Way back in post #7 Bazz referred to LZ1AQ:
http://www.lz1aq.signacor.com/
I bought one of his amplifier kits and made up a loop from 2.5mm cable (I think). It is cleverly fed by Cat 5 cable, one of the pairs being used for the balanced signal and others for power and a switch signal which allows selection of either the loop or a short vertical. It's amazing sometimes to switch between the two aerials. I find the system very effective in reducing RFI across a wide spectrum.
LZ1AQ goes into a lot of theory on his site which I find very plausible. He comes from an academic and practical engineering background and has clearly spent a lot of time on research. He does consider the benefits of a copper pipe loop as against a simple wire and does a thorough analysis of the needs of the all important preamplifier. There are also a lot practical recommendations. All very well worth reading.
Ian