LOG PERIODIC DuHamel – University of Illinois  altered the bi-fin antenna linear polarization, satisfied ANGLE PRICIPLE, but not TRUNCATION PRINCIPLE. needed to cause currents to fall off more rapidly introduced discontinuities – “teeth” in fins – to increase radiation and increase decay of “I “

LOG PERIODIC (CONT) To satisfy ANGLE PRINCIPLE concept of Rumsey, teeth were made along circular arcs so their length was determined by an angle BUT HOW TO DETERMINE SPACING ?

LOG PERIODIC (CONT) Looking at equiangular spiral on a radial line from center the spacing of the arms are in a constant ratio So radii of successive teeth were chosen such that

LOG PERIODIC (CONT) Resulting structure is not exactly frequency independent, but it should have identical characteristics at a discrete number of frequencies Example If the p-th tooth is at then the (p-1)th tooth is at

LOG PERIODIC (CONT)  so identical performance at as long as the structure is accurate at the feed and there are negligible currents at the truncation point.

LOG PERIODIC (CONT) Fig. 11-6 Planar and wire logarithmically periodic antennas

LOG PERIODIC (CONT) Since performance is periodic function of the log of the frequency LOG-PERIODIC ANTENNAS Experimentally, they found that the variation in pattern between the periods were relatively minor for designs where  was not too far from 1.0

LOG PERIODIC (CONT) Still this periodic antenna was bi-directional and had a broad pattern z BI-FIN had fields polarized in dir. Some models of log-periodic were found to have linear polarization in the dir. (along dir. of teeth instead of perpendicular to them) z x x BI-FIN LOG PERIODIC

LOG PERIODIC (CONT) Isbell – ignored restrictions on planar structures by Babinet. folded planar L-P structure into a wedge got unidirectional pattern off the tip same bandwidth -dir polarization could be made straight (trapizodal) most currents were in teeth whose length was near [see model] Also found they could only just outline the periphery of the sheet structure with a wire and it would still work

LOG PERIODIC (CONT) Fig. 11-6 Planar and wire trapezoidal toothed log - periodic antennas

LOG-PERIODIC (CONT) Fig. 11.8 Linearly polarized flush mounted cavity-backed log-periodic slot antenna