Phasing can be done with any two antennas. I've done that with the MFJ 1026 phaser unit. It wasn't clear what Clark was doing or why the physical arrangement was important. Same for the purpose of the capacitor.
There was a good article in the October 2010 QST by Tony Preedy, G3LNP, "An Orthogonally Steered Antenna That Reduces Interference" wherein a K9AY loop is modified so that the rear side null elevation angle can be steered. The entire loop could be rotated in the azimuth plane, so a null could be placed almost anywhere. Some pretty amazing QRM reduction was possible. This system was designed to reduce more distant QRM. No evaluation was done on local QRN. My K9AY might be modified this way soon.
Two loops enable use of a "phaser" like Jon's DXE NCC-1 to combine their outputs. RF from the loops can then be manipulated in phase and amplitude to enhance rejection or maximize desired signal.
Oddly, there is a lengthy article by a local boy on loops for DF in August 1940 QST.
