s we have discussed in a group of articles in antenneX, the Prismatic Polyhedron, or Pn, is a derivative of the simple dipole, albeit with a more complex form and feed arrangement. See, Prismatic Polyhedron Measurements by David Jefferies in antenneX, November, 2002.[i]
The principal characteristic of this antenna, well documented in the above article, is its extremely wide bandwidth. This parameter has been extended, with NEC modeling and subsequent construction of prototypes, to a 5:1 frequency range. That is the range over which the SWR is equal to or less than 2:1.
In order to try to glean why the Pns perform as they do, we investigated a class of antennas, the Plate Dipoles, which were found to be most analogous to the Pns. The results of the studies were published in two articles in antenneX.[ii] These antennas were dipoles, having very wide bandwidths, where each monopole half is constructed with a rectangular sheet of metal instead of a rod. Jefferies surmises that this wide-bandwidth phenomenon is largely the result of a coupled parallel dipole effect" which, due to the distributed C and L over parallel plates of large areas, results in low stored electrical and magnetic energy and, in turn, results in the narrow, cyclic, excursions in the Rin and Xin which are characteristic of the Pn class of antennas. The plate dipoles have wide parallel conductive edges separating the two halves of the dipole. The Pns may be thought of as being skeletonized plate dipoles with most of the metal etched out, leaving only a thin outer matrix.
Recently, in an exchange of e-mail with Dave Cutberth, another one of antenneXs contributors, Dave proposed that the Pn was simply a form of the well-known cage dipole. This is understandable due to the superficial similarity in form between the two antennas. David Jefferies thought this was a reasonable conjecture, and set a project student to construct a 1-GHz cage dipole. To test this conjecture further, David suggested that I investigate the similarities and differences between them by NEC simulation, especially in the parameter which I feel is most important, that is, in bandwidth.
My belief, based on the previous work with Jefferies, was that the cage dipoles lacked the basic design elements necessary in obtaining the very wide bandwidths found with the Pn class of antennas; namely the individual radiators which are fed in phase by common wide, parallel, wires arrayed as transmission lines from the central feedpoint to each of the radiators. These are well illustrated in the photographs found in Jefferies recent article where he shows the evolution of these antennas from a dipole to a P2, a P3 and a P4.
In order to compare like-with-like antennas, I used the same wire diameter - 1.5 cm - and strove to keep the heights and widths of the antennas I compared generally the same. I modeled the performance of two sets of antenna pairs. These were: 2- and 3- radiator Pns or P2s and P3s (Figures 1 and 2) with 2- and 3-wire cage dipoles (Figures 3 and 4). The two-wire antennas were planar while the 3-wire antennas had wires arranged in a triangular polyhedron or prism.
All of the antennas that were studied were modeled over a frequency range of 300-900 MHz. There is no doubt as to the ability of NEC to deal with these antennas since many such have been built by myself and Jefferies students and, if anything, their performance exceeds that predicted by NEC.
Table 1 - Dimensions are in meters, frequency is in MHz
The two-radiator Prismatic or P2, as shown in Figure 1, has its SWR frequency sweep illustrated in Figure 5. Note the cyclic variation in the R and X in Figure 6 which is characteristic of the Pn class. The dimensions, Rin, and bandwidth data are found in Table 1. The simple P2, composed of relatively thin wire (0.015 wl), has a bandwidth of 2.6:1.
5 - P2 - SWR sweep
6 - P2 - R/X sweep
The three radiator or triangular Prismatic or P3, illustrated in Figure 2, performed as in Figures 7 and 8. The antenna with the dimensions in Table 1 had a bandwidth of 2.8:1 or 300-850 MHz. Note that with thicker (in terms of wavelength) wire at higher frequencies, this antenna has a measured bandwidth which is somewhere around 4:1.
7 - P3 - SWR sweep
8 - P3 - R/X sweep
Table 2 - Dimensions in meters, frequency in MHz. Overall height includes the heights of the two monopoles, the feed segment of 1.5 cm and the separation distance between them
The antennas in Figures 3 and 4 have the same shapes and general sizes of the Ps save for the fact that the radiators are jointly fed, typically for cage dipoles, in a triangular or conical apex arrangement at the feedpoints. The wires emanating from the feedpoint are not parallel as in the Pns. Because of this, the total conductive mass or, more importantly, surface area of the cage dipoles, is greater than that in the Pns. One would assume that this ought to be beneficial to their performance. But wait.
