This may be the cheapest, lowest, no-radials-required, no fancy networks, way to get great gain on 40M. If you have some rope, trees, junkbox wire, a couple of DPDT relays, and some coax laying around, you have all it takes to build this antenna.
Here’s a two-element array of wire verticals for 40M — read about using passive phasing, then Christman phasing. Includes a handy online calculator for phasing line lengths, and audio clips of the array in action.
Sometimes when my antenna is at 25′, I do quite well into Canada and the U.S — perhaps even better than with the antenna at 45′. Terrain analysis helped end the mystery of height vs. performance at this ridge-top QTH.
This handy calculator will determine the lengths of coax feedlines and phasing line for Christman phasing — great for verticals spaced 1/4-wave apart. [Note: will not display if you use Internet Explorer].