For an excellent discussion of delta loops close to the ground and the effects of different feed points - all done in the pre-computer model days - see G3AQC's article in 1974 RadCom (May '74 I think, reference is in ON4UN's book, all editions). BTW - Laurie, G3AQC, made the first two way transatlantic QSO on 136Khz earlier this year. 73 Chris G3SVL