Some Log Periodic Notes
The purpose of this article is to review bit of history and to encourage further experimentation with the Yagi /log periodic designs.
The Log Periodic Antenna came about in one way by the necessity to simplify high gain receive cable TV systems. For example, the legendary Oliver Swan, working on remote sites in Mexico, was using large wire modified parabolic reflectors focused on multiple Yagi beams to amplify stateside TV signals for cable systems. All of these channels had to be combined before distribution. You can imagine the number of beams, messy feedline and impedance matching tangles. He then reasoned, “Why not make one pickup antenna (Yagi) at a focal point of the big dish?”
Well it worked, by connecting all elements in line, starting from the lowest TV band to the highest pointed at the “dish”. He now had one transmission line to contend with. Many cable companies began doing the same more or less.
The University of Ohio Engineering Department began trying different combinations of Yagi-type antennas, spacing and element lengths finding whatever worked. The data was recorded and passed to a Mr. Carroll, who was in charge and being the most skilled mathematician of the team. His job was to “make sense” of all this data. After finding a general pattern which seemed to fit most cases, the next step was to name this array and being a mathematician, he chose “log periodic”.
Later, improved designs followed. George Smith, W4AEO built many wire-type log periodics. Perhaps most successful was a monobander for 20 meters with a parasitic reflector and one director in front of the log periodic. During the late 1960’s very poor propagation times, this combination gave the best signal reports to Australia from his home QTH.
My Version
I took this idea another step by building a two-meter LP and placing a director .1 wavelength in front of each element with each director being 5% shorter than the LP element. My thinking at the time was there might be so much lowering of impedance, matching might be difficult. So I made each LP element a three-wire folded dipole. I wanted to put parasitic reflectors behind the LP but had mechanical problems, though I knew it would be nice. Testing was done with signal generator connected to a receiver and a dipole arrangement with a calibrated attenuator to get gain and pattern measurements. This range was 7.5 wavelengths long.
D1=-3% of L1 Spaced
.1 wavelength of L1
D2=-3% of L2 Spaced
.1 wavelength of L2
D3=-3% of L3 Spaced
.1 wavelength of L3
D4=-3% of L4 Spaced
.1 wavelength of L4
- Note: Only alternate LP elements are “cross connected”. All Yagi elements are electrically isolated from LP, boom and dipole. There is no phasing loop at the longest end of the LP dipole.
- Simple design = the longest LP dipole (L1) is (Fr x 30% of center of frequency of interest). Each succeeding LP = -5%. For example, L2 -5% of L1, etc. Frequency of last LP should be 1.5 times frequency of L1.
- All of this can be plotted on graph paper. The boom length for 75/80 meters is 85’ +/- with reflector, which is close to a simple wide-spaced 3EL Yagi.
- The final balun ratio must be found by measurement.
More Gains
Having since moved from that original location, all data has been lost. However, I do recall a gain of 12 dB or more even at zero elevation angles as Mr. Carroll had given his gain figures for the basic LP of a few elements. Since that time many hams have found that alternate crossing of feedline elements gave much more gain. George Smith claimed better phase matching of elements and no perceivable benefits of a loop at the end (longest element). Having tried it myself, I agree with this conclusion.
Years later I acquired “The Antenna Engineering Handbook” second edition by Richard Johnson, Henry Jasik, McGraw-Hill. Patent #3257,661 (The Clamped Mode) and extended aperture, Patent #3618,110. This clamped mode gives three dB more gain with the same number of elements and NO increase of boom length. Sure it is a bit wider, but anyone wanting lots of gain, cheap construction using just wires and working at below 75 to 100 feet of height and still get the classic Yagi pattern, try this LP.
In Conclusion
The LP is a very forgiving antenna and made of long wire, a very rugged antenna, which here in northern Maine had understood, ice, sleet, wind, for nearly nine years. The 5 element LP periodic with Yagi elements should be in the 12 – 16 dB range, even at less than optimal height. A straight Yagi design is influenced too much by ground reflections.
Originally posted on the AntennaX Online Magazine by L. B. Cebik, W4RNL
Last Updated : 16th May 2024