Whip antenna
The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed.
Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use.
Whips are the most common type of monopole antenna, and are used in the higher frequency HF, VHF and UHF radio bands.
[1] Whip antennas less than one-half wavelength long, including the common quarter wave whip, have a single main lobe, and with a perfectly conducting ground plane under it maximum field strength is in horizontal directions, falling monotonically to zero on the axis.
This is because they transmit (or receive) equally well in all horizontal directions, while radiating little radio energy up into the sky where it is wasted.
The gain and input impedance of the antenna is dependent on the length of the whip element, compared to a wavelength, but also on the size and shape of the ground plane used (if any).
A quarter wave vertical antenna working against a perfectly conducting, infinite ground will have a gain of 5.19 dBi and a radiation resistance of about 36.8 ohms.
[3][4] The gain will be somewhat lower than a dipole, or a quarter-wave whip with an adequate size ground plane.
) has somewhat higher gain than a quarter wave whip, but it has a current node at its feedpoint at the base of the rod so it has very high input impedance.
The maximum horizontal gain of a monopole antenna is achieved at a length of five eighths of a wavelength
In a whip antenna not mounted on a conductive surface, such as one mounted on a mast, the lack of reflected radio waves from the ground plane causes the lobe of the radiation pattern to be tilted up toward the sky so less power is radiated in horizontal directions, undesirable for terrestrial communication.
[5] Also the unbalanced impedance of the monopole element causes RF currents in the supporting mast and on the outside of the ground shield conductor of the coaxial feedline, causing these structures to radiate radio waves, which usually has a deleterious effect on the radiation pattern.
The radiation resistance of a quarter wave ground plane antenna with horizontal ground wires is around 22 ohms, a poor match to coaxial cable feedline, and the main lobe of the radiation pattern is still tilted up toward the sky.
Often (see pictures) the ground plane rods are sloped downward at a 45-degree angle, which has the effect of lowering the main lobe of the radiation pattern so more of the power is radiated in horizontal directions, and increases the input impedance for a good match to standard 50-ohm coaxial cable.
To match 75-ohm coaxial cable, the ends of the ground plane can be turned downward or a folded monopole driven element can be used.
To reduce the length of a whip antenna to make it less cumbersome, an inductor (loading coil) is often added in series with it.
The helix distributes the inductance along the antenna's length, improving the radiation pattern, and also makes it more flexible.
Another alternative occasionally used to shorten the antenna is to add a "capacity hat", a metal screen or radiating wires, at the end.
