The Flat-Plane Reflector for 432 MHz
The search for a good wide-band vertically polarized antenna with utility-level performance goes on and on, with various candidates being offered as the current champion. Perhaps a review of some alternatives might be useful, starting with a Yagi as a usable standard, and then looking at some flat-reflector alternatives, and closing with a modest corner reflector.
The following table gives the dimensions in inches:
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6-Element Wide-Band 435-MHz Yagi
Element Length Space from
Inches Reflector (Inches)
Reflector 13.46 -----
Driver 12.32 5.94
Director 1 11.10 9.72
Director 2 10.79 16.18
Director 3 10.20 24.94
Director 4 9.80 33.94
Elements are 0.5" diameter aluminum and should be considered well insulated and
isolated from any conductive boom material.
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The Yagi provides a very low SWR across the entire band, as evidenced by the 50-Ohm SWR plot in Fig. 2. For this antenna and those to follow, all data is from NEC-4 models of the antenna 20' above average ground.
The following table gives a spot check of performance across the band when used as a vertically polarized antenna. At 20' up, all of the antennas have TO angles (elevation angles of maximum radiation) between 1.5 and 1.6 degrees, assuming a clear path and flat, uncluttered ground.
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6-Element Wide-Band 435-MHz Yagi Performance
Freq. Gain Front-Back Beamwidth Feedpoint Z 50-Ohm
MHz dBi dBi degrees R+/-jX Ohms SWR
420 15.86 18.26 65.2 44.5-j 9.6 1.27
435 16.62 25.79 59.6 45.1+j 4.3 1.15
450 17.25 17.32 53.4 50.7+j 9.7 1.21
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Although the Yagi exhibits a very flat SWR, some other figures show considerable variation, even within the relaxed standards one might associate with a utility antenna. For example, the gain varies by almost 1.4 dB, while the front-to-back ratio varies by almost 8.5 dB. Even the half-power beamwidth varies by almost 12 degrees from one end of the band to the other.
As well, Fig. 3 exhibits a considerable variation in the azimuth pattern, especially among lobes other than the main forward lobe. As we raise the frequency, the single rear lobe at 420 MHz splits into two lobes that gradually work forward until at 450 MHz we have a new single rear lobe with two prominent rearward sidelobes. The forward sidelobes are down from the main forward lobe by only 14 to 15 dB across the band.
The Yagi's strong suit is gain, which averages about 16.5 dBi. However, some wide-band vertical antenna users may wish to achieve better patterns with less variation in operating properties as we change frequencies. If we take the 3' by 1' dimensions of the utility Yagi as a general building limit guide, then we must begin to think of other antenna arrangements.
For all but one of the antennas that we shall examine, it makes no difference whether we construct the reflector from a series of vertical bars about 2" apart, from a screen mesh, or from a solid panel. (The double-quad version will require either a screen or a solid panel.) Construction considerations, along with the ease of obtaining materials, will largely determine the selection of the flat-reflector type.
A flat reflector has a special advantage: we may mount it close to the supporting mast, thus easing certain (but not all) challenges of structural durability.
The driver dipole is 0.5" in diameters and spaced 4.7" from the reflector. One of the flexibilities offered by the flat-reflector + driver system is that you can adjust the feedpoint impedance of the array by juggling the reflector-driver spacing and driver length for any vale between 50 and 100 Ohms without serious harm to the overall performance figures.
Fig. 5 shows that we can obtain a 1.5:1 SWR maximum across the entire band. Should you wish to emphasize only part of the band, you may adjust the driver spacing and length to move the SWR curve in either direction.
The following table provides modeled performance figures for the single-driver array.
