Why an inverted L?
A quarter-wave vertical for 160m would be about 38 m tall, which few people can manage. An inverted L goes up as high as you can, then bends over and runs horizontally for the rest of the length. The vertical part does most of the radiating, at the low angles needed for DX, while the horizontal part acts as top loading and adds some high-angle radiation for shorter contacts. It's one of the most effective and popular antennas for 160m and 80m from an ordinary garden.
Horizontal = total − vertical
The bend changes the resonant length a little, and nearby trees and buildings change it more, so cut it long and trim it while watching an analyser.
The longer version with a capacitor
A quarter-wave inverted L over radials has a feed impedance of roughly 20 to 40 Ω, depending on how much of it is vertical and how good the ground system is. A popular trick is to make the wire longer, about 0.3 wavelengths (around 46 m on 160m). Its feed impedance then has a resistance close to 50 Ω plus some inductive reactance, which a variable capacitor in series with the feed cancels. A few hundred pF at a voltage rating of a kilovolt or more is typical at 100 W. This often gives a better match and a little more of the current in the vertical section.
Radials
Like any vertical, an inverted L needs a good ground system. Lay as many radials as you can: even 8 to 16 radials of 10 to 20 m make a big difference on 160m, compared with a ground stake alone. With elevated radials, two to four tuned quarter-wave radials work well. See the quarter-wave vertical calculator for the radial table. Fit a choke on the coax at the feedpoint so the coax doesn't become part of the antenna.
Questions
How long is a 160m inverted L?
About 38.5 m of wire in total at 1.85 MHz for a quarter-wave version. With a 12 m vertical, the horizontal section is about 26.5 m.
How high should the vertical part be?
As high as you can manage. Every extra metre of vertical improves low-angle radiation for DX. 10 to 15 m is common on 160m.
Do I need radials for an inverted L?
Yes. The ground system is half the antenna. Lay as many radials as possible, or use two to four elevated tuned radials.
What capacitor do I need for a long inverted L?
Typically a few hundred pF, adjusted for the best SWR, rated at 1 kV or more for 100 W.