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Fall 2002 Issue |
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Log-Periodic
Dipole Arrays |
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One thing about the VHF+ ham bands: There are lots of them, which can be a problem if you don’t have enough antennas. Enter the log periodic. K1ZJH gives an over view of this antenna and briefly reviews some of the more popular models.
By Peter Bertini,* K1ZJH |
 Photo 1. The author’s antennas required only an additional several feet of mast space above the HF antenna. Note that the Rover is nestled below the top-mounted KMA-4113. |
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Here’s something for a budding VHF/UHF enthusiast to consider before spending lots of money on single-band Yagis. Amateur VHF/UHF band assignments span from 50 through 1300 MHz. Within this spectrum, there are no less than six amateur bands to use and explore, or seven bands if you include the British 4-meter 70-MHz band! Things have changed a lot in the past several years. Many HF rigs now sport 6- and 2-meter and 70-cm band multimode coverage out-of-the-box. Perhaps you are one of those HF rig owners with a hankering to explore those additional bands—at minimal cost, of course—to see if VHF+ operating is your cup of tea. For many of us on limited ham budgets, being able to afford six single-band antennas, along with the requisite individual coax runs, connectors, and related tower-mounting expenses, is just not feasible . . . sigh. Hence, those elusive frequency allocations remain just out of reach! For years, my modest VHF/UHF antenna farm consisted of individual long-boom Yagi antennas for the 6-meter, 2-meter, and 70-cm bands. After 15 or 20 years most of these antennas needed serious attention: Failing coax-connector seals, overweight birds, Father Time, and some close encounters with wind-driven tree branches had taken their toll. I enjoy VHF operating, but lately my VHF operating is growing more casual, with some infrequent contesting throw in. My need was for a single antenna that would cover all of the VHF/UHF bands, with gain and directivity. I also wanted something that could be handled on a modest fold-over tower, requiring minimal maintenance. Enter The LPDA!
First things first: What is a log-periodic
antenna? Well, this antenna has been around for years. Many 12-channel VHF
TV antennas are log-periodic designs, and some early KLM VHF/UHF antennas
used log-periodic drivers to achieve full-band coverage. You can view them
as being a series of dipole antennas, each cut for a slightly different
frequency, all being fed, transposed fashion, from a single feedline. At
any frequency within the antenna’s operating range, a longer element
serves as a reflector, while one or more of the forward shorter elements
work as directors. Some limitations are placed on the minimum number of
elements needed to do the job—too few—and the SWR will vary widely across
the design range! There are several different log-periodic antenna
variations, with the Log Periodic Dipole Array (LPDA) being most popular
among amateurs. Figure 1 shows a schematic of how the LPDA looks
electrically.
Click here to subscribe to VHF and read more about Log-Periodic Dipole Arrays. _________________ © Copyright 2002, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.
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