Publications Departmentby Edward R. Cole, AL7EB (al7eb@ptialaska.net)
SETI League volunteer Regional Coordinator for Alaska
This article documents research done by Dr. Darrel Emerson, AA7FV, Kitt Peak Observatory: demerson@nrao.edu
Several studies of the axial-mode Helix, created by Dr. John Kraus, W8JK, at Ohio State University, indicate that an antenna of 7 wavelengths and approximately 1.05 wavelength circumference will produce gain of ~15 dbi (Kraus' early formulae appear to overestimate the gain of a helix). Several studies have been done: King and Wong, Lee and Wong...Darrel has a web page which investigates the Helix and shows several graphs. NEC analysis is also included.
Mike Cook, AF9Y created a 437 Mhz tapered Helix design based on these studies (which could be scaled to 1420 Mhz), and can seen on his web page. This is essentially the antenna I built for 437 Mhz with one difference: I used a 15 inch aluminum pizza pan for a reflector. The AF9Y Helix is fool-proof, if scaled. The tapered design improves circularity, as the standard Kraus Helix exhibits a little assymetry. I would use a reflector screen, however, to ensure good G/T. I've toyed with the idea of building one of these to compare with a dish.
The spacing between helices is similar to spacing of yagi antennas. The goal is to have their effective aperatures just touch to maximize gain. A small overlap of aperature is usually done to minimize sidelobes at a small sacrifice of gain. Aperature of an ideal dish is equal to its physical size, whereas antennas such as yagis and helices the aperature is usually greater than the physical dimension. A good antenna book will give the relationship between gain and aperature. I suppose NEC can do the same.
The rule of thumb for long (≥2 wavelength) yagis is to space antennas of an array at half the boom length. Therefore for a 7 wave helix spacing would be 3.5x21cm=73.5cm. This is about 2.4 feet. An array of 16 of these helices would be 7.2 foot by 7.2 foot square and the ground plane would be 5 inches larger (or 7.65 feet square). From Kraus, the number of turns = boom length/S, where S=Ctan(a). S is the turn spacing, C is the turn circumference, and a is the turn pitch angle. For C=1.05 and a=12.5 deg, S=.23 wavelengths. 7/.23=30 turns. AF9Y claimed 15.9 dbi gain from a 23 turn helix, so maybe a 30 turn helix would do a bit better. Kraus' book "Antennas", gives a good example of how to feed four helices from a common 50 ohm coax feed.
In summary, Darrel Emerson reports: "The modelling studies, and recent papers in the literature, all agree that the simple formula for gain of the helix vs. length derived by Kraus is too optimistic. King and Wong, and Lee and Wong, are the classic references."
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