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Flat Reflector with Single Dipole Driver Performance
Freq. Gain Front-Back Beamwidth Feedpoint Z 50-Ohm
MHz dBi dBi degrees R+/-jX Ohms SWR
420 13.36 15.66 84.4 40.4-j15.5 1.50
435 13.42 16.24 84.0 48.7+j 3.1 1.07
450 13.44 16.79 83.8 58.5+j21.5 1.53
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Across the entire band, the gain varies by less than 0.1 dB, although the average value is 2 dB below that of the Yagi. The front-to-back ratio varies by just over 1 dB, while the beamwidth varies by just over a half degree. Hence, the user can expect virtually identical performance from the antenna anywhere in the band. For that reason, a single sample azimuth pattern suffices to illustrate the well-controlled pattern created by the antenna. See Fig. 6.
The reflector dimensions for the antenna just shown are 24" by 20". If we enlarge the reflector to 44" by 32", creating just over double the surface area, we increase the gain at 435 MHz to 14.22 dBi and the front-to-back ratio climbs to 19.18 dB, with a beamwidth of 82.8 degrees. These are gains over the smaller model of about 0.8 dB for gain and 3 dB for the front-to-back ratio, with a beamwidth loss of just over 1 degree. Fig. 7 shows that the rear portion of the pattern draws inward without creating problems for the forward part of the pattern.
As Fig. 8 shows, the SWR curve does not significantly change despite the increase in reflector area.
The single-dipole-driver version of the flat-plane reflector array provides smooth performance, but remains noticeably lower in gain than the Yagi. However, the flat-plane reflector lends itself to a number of interesting driver variations.
A pair of dipoles at least 1/2 wavelength apart and fed in phase will provide considerable bi-directional gain over a single dipole. If we place these 2 dipoles in front of a flat reflector, we may accrue most of this gain in the form of a stronger main forward lobe for a flat-plane reflector array.
For our design experiment, let's use the same small 24" by 20" reflector. Then the final array will have the appearance of the sketch in Fig. 9.
The dipoles are longer and spaced further away from the reflector than the single dipole in our first attempt at a flat-plane reflector array. These dimension arise from the requirement for in-phase feeding of the dipole pair. The dipole length and spacing emerge from design goal of achieving a 100-Ohm resonant impedance for each dipole. One of the flexibilities of the array (which also applies to the corner reflector) is the ability to adjust the driver element(s) spacing for virtually any desired impedance in the 50 to 100 Ohms range, with suitable changes in length to eliminate reactance at the design frequency.
The 100-Ohm dipoles are nearly an exact match for a 100-Ohm transmission line running from each dipole to a center point, where they join in parallel. The result is a 50-Ohm impedance for the main feedline. We achieve a very broadband array in terms of SWR by matching the "phasing" line to the dipole. Had we used 50-Ohm dipoles with a 75-Ohm line to transform the impedances to 100 Ohms, we would shrink the bandwidth somewhat, since the required 1/4 wavelength lines would be the required length for only a small part of the 30-MHz bandwidth.
Constructing the required dipoles and their phase lines is likely best done with a version of a glass board. Given the match of dipole impedance to line impedance, the 100-Ohm lines can be any length, so long as the lines to each dipole ar identical. Strips on each side of the board would allow straight-line phase line design. The dipoles might also be composed of strips on the board. However, translating the 0.5" diameter round conductors of the NEC-4 model into flat strips and finding the exact line width for a 100-Ohm impedance with whatever might be the exact thickness and composition of the glass board very likely would call for use of an FDTD program--well beyond my current economic means, since virtually all FDTD programs are proprietary and expensive.
Should you be able to meet the physical challenges of construction for the dual-dipole flat-reflector array, you will achieve an SWR plot for the 420-450-MHz range like the one in Fig. 10. The parallel reactances of the dipoles as you move away from the design frequency grow at a slower rate than would be the case for a single driver. Consequently, band-edge SWR values well below 1.25:1 are certainly possible. The following table provides the performance samples that we have gathered so far for each antenna.
