HF horizontal loop antenna in a small back yard

The back yard in our new house is very small and there is an intermittent interference source that has plagued me since setting up here. Phil, VK7JJ, is a great advocate for horizontal loop antennas which he uses to attain world-beating WSPR reception. Phil encouraged me to fill the available space with a horizontal loop.

The area between the back of the house and the front of the garage is about 5m x 3.6m and at the right you might be able to make out a loop made with thin black hookup wire. This antenna was fed from coax from the operating position via a common mode choke.

There's a small verandah with a wooden pole which is convenient for mounting baluns etc out of the rain. Here's the first attempt.

Later I did a better job and used an electrical junction box. 

The base nails to the wood and then the top is screwed on to that. Not waterproof but fine for my use. 

The loop works well on 20m but wouldn't do 40m. I'm not sure why at this point, but the loop's signal to noise seems indistinguishable from that of the 20m dipole along the fence line.

I've been improving the dipole by attaching one end to the tripod on the garage roof and I think the height advantage evens things up. One idea is that I might put more poles on the garage to hold up one end of a loop and as well as increased area I'll get some more height.