I decided to make my own K9AY antenna system and controller. Basically, after looking at the currently available commercial version, I figured I could make a better unit for less money and be happy. The basis for the K9AY antenna is that it has a steerable null. The gain around the antenna is close to unity, except for the terminated side of the loop, which has a deep null.
This can be switched around using a combination of relays that change the loops and termination. This comes in very handy for MW and SW listening, when co-channel stations can create annoying interference and hetrodynes. I have had good success pulling many stations out of the muck, especially in the AM band using this antenna. This antenna requires a good ground to work against. For optimum installations, I would recommend placing two radials under each side of the loops. This will keep the ground conductivity below the antenna fairly constant, thus the value of Rterm will remain consistent for each band. My other idea is to add a preamp right at the antenna to overcome transmission line loss and the loss from a 4 port passive receiver coupler. Something around 10 dB, low noise (obviously), low parts count and rugged. I decided that a Norton preamp was a good design, with only one active device, a common 2N5109 BJT. Most of the time, this preamp is switched off and out of the circuit. There have been several occasions, however, where an extra 10 dB made the difference between no copy and good copy.