The Pros and Cons of Log Periodic Antennas
An Introduction to Log Periodic Antennas
There are several different types of log periodic antennas, such as the planar log periodic, zig-zag periodic, slot periodic antenna, V LP antenna, trapezoidal and dipole LP antenna. The log periodic dipole array antenna is abbreviated to LPDA for short; this is the most common type of log periodic antenna.
In all cases, the longest element of the log periodic antenna is at the rear of the array. The log periodic antenna is similar to a yagi antenna but not exactly the same. For the log periodic antenna, the driven elements decrease in size where none are the same length. For a yagi antenna, most of the elements are of the same length. The highest frequency the antenna can receive is a function of the shortest element at the front. The element at the back is half a wavelength of the lowest frequency in operation.
What are the advantages and disadvantages of log periodic antennas?
The Advantages of Log Periodic Antennas
They have very wide bandwidths. Log periodic antennas have much broader frequency bandwidths than Yagi antennas. One log periodic antenna may work for both HF and UHF frequencies. A log periodic antenna can be used for EMC measurements, when it is necessary to scan a wide band of frequencies.
Log periodic antennas have the same radiation resistance over their frequency range. This gives it the same SWR whether at the low point of its frequency range or the high end.
LP antennas also have the same gain and back to front ratio, and they have high forward gain. In contrast, a yagi antenna would experience degradation of its gain factor or front to back ratio as the frequency shifted from the one the antenna was optimized for. Three to six dB gain over a 2:1 bandwidth is reasonable with a log periodic antenna.
Their feed point impedance is mostly constant.
Because log periodic antennas have elements of different lengths as part of their design, it is easy to make changes in its frequency with relatively little impact on its electrical characteristics. Adding elements to a log periodic dipole antenna increases its bandwidth.
Log periodic antennas typically have low SWR, rarely greater than 2:1. You may be able to achieve an SWR level better than 1.3:1.
Since a log periodic antenna electrically acts like an array of yagi antennas, using a single log periodic antenna can replace multiple yagis.
Disadvantages of Log Periodic Antennas
They have very low gain. They have less gain than a yagi antenna of the same size. And they have low gain per unit of weight or wind load.
In order for a log periodic antenna to have very good VSWR performance, it needs to be very large. For smaller log periodic antennas, the VSWR performance is not very good. This can be offset by adding a yagi antenna to the receiving array.
Observations about Log Periodic Antennas
Log periodic antennas are beam antennas, not omni-directional. And they have a low polarization ratio as a result.
Elements in a log periodic antenna may be sloped forward or simply sit perpendicular to the main feed. Herringbone Log Periodics, log periodic antennas with swept forward elements, work well on their 3rd harmonics. Since TV channels 7 through 13 are the 3rd harmonic of TV Channels 2 through 6, making herringbone log periodic antennas popular as TV antennas.
Parasitic elements are sometimes used to enhance the gain or front to back ratio of a log periodic antenna.
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