Yagi-Uda antennas include one or more director elements, which, by virtue of their being arranged at approximately a quarter-wavelength mutual spacing and being progressively slightly shorter than a half wavelength, direct signals of increasingly higher frequencies onto the active dipole. (See also log-periodic antenna.) Thus, the complete antenna achieves a distinct response bandwidth determined by the length, diameter, and spacing of all the individual elements; but its overall gain is proportional to its length, rather than simply the number of elements.
All the elements usually lie in the same plane, typically supported on a single boom or crossbar. The parasitic elements do not need to be
coplanar, but can be distributed on both sides of the
plane of symmetry. Many Yagi-Uda antennas are designed to operate on multiple bands; the resulting design is made more complicated by the presence of a resonant parallel
coil and
capacitor combination (called a "trap") in the elements. Traps are used in pairs on a multiband antenna. The trap serves to isolate the outer portion of the element from the inner portion for the trap design frequency. In practice, the higher frequency traps are located closest to the boom of the antenna. Typically, a triband beam will have 2 pairs of traps per element. For example, a typical triband Yagi-Uda beam covering the 10, 15 and 20 meter bands would have traps for the 10 and 15 meter bands. The introduction of traps is not without cost—due to their nature, they reduce the overall bandwidth of the antenna and overall efficiency of the array on any given frequency, and radically affects its response in the desired direction.