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Flat Reflector with a Dual-Dipole Driver Performance
Freq. Gain Front-Back Beamwidth Feedpoint Z 50-Ohm
MHz dBi dBi degrees R+/-jX Ohms SWR
420 14.93 16.52 53.4 56.4-j 7.6 1.21
435 15.11 16.81 52.0 47.5-j 4.0 1.10
450 15.25 17.06 50.6 41.8+j 1.0 1.20
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Across the band, the gain varies by about 0.3 dB, with a 0.5 dB front-to-back ratio variation. The beamwidth changes by less than 3 degrees, and is the only variation significantly higher than for a single dipole.
If we use the small reflector for the array, we pay a cost for the added gain over a single dipole (about 1.7 dB). As Fig. 11 shows, we acquire two rearward quartering sidelobes similar in strength to the main rear lobe. The rearward lobes derive in part from the dipole spacing: greater than 1/2 wavelength in order to achieve maximum gain. With such wide spacing, the individual dipoles move quite close to the edge of the reflector plane, reducing its benefits.
We may better control the rearward lobes and obtain further forward gain by using a larger reflector. If we place the array--with no changes in dipole length of spacing--ahead of the 44" by 32" reflector, we obtain the much better controlled pattern of Fig. 12. all rearward lobes are down by 20 dB. In addition we add another 1.35 dB to the gain for a total of 16.45 dBi at a half-power beamwidth of about 46 degrees. At this size reflector, the array become very competitive with the 6-element Yagi that we employed as a standard against which to measure performance. Although the flat-plane reflector dimensions are greater than those of the Yagi, the array requires no special consideration of its distance from a supporting mast. In addition, the greatest support occurs in the region of greatest mass for the array, a feature that offers lower bending moments in the wind.
The dual-quad is a UHF adaptation of dual loops that have been common for many years in the lower HF region. Dual deltas, dual rectangles, and dual half-squares (called bobtail curtains) have a long history of providing considerable bi-directional gain over their single-section counterparts. Like the antennas used at lower frequencies, a UHF dual quad is subject to the same rules of composition.
A dual quad loop, when composed of two squares, has a feedpoint impedance of about 80 Ohms. To lower this impedance to the design figure of 50 Ohms, we must change the length (side-to-side) to height (bottom to top) ratio of each loop from a square 1:1 ratio to a value of about 1.4:1. As shown in the antenna sketch, the loops in the final design are 11" by 8.6", about 1.28:1. The alteration from the free-standing ratio arises from the presence of the reflector. The reflector-to-driver spacing of 4" dictates loop alteration to maintain the 50-Ohm feedpoint impedance. The design shown in the sketch is subject to alterations in spacing, if you also change the loop dimensions to sustain the desired feedpoint impedance.
As a single-driver system, the dual-quad array cannot produce the exceptionally flat SWR curve of the dual-dipole array. As the curve in Fig. 14 reveals, the array is capable of holding a 50-Ohm SWR well under 2:1 across the band. This curve applies to the 4-mm (0.1575") diameter loop element used in the design example. Increasing the loop element diameter will yield a flatter SWR curve, but at a cost of increased loop circumference and a change in the length-to-height ratio. Practical considerations, such as the feasibility of bending the loop element material into the desired shape, may well limit the diameter of the element. In contrast, there is no practical limit to the diameter of the dipoles used in the preceding array designs.
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Flat Reflector with a Dual-Quad Driver Performance
Freq. Gain Front-Back Beamwidth Feedpoint Z 50-Ohm
MHz dBi dBi degrees R+/-jX Ohms SWR
420 14.71 28.99 54.8 47.0-j33.2 1.96
435 14.81 29.61 54.0 55.2-j 7.6 1.19
450 14.89 29.78 53.2 65.4+j17.8 1.50
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The table of sampled values reveals why the SWR curve for the dual-quad driver is steeper than for the other arrays: the reactance at the feedpoint changes more rapidly as we change frequency within the operating passband. However, the gain and front-to-back ratios are very stable, changing by under 0.2 dB and under 1 dB, respectively. The beamwidth variation over the entire band is only about 1.5 degrees.
Fig. 15 shows the azimuth pattern of the array at mid-band 20' above average ground at an elevation angle of 1.6 degrees. The very well-behaved side and rear lobes deserve special attention. We may obtain a high front-to-back ratio only by use of a screen or solid reflector. If we use a reflector composed of vertical bars, the front-to-back ratio will decrease by as much as 8 dB. The quad loop driver is only dominantly vertically polarized. Some horizontally polarized radiation remains owing to the driver shape. A reflector composed only of vertical bars is relatively ineffective on such radiation.
As well, the use of a larger reflector--the 44" by 32" sample--does not yield the dramatic improvement that it did for the dual-dipole array. The small-reflector gain values are similar for the two arrays. However, the larger reflector yields only about a 0.5 dB gain addition for the dual-quad driver (compared to the 1.35 dB addition for the dual-dipole array). Hence, even with a large reflector, the dual-quad array shows a full dB gain deficit relative to the 6-element Yagi uses as a standard.
The individual planes--whether composed of vertical bars, a screen, or a solid surface--is about 16" high and 22.6" long. The resulting aperture or distance from one extreme edge to the other is 32". The driver is a 0.5" diameter element that is 11" long and spaced 8.5" from the reflector apex.
As shown in Fig. 17, the corner reflector is capable of good SWR bandwidth. The use of a "bow-tie" or bi-conical driver elements can increase the operating bandwidth of the array. As well, increased spacing and the consequential increase on the feedpoint impedance to the 100-Ohm range can also increase the SWR bandwidth.
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Corner Reflector with a Dipole Driver Performance
Freq. Gain Front-Back Beamwidth Feedpoint Z 50-Ohm
MHz dBi dBi degrees R+/-jX Ohms SWR
420 14.75 35.54 60.0 41.8-j14.9 1.45
435 14.91 36.05 58.4 52.6+j 4.7 1.11
450 15.04 36.99 57.0 65.4+j23.7 1.63
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Part of the reason why the corner reflector SWR curve is flatter than that of the dual-quad flat-reflector array owes to the smaller excursion of reactance across the band. The gain figures for the corner array are comparable to those of the dual-quad array, but the front-to-back ratio continues to improve. The variation in gain is only about 0.3 dB, with a 3-degree change in beamwidth.
Fig. 18 shows the mid-band azimuth pattern for the corner array, a version of a high-gain cardioid pattern. Increasing the reflector size can increase gain quite dramatically. If we increase the reflector size to achieve a 44" aperture with a 32" height, we achieve a mid-band gain of 16.45 dBi, once more highly competitive with the Yagi. Fig. 19 shows the resulting higher-gain azimuth pattern,
One of the interesting facts about the corner reflector is that it shows parasitically tuned behaviors. Consequently not all performance parameters increase uniformly or remain constant. The front-to-back ratio of the larger reflector model is lower (26.49 dB) than for the smaller reflector, despite the rise in gain. Moreover, the performance changes show some periodicalness: they either rise and fall or change the rate of rise as we increase the reflector dimensions either in height or in aperture. See the notes on corner reflectors for further modeling experiments (Corner Reflectors Revisited: Parts 1-3).
Despite the variations in the rate of gain change and the rise and fall of front-to-back values, increasing the reflector size generally increases gain. The best gain value that I have modeled would translate into a 20-dBi gain over ground, although the reflector may reach prohibitive size for the 420-450-MHz band. Nevertheless, the construction difficulties are likely no greater--even if quite different qualitatively--than those associated with long-boom Yagis of the same performance potential.
The notes have omitted many details that would be a normal part of any antenna design project prior to actually constructing an array. Many facets of design work for the future have received warnings or hints, but much more remains to be done before we can draw any definitive conclusions about any of the arrays discussed. Nevertheless, the flat-plane-reflector array can be a useful alternative to corner-reflector arrays and Yagis as a utility antenna for the 420-450-MHz band. The dual-dipole version of the array is especially interesting, since it can--with a larger or perfected reflector--compete easily with the best of wide-band Yagi designs and with corner reflectors with comparable reflector areas.
Updated 11-08-2001. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.
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