Such antennas are very widely used for cave radio work.  I understand they approximate to a vertical loop, but the analysis of the propagation mechanism is not simple.  

Some of the theory can be found in David Gibson's PhD thesis: "Channel Characterisation and System Design for Sub-Surface Communications" (Leeds, 2004).  Theory and practice have been discussed extensively in the Cave Radio & Electronics Group Journal.  There is a bibliography of this which can be searched at http://www.sat.dundee.ac.uk/bin/cregsearch

73 John F5VLF