Simple copper pipe loop for 40m

After several false starts, including complex schemes involving stepper motors and long rods, I've settled on a very simple design for a loop that nicely covers 40m.

The loop is about 1m in diameter, a cat is shown for scale.

Contrary to some other designs, I've put the coupling at the top. Two turns on a FT-114-43 toroid. (Since this picture I've anchored things with some cable ties and removed one turn to get a lower SWR).

The tuning is at the bottom. I needed an extra 180pF to get this on to 40m. A larger knob is better, a reduction drive would be even better. (Tuning is very sharp).

Here's a rough drawing of the circuit:

Sitting at the table in the back yard yields decent reception - not as good as the full size dipole - but not bad.

The 3m length of copper pipe was a standard cut length at the local hardware store.

To find where the loop was resonant and make sure it was working, I used a miniVNA pro  and the vna/j software. I'm running Ubuntu Linux and ran in to some problems getting vna/j running. The trick is to remove the open source Java implementation and install the version from Sun. Also there's the JNI libraries that are required the Arduino IDE that needed to be removed to get it running.

Here's the plot showing the SWR dip at 7.1Mhz to 1.4:1. The KX3 tuner nulls out the mis-match without problem.

I've transmitted up to 50W SSB without any bad things happening.