A 3-element Yagi designed for the 20 m band. In this shorten
version using traps, each element is about 5 m long "only". The
mast hardly maintained with guy wires is about 8 m high and is equipped with
a light rotator. This is the "standard" installation to work DX
stations in good conditions. Document Kim, DS5URW.
Notice that the maximum gain and FB ratio do not occur
at the same frequency. This is true in general for 3 element designs. By making
the Yagi longer (adding more elements) and controlling the length and spacing
of each new element, it is possible to bring the frequency of maximum gain
and FB ratio closer together.
The operating bandwidth of a Yagi is often defined as the range of frequencies
over which the FB ratio is greater than 20 dB. In the chart above, the range
of frequencies over which the FB ratio is greater than 20 dB is 0.985 f0 to
1.01f0 or 2.5% of the design frequency. This is a typical bandwidth for a
Yagi array. It is possible to widen the operating bandwidth by lengthening
the array and adding elements, although this improvement normally comes at
the expense of forward gain.