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This is post 2358 in a continuing series of simple ham radio antennas.
If you want a simple, inexpensive 40 meter antenna with some gain (about 3 dBi), please try the "classic half-square antenna."
In this video, Callum (M0MCX) discusses the theory and construction practices of this antenna.
I've built several of these half-square "aerials" for both 20 and 40 meters, and they work very well broadside to the vertical array.
Callum does an excellent job of outlining the pros and cons of this easily-made antenna. Here are few of his remarks: