For portable radio operation, I like End-Fed Half-Wavelength Antennas (EFHWA, pronounced "EF-WAH"). This type of antenna is similar to the common half-wavelength dipole, but with one significant advantage. A dipole has its feedpoint (where it connects to the radio) in the middle of the antenna, but an EFHWA's feedpoint is at one end. This makes it very convenient to throw the antenna up in a tree and connect the bottom of it to your radio. Here are instructions for making a multiband end-fed half-wavelength antenna that works on 17, 20, 30, and 40-meter bands.
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