In addition to the lack of parallelism, the feed arrangement increases the distance between the monopole wires in the cage or widens their feedgaps. This, in turn, reduces the capacitive coupling between monopole ends, a factor which also appears to be significant in wideband performance.
I optimized each cage design by varying the overall perimeter size, the aspect ratio (AR) which is the ratio between the long vertical dimension and the narrower horizontal one, and the separation between each of the monopole halves. I targeted whatever Rin gave the widest bandwidth. The sizes and the performance data can be seen in Table 2. The 2 wire cages performance curves are in Figures 9 and 10 while the 3 wire cages are in Figures 11 and 12.
Figure 9 - 2-wire Cage Dipole - SWR sweep
10 - 2-wire Cage Dipole - R/X sweep
11 - 3-wire Cage Dipole - SWR sweep
12 - 3-wire Cage Dipole - R/X sweep
While the bandwidths are quite wide, they do not compare favorably with those of the Ps. The 3-wire cage dipoles bandwidth figure of 2.1:1 does not approach that of the simple P2.
After I had finished modeling the 3-wire cage dipole, I changed its dimensions to those of the P3 and made the feedwires parallel - as in a typical P. To clarify this further, the wires were not brought to a conical apex but were orthogonal to the radiators and parallel to each other. The only change I made was to the 6 wires coming out of the feedpoint to the radiators. These I made 1.5 mm in diameter to make them comparable to those found in the transmission lines in the P3.
13 - 3-wire Cage Dipole modified into a P3 - SWR sweep
Figure 13 clearly shows that, when the cage dipole is transformed into a Pn, the bandwidth far exceeds that of any cage conformation. This study further shows that modeling the Pn with real wires as transmission lines yields the same results as with the TL (transmission line) facility in NEC.
more comparative observations:
The cage dipoles have gains which exceed those of the comparable Ps by about 0.5 dB. But note in Tables 1 and 2 that the cages have overall heights which are greater than the Ps. The 3-wire cage dipole is 25% taller than the P3, and this is reflected in the frequency at which it becomes electrically too long and begins to show an elevation pattern that squints upward. The elevation pattern at 900 MHz is shown in Figure 14 and indicates that, above this frequency, most of the radiation will be at the higher secondary elevation lobes and not at the primary one at the horizon. So, even if we can find a design which will have a useful SWR bandwidth of 3:1, it will become useless, from a pattern point of view, at that point.
14 - Cage Dipole - 900 MHz elevation pattern
As we have pointed out in past articles, the end-loading of the Pns gives them a far better low frequency response and can lead to considerable shortening. This shortening of the radiators, in turn, pushes the point at which the undesirable elevation pattern squint occurs further up in frequency. Therefore, the P3 studied here has useable radiation patterns up to 1200 MHz or 4:1.
As mentioned in earlier articles on the Pns, this is the major advantage of the Pn class over the well-known biconicals. Those antennas also have very wide bandwidths but begin to squint at about 2.5 times the lowermost useable frequency or about when they become 1.25 wl long.
Due to what Jefferies has conjectured to be the parallel dipole effect, the parallel transmission wires in the Pn class of antennas confer the wide bandwidths that they are designed for. Cage wire arrangements of regular dipoles do indeed widen the bandwidth - a fact which has long been known. However, even with very wide cages such as those modeled here, their bandwidths do not approach those of the Pns. Moreover, although they have fractionally greater gain, their useable pattern bandwidths are limited to about 3:1 while those of the Pns can exceed 4:1.
Cage dipoles may superficially seem to be quite similar to the Pns but they are not from the point of view of the most important design criteria and performance. And they are no less difficult to construct than the Pns.-30-
[i]. This article by Jefferies lists all of the relevant references to the plate dipoles and Prismatics discussed here.
[ii].Plate Dipoles and Prismatics, Dan Handelsman and David Jefferies, antenneX. #44 in Archive V and Plate Dipoles and Prismatics - 2, David Jefferies and Dan Handelsman, antenneX, #56 in Archive V.
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!).
Dr. David J. Jefferies
School of Electronic Engineering, Information Technology and