by Neville an original AX Member *RIP
The Log Periodic Directional Array (LPDA) is intended to achieve a very wide bandwidth, whereas the Yagi has a very narrow bandwidth but achieves greater gain dependent on element spacing and number of elements.
The traditional Multi Element Yagi and LPDA designs look very similar at first glance, as both consist of a number of dipole elements spaced out along a support boom.
The Yagi, however, has only a single dipole “Driven Element” connected to the Coax transmission line, usually the second one from the “Reflector” at the back of the array. There is usually a method of matching the 50 Ohm Coax to the required 50 Ohm load of the much higher load of ?the Half Wave Driven Element. This is usually by a Capacitive mechanical ?adjustable “Gama” Match or a Reactance tuned solid wire “HairPin” or “Beta” match which is a better method as it is less affected by weather conditions or moisture. [ Coincidentally it is similar to the LPDA Zig-Zag solid wire tuning?method except for the 1:1 Balun.]
The other Yagi dipole elements on the boom are ?passive elements, with their two sides shorted, acting as Directors or Reflectors depending on their slightly different lengths and position relative to the “Driven Element”.
The difference between the LPDA and Yagi becomes obvious when examining their electrical Matching connections; Yagi’s lack the zig-zag connection between the elements.
Every element in the LPDA design is “active”, that is, connected electrically to the feedline along with the other elements, though at any one frequency most of the elements draw little current from it. Each successive element is connected in opposite phase to the active connection running as a transmission line along the boom. For that reason, that transmission line can often be seen zig-zagging across the support boom holding the elements.
Another visible difference is the spacing between the elements, which is normally constant in the Yagi, but becomes exponentially wider along the LPDA. Although both directional, the LPDA is intended to achieve a very wide bandwidth, whereas the Yagi has a very narrow bandwidth but achieves greater gain.
In general terms, the log-periodic design operates somewhat similar to a series of three-element Yagis, where each set of three consecutive elements forms a separate antenna with the driven element in the center, a director in front and reflector behind. However, the system is somewhat more complex than that, and all the elements contribute to some degree, so the gain for any given frequency is higher than a Yagi of the same dimensions as any one section of the log-periodic. However, it should also be noted that a Yagi with the same number of elements as a log-periodic would have far higher gain, as all of those elements are improving the gain of a single driven element.
The LPDA normally consists of a series of dipoles known as “elements” positioned along a support boom lying along the antenna axis. The elements are spaced at intervals following a logarithmic function of the frequency, known as d or sigma. The length of the elements correspond to resonance at different frequencies within the antenna’s overall bandwidth. This leads to a series of ever-shorter dipoles towards the “front” of the antenna.
In its common use as a television antenna, it was common to combine a log-periodic design for VHF with a Yagi for UHF, with both halves being roughly equal in size. This resulted in much higher gain for UHF, typically on the order of 10 to 14 dB on the Yagi side and 6.5 dB for the log-periodic.
An interesting Antenna Website with great information worth reading if building any beam antenna,