Like someone else said - instead of a trencher, why not try a edger.
You can buy an electric edger and a long extension cord cheap enough -
probably cheaper than renting a trencher.
I heard one person say he just purchared a new chain for his chainsaw - and
used the old chain to make the trenches for radials.
Another thought - don't do radials at all - hire a front-end-loader to bury
a few chain-link fence sections and tie ground to them.

Be sure to grunt like the tool-man when installing the radial system - its a
tradition.