Electrically Short Whips
To reduce the length of a whip antenna to make it less cumbersome, an inductor (loading coil) is often added in series with it. This allows the antenna to be made much shorter than the normal length of a quarter-wavelength, and still be resonant, by cancelling out the capacitive reactance of the short antenna. The coil is added at the base of the whip (called base-loaded) or occasionally in the middle (center-loaded). In the most widely-used form, the Rubber Ducky antenna, the loading coil is integrated with the antenna itself by making the whip out of a narrow helix of springy wire. The helix distributes the inductance along the antenna's length, improving the radiation pattern, and also makes it more flexible. Another alternative to shorten the antenna is to add a "capacity hat", a metal plate or screen, at the end. However all these electrically short whips have lower gain than a full length quarter-wave whip.
Multi-band operation is possible with coils at about one-half or one-third and two-thirds that do not affect the aerial much at the lowest band but create the effect of stacked dipoles at a higher band (usually x2 or x3 frequency).
At higher frequencies (2.4 GHz, but Military whips for 50 MHz to 80 MHz band exist, and are standard issue for the SINCGARS radio in the 30-88 MHz range), the feed coax can go up the centre of a tube. The insulated junction of the tube and whip is fed from the coax and the lower tube end where coax cable enters has an insulated mount. This kind of vertical whip is a full dipole and thus needs no ground plane. It generally works better several wavelengths above ground, hence the limitation normally to microwave bands.
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