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Long Wire Antenna
The simplest and least expensive portable antenna is the "long wire." As with any antenna, the wire should be resonant at the radio's operating frequency to allow the most efficient radiation of a transmitted signal. Although a random length of wire is likely to work somewhere within the amateur radio bands, with a portable trans-match antenna tuner, you should find resonance and desirably low standing wave ratio (SWR) on most of the HF bands. Use a trans-match antenna tuner whenever you need to make an antenna more versatile than its built-in design allows.
Classic Halfwave Dipole Antenna
Many portable HF radio operators use QRP (low-power) equipment, which often transmits only on a single frequency band. The classic half-wave dipole antenna is ideal for single-band operation. The signal is sent from the radio by a transmission line to a center "feed point" on the antenna, from which two wire legs of equal length stretch out horizontally. The ideal length of the antenna is determined by a formula: 468 / frequency in MHz = length in feet. For example, a classic dipole for 14 MHz needs to be 33.43 feet in long. You can suspend a dipole antenna from trees using ropes, so that it's as high and as horizontal as possible.
Sloper Antenna
It's not always easy to put up a dipole antenna to achieve an ideal horizontal configuration. But if you can get at least one end of the antenna as high as possible into the air, you can attach the other end to a point closer to the ground. This "sloper" antenna will "aim" a signal in the direction of its lowest point, so be sure to orient the antenna in the direction you prefer.
Inverted Vee Antenna
Another dipole configuration is the "inverted vee," which has its center feed point as high as possible above the ground with its two legs sloping towards the earth's surface. This antenna is less directional than the sloper, but because of its special orientation, it needs to be cut about 4 percent shorter than the classic dipole.
Vertical Ground Plane Antenna
Vertical ground plane antennas have very low angles of radiation, such that radio waves traveling from them hit the ionosphere (which reflects radio waves) at angles conducive to long-distance (DX) worldwide communication. In areas with highly conductive earth grounds, ground radial wires at the base of the vertical aren't necessary. A quarter-wave vertical wire suspended from a tall tree in wet soil will perform wonderfully on its resonant frequency as well as on harmonic (multiple) frequencies. Most manufactured mobile whip antennas are variations of the ground plane antenna.