A NEW ANTENNA - THE PRISMATIC POLYGON

ew antennas rarely arise from a specific need and a directed solution. They are more like medical breakthroughs and likely to be happenstance. Look at Alexander Fleming’s discovery of Penicillin. He wasn’t looking for an antibiotic. He was examining a culture medium when he noted that a mold had inhibited bacterial growth. The antenna I am about to reveal does not have the import of Penicillin. It however is very useful and illustrates how fortuitous events occur. And it illustrates how “laymen” or non-professionals can come upon a concept that was right there for the plucking all along — if anyone had seen it earlier.

As some of you may be aware, I have been playing with all sorts of variants on the full wave loop. Among the antennas that I had examined were multiple loops that were attached to each other linearly — as in Figure 1. In “antennese” this is a triple full wave symmetrical linear planar loop that consists of 3 full-wave loops attached end to end. It has four radiators. You may remember it from an article here in antenneX called Multi-loop Rectangles and Compact Arrays.

One night, in the throes of insomnia, I started playing with geometrical arrangements of such linear loops. The loop in Figure 2 stuck in my mind. This was the simple planar triple loop folded back on itself so that it became a three-dimensional figure. Just for the fun of it, I modeled it with NecWinPlus the next morning. Please ignore the feed arrangement that we will get to shortly.

Figure 1: MULTI-LOOP LINEAR
PLANAR RECTANGLE – Triple Loop
prisfig1.gif (2606 bytes)

Figure 2: PRISMATIC TRIANGLE

prisfig2.gif (2479 bytes)

The Prismatic Polygon:
What immediately struck me about this antenna was that it had a VERY wide bandwidth. I fed one of the vertical wires — since I had it oriented as a vertically polarized radiator — and noted that, unsurprisingly, it had directivity in the direction of the fed wire. As with all my first models, I had designed it very simply as consisting of full wave loops of 1-meter perimeter at 300 MHz. Right at the beginning, I noted that there was almost nothing that I could do to it, dimensionally, to limit its bandwidth to less than one octave or over 2:1.

Later on, after I filed a patent on the design, I was looking for a name for the antenna. I originally called it the Cubic Polygon but thought that this might be confused with the Cubical Quad. David Jefferies was kind enough to come up with a perfectly descriptive name, the “Prismatic Polygon”, since it looked like a prism with a polygonal cross section.

The original design was a Prismatic Triangle since it consisted of three full wave loops arranged as a triangle. This is the antenna in Figure 2. Later on, I modeled antennas with 4-12 rectangular sides - Prismatic Squares to Prismatic Dodecahedrons.

Evolution of the design:
These antennas have many interesting properties. First, I found that I could negate the pattern irregularities by stacking two of these as elements in a two-element collinear array fed in phase. If I rotated the two elements with respect to each other so that the two radiators being fed were at 180 degrees, I could make the radiation pattern more circular and offset the front to back directivity.

I also noted, with stacked Prismatic Hexagons as in Figure 3 that, by feeding the same vertical wires in each of the elements, the antenna became directive with a 9 dBi gain and a cardioid pattern which had a very deep null off the back. This antenna could be electronically rotated, by changing the feed wire, for use as a scanning array.

Bandwidth:
While all of these antennas had very wide bandwidths to start off with, in the order of 3:1, I came across another variation in the feed arrangement that contributed to even wider bandwidth. This was done by feeding all of the vertical wires — in vertically polarized antennas as shown in Figure 2 — from a common point at the antenna center. The bandwidth now routinely exceeded 3:1 and, in some cases, approached 4:1 or two full octaves!

A typical SWR sweep is shown in Figure 4. This pattern, depending on wire diameter, holds across the entire spectrum from low-HF to UHF. Figure 5 illustrates a sweep from 1.8-6.6 MHz for an antenna that I modeled for L.B. Cebik, W4RNL. This is a single element Prismatic Square where the lower wires are 5 meters above ground.

Size:
The largest dimension, the vertical one in the case of vertically polarized antennas, is approximately .25 wl at the lowest operating frequency or about .35 wl at the design frequency which is about 20% higher.

The Prismatics may be called cubically compact designs since the maximum breadth is so small. On the other hand and unlike comparable vertical dipoles, there is a significant horizontal dimension. You can’t get something for nothing.

Gain:
A single element, fed at one radiator, has a gain of about 4+ dBi with directivity. Stacking two such elements collinearly and feeding opposite radiators in phase results in an almost circular Az pattern with gains of 5-7.5 dBi over the full bandwidth of about 3:1.

In the case of the “all-fed” version - with all of the radiators being fed from a common feedpoint at the antenna center - the gain of a single element is that of a dipole at the lower end of the passband and is about 2.5 dBd at the high end.

The significant contribution of the “all fed” antenna is that the gain bandwidth exceeds the SWR bandwidth. That is, the major radiation lobe remains at the boresight over the entire, very wide, bandwidth. That is over close to two full octaves. There is some minor loss of circularity at the high end, but the lowest gain does not fall below the gain at the low frequency limit.

Feedpoint Impedance:
Single feedpoint impedances vary, with geometry or the aspect ratio between the radiator height and the width of the rectangle and with the number of radiators, from 200-600 ohms. “All-fed” versions can be designed for almost any common input impedance between 25-100 ohms. This again depends on the geometry and the Zo of the transmission lines connecting the common feedpoint to the radiators.

Prototype Antennas:
I built Prismatic Triangles and Squares — with 3 and 4 radiators respectively — for 50-150 MHz, 140-450 MHz and 800-2600+ MHz. These were actually limited as to their bandwidths due to design compromises: a Zin of 50 ohms and the fact that I used TV 300 ohm twinlead to connect the common feedpoint to the radiators. The antennas worked almost exactly as modeled. This confirmed the validity of the NEC simulation for this particular type of antenna.

The ones for the ham bands are easy to build — I used “plumber’s delight” construction with copper pipe and fittings. The picture at the top of this page shows an early model of a Prismatic Octagon for 50-150 MHz with four fed wires that I have since disassembled. The square version can be constructed with the same fittings and works better.

Theoretical Analysis:
The best analogy is to the Quad Loop. Figure 6 shows how that loop is constructed with two bent dipoles that are attached at their ends. Similarly, each radiating element of a Prismatic can be thought of as a dipole with capacitive end-loading by the horizontal, or circumferential wires as David Jefferies calls them.

Taking the simplest form of a Prismatic, with a triangular cross section and with the radiators located at the apices, the antenna can be viewed as having elemental short-monopole radiators — three above and three below the feed-point — arranged not in a line, but as a 3-dimensional array. Array antennas are often thought of as elements arranged in a line or a plane, but it is much rarer to consider them arranged on a 3-D lattice. David Jefferies says that he had suggested to me much earlier that this would be a fruitful line of research for increasing the bandwidth of an array. I didn’t pick up on it at the time. Widening of the bandwidth is due to coupling between resonant elements having identical center frequencies. The broadening in a 3-D array is similar to the way energy bands arise in a Silicon semiconductor, for instance.

David Jefferies suggested that the bandwidth should depend on the coupling between the radiating elements, which is, in turn, related to the number of radiators, the distance separating them and the wire size.

The “all-fed” version can be successfully thought of as a number of radiators that are end-loaded. In this special case, there is a zero-current point at the center of each of the horizontal wires connecting the radiators. The antenna performs identically if the wires are “opened up” at these points. It also functions, albeit with a slight loss of performance, if these wires are eliminated entirely.

David Jefferies, John Belrose, VE2CV and Ron Sebol, W3HXO all pointed this out to me when they looked at the design. A simple three-dimensional arrangement of dipoles, which are all fed in phase from a common point, has a bandwidth far in excess of that of a single dipole. This is what the geometry does. But, similarly to the Quad and its two-dipole analog, the whole Prismatic package delivers more than the sum of its parts.

The circumferential wires have five raisons d’etre. These are: they end-load the dipoles to enable considerable shortening; they widen the bandwidth to some degree; they raise the feed-point impedance of the array over that of a similar array of naked dipoles; for some reason they prevent a squint or nosing up of the elevation lobe at the highest limits of the bandwidth; and, most importantly, they enable the use of thinner wires for the radiators.

The latter becomes important when constructing the antennas. For each configuration there is an optimal wire diameter. And this optimal diameter is inversely related to the number of radiators. For instance, I could not get a Prismatic Triangle to cover 140-450 MHz while using 5/8" OD pipe. The antenna easily did so, with the same plumbing, when configured as a Prismatic Square.

When I modeled the Square arrangement for use at 1.8 - 7.2 MHz, four “naked” dipoles fed similarly to those in a Prismatic Square needed radiators far greater in diameter than did the Prismatic.

Summary:
I have just described a new class of antenna that is composed of a three-dimensional arrangement of rectangles. The hallmark of the antenna is an extremely wide bandwidth. There are many possible uses depending on the feed arrangement. It can be used in its single feed or “rectangular form” for directivity and for a high feedpoint impedance or in its “all-fed” or “dipole analog” form for a wider bandwidth and a circular pattern. –30-antenneX June 2001 Online Issue #50

BRIEF BIOGRAPHY OF AUTHOR

Dan Handelsman, N2DT
Dan Handelsman, N2DT was first licensed as WA2BCG in 1957at age 13. He became interested in antennas at that time when he had to figure out a way to operate from the 6th floor of his apartment house. This resulted in a mobile whip being stuck out from a window without a counterpoise. At that point he became an "expert" in TVI. He was licensed as N2DT in 1977 and is a DX'er and contester. He is now playing with experimental antennas and low power.

Professionally, he is a Pediatric Endocrinologist and holds M.D. and J.D. degrees and is Clinical Professor of Pediatrics at the New York Medical College. As far as his antenna work he is an "amateur" in the truest sense of the word (Dan's words!).