Which of the two will work more effectively for UHF/VHF TV transmission and reception?
Log-periodic antennas are designed to work across a wide range of frequencies; Yagi-Uda is optimised for one frequency. Yagi-Uda will give higher gain at its optimal frequency; log-periodic will have lower gain across a wide range of frequencies. So the answer depends on the bandwidth and gain you require.
Log-periodic antennas are designed to work across a wide range of frequencies; Yagi-Uda is optimised for one frequency. Yagi-Uda will give higher gain at its optimal frequency; log-periodic will have lower gain across a wide range of frequencies. So the answer depends on the bandwidth and gain you require.
On lower frequencies (HF, VHF) a disadvantage of the Log-Periodic Dipole Antenna (LPDA) over the Yagi-Uda antenna (Yagi) is the number of elements needed, and therefore weight and wind load when installed outside.
Yagi-Uda antennas can be designed to show "constant" (within 1 or 2 dB) over 15-20% bandwidth, LPDA's can span 1 or 2 octaves.
LPDA's can achieve free space antenna gains of 6-10 dBi, realizing higher gains is difficult. Whereas Yagi's can be designed to yield gains of up to 20 dBi, even more. But just like with all antennas: that gain comes at the cost of required space (boom length) and design effort.
Log periodic antenna is so named because its characteristics, could be gain or impedance, shows a periodic variation with log of the input frequency. The principle of operation may be explained by considering a log periodic dipole array. It contains a large number of elements. Both the length and the spacing between the elements are linearly tapered. Hence at a particular frequency some of the dipoles have lengths around 1/2 lambda. This set of dipoles defines the radiation zone of the antenna and take part in radiation. The set of dipoles whose lengths are larger than 1/2 lambda acts as reflector where the dipoles having smaller lengths behaves as directors. Now the operation is similar to the Yagi Uda array. With change in input frequency the location of the radiation zone changes. Corresponding to this new radiation zone the sets of reflectors and the directors also change. Larger band width is obtained because for a large range of frequencies there exists a set of dipoles whose lengths are nearer to the resonating length. Normally the number of elements in a radiating zone is around 3 where in a Yagi Uda array there is only one active element. In a log periodic array with a change in operating frequency the radiation zone changes its location and this is the main reason why it has a very large band width.
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