By Dr. Glen Dash, Ampyx LLC
In 1957, R.H. DuHamel and D.E. Isbell published the first work on what was to become known as the log periodic array. These remarkable antennas exhibit relatively uniform input impedances, VSWR, and radiation characteristics over a wide range of frequencies. The design is so simple that in retrospect it is remarkable that it was not invented earlier. In essence, log periodic arrays are a group of dipole antennas of varying sizes strung together and fed alternately through a common transmission line. Still, despite its simplicity, the log periodic antenna remains a subject of considerable study even today.
The log periodic antenna works the way one intuitively would expect. Its "active region," -- that portion of the antenna which is actually radiating or receiving radiation efficiently -- shifts with frequency. The longest element is active at the antenna's lowest usable frequency where it acts as a half wave dipole. As the frequency shifts upward, the active region shifts forward. The upper frequency limit of the antenna is a function of the shortest elements.
Log periodic designs vary, but the one most commonly used for EMC work is the Log Periodic Dipole Array (LPDA) invented by D.E. Isbell of the University of Illinois. The LPDA we will discuss in this article covers a frequency range of 200 to 1000 MHz. We did not actually build it, but we did simulate its operation on a Method of Moments simulator.
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