising to Divisional Head, six years as Technical/Engineering Director for MEL the Thorn-EMI in Crawley, and 13 years as Professor of Electronics, Head of Department and Dean of Engineering at the University of Surrey. Overlapping this, he was for 10 years Founder and Research Director of Toric Limited, an SME supplying "embedded-jitter-suppression" solutions to the semiconductor industry. Underhill Research Limited is a very busy semi-retirement activity involving research and presenting conference papers around the world (Turin and California this year.)