Click Thumbnails For Larger Image
The schematic above shows the layout of my inverted U antenna.
It was originally set out as a quarter wave Inverted L tuned against ground. In an effort to reduce residual band noise, I introduced a tuned counterpoise and removed any connection with ground from the system. Shortening the top horizontal section and bringing it down vertically at the far end also helped reduced band noise and had a marked effect on the radiation performance.
The tuned system is resonant mid-band without the need for any additional tuning or matching but I have introduced a variometer at the counterpoise feed point allowing for a little frequency "tweaking" when required.
On the down side, resonance moves lower in frequency when there is rain on the counterpoise.

The system tunes and works well on the 80M and 60M  bands with the help of a simple L Match to alter the load impedance.

Conclusion:  I can definitely see a marked improvement on 160M over the Inverted L at around 40 feet.
Not much is written around this configuration but the analysis appears to show between 1 and 2 db gain over the "L".  I am unsure just how the two vertical sections of the antenna pan out but it seems to do the job on 160M.

See the analytical charts depicted below:
Results from antenna-analyzing tool:  
Click the thumbnails below for a larger image.
G4AXP - Antenna_2
G4AXP - Antenna_1