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An item of correspondence in the January, 1960 issue of Proceedings of the IRE(1) under the above title caught the eye of a number of u.h.f. antenna experimenters, and generated a fair amount of "Why don't we use this?" mail for the writer. The best way to answer these inquiries seemed to be to try the idea, since we already had a tabletop antenna setup for 1215 Mc. that could be adapted readily to the backfire principle.
For those who do not have access to antenna-expert Ehrenspeck's letter, the idea - is that a plane reflector can be mounted in front of what is normally the forward director of a Yagi antenna, to reverse the directivity, narrow the beam pattern and increase the gain. In an example cited, the gain of a Yagi two wavelengths long was increased by 3 db., while the minor-lobe content was substantially reduced and the main lobe sharpened from 41 to 20 degrees. This resulted from the use of a reflecting disk two wavelengths in diameter. A Yagi with a 2.5 wavelength boom and a plane reflector 4 wavelengths in diameter gave a gain of 20.5 dB over isotropic, an improvement of 6 dB over a Yagi of similar length without the plane reflector!
Anyone who has tried to build up the gain of an array that already has 14 db. knows that decibels come hard at this point. Adding 6 dB to a 14.5 dB Yagi looks almost like black magic, at first glance. Can amateurs expect to duplicate this feat? Will the end justify the means?
Like other ways of improving antenna performance, backfire gain is no something-fornothing proposition. Note that a reflector 4 wavelengths in diameter was needed for the 6 dB improvement. This rules out use of the idea by most amateurs, except for frequencies above 420 Mc - and a reflector 8 feet in diameter would be needed at that frequency. A few extra Yagis in phase, or a large-but-light collinear array, might be a better solution to most hams' need for higher gain.
Fig. 1. The backfire Yagi employs a large reflecting plane out in front of a conventional system to reverse the directivity. Because this makes each element work twice, and effectively doubles the length of the Yagi, a gain of 3 dB or more is possible.
The backfire idea suggests potential applications that some amateur u.h.f. enthusiasts might find attractive, however. One is the use of a single reflecting plane for two or more antennas. A 432-1296 combination might be interesting, for example. An array of 16 driven elements in phase for 432 Mc would require a screen reflector roughly 8 by 10 feet in size. This would make a wonderful plane for a really long (in terms of wavelength) 1296 Mc Yagi, on the opposite side of the reflector. This might be a relatively simple way of obtaining an honest 20 dB gain at 1296 Mc - a feat that is by no means as easy as some might think from having listened to "gain" figures that are often bandied about in amateur circles.
We tried backfire reflectors of various sizes with 1215 Mc Yagis of from 3 to 9 elements, and found it possible to develop appreciable gain with any Yagi, provided a sufficiently large plane was used. How much gain, did you say? After long experience in working with antennas in confined spaces, we are a bit chary with gain figures. Without a properly-designed antenna range, you can't measure gain with any degree of reliability - and there is no antenna range in the ARRL Lab. Possibly some sunny day next spring we might do a little better up on our flat roof. To more than merely reverse the directivity of a Yagi we had to use a sheet of aluminum 20 by 28 inch (almost 2 by 3 wavelengths) in size. A sheet 34 by 36 inch (about 3½ by 3¾ wave-" lengths) gave a gain indication close to 3 dB.
With reflections from all over the place it is impossible to get a suitable reference-antenna reading with a dipole. This trouble drops off with increasing directivity in the reference antenna, but it is never eliminated. Our estimates of gain are, therefore, likely to be on the low side most of the time, and inaccurate always, but they do show that the idea works, and that it is not a particularly fussy business to get at least some benefit from the installation of such a plane reflector.
We did not get anything that looked even close to 6 dB, and we suspect that there is more than meets the eye in the IRE item to be taken care of before one does achieve such a gain. It is not entirely clear, for example, just what should be done in the way of element lengths and spacings when one goes to the backfire principle. It seems fairly obvious that you do not simply make up a Yagi for optimum performance in the forward direction, add a backfire reflector, and sit back and enjoy a 6 dB improvement in the reverse direction. This is borne out by the writer's attempts to get a backfire array really going. It took some playing around with element lengths and spacings beyond that needed to make a Yagi work in the conventional manner.
Spacing of the reflecting plane away from the forward director for maximum gain varied with the element spacing, element length, and number of elements used. Furthermore, we found it difficult to get very much more out of a backfire system than could be obtained with the same number of elements, when the reflecting plane was used to replace the usual parasitic reflector, leaving the array with its normal directivity. Very possibly this indicates that we have not yet hit on the optimum element parameters for a backfire system - we merely mention this in known amateur tendency to go off the deep end in a burst of enthusiasm over any new antenna idea.
Whether the backfire Yagi is worth the effort involved is something we are not prepared to say at this writing. Possibly come spring, and the urge to work outside again, we might be able to rig up a test setup less fraught with ham traps than our basement Lab, and thus come to some better conclusions about its basic merit. Meanwhile, it's something interesting to play with, and we commend it on that basis to anyone who is intrigued by novel ideas in antenna design.
Ehrenspeck states a belief that the special forte of the backfire Yagi is the gain range around 20 dB. For much higher gain, a parabolic reflector may be desirable, and for 10 to 15 dB, stacked Yagi or collinear systems seem to hold the advantage. In favor of the idea it should be pointed out that much of the trouble in large amateur v.h.f. and u.h.f. arrays develops in the phasing systems. If these mechanical and electrical hazards can be avoided by using the backfire reflector to build up the gain of a single Yagi by a respectable amount, it may be worth consideration the next time you set out to revamp your antenna systems for those "coming" frequencies above 420 Mc.