Allowing amateur radio operation from an apartment or balcony.
A description and demonstration of an ultra-simple but 100 watt capability 7 MHz magnetic transmitting loop for units and apartments. There are no hard to get parts or complex workshop skills required to build. Instead of the expensive vacuum variable capacitor, the loop is brought to resonance with a length of RG213 coax (approx 1.85 metres for a 3m circumference loop for 7 MHz). The sliding capacitor's dimensions are not critical but are approximately 6 x 8 cm.
As demonstrated performance is excellent and would be significantly better than lighter and more portable loops. It would thus make an excellent project for apartment or unit dwellers who have limited room for other HF antennas.
CAUTION: Hazardous high voltages can develop across the capacitor on transmit with this (or any other) magnetic loop. Hence the loop should be operated in an out of reach location. For the same reason the loop should not be adjusted with RF applied even if only QRP.
A full article on this loop appears in November 2014 Amateur Radio magazine. Also some notes at http://home.alphalink.com.au/~parkerp/projects/projloop.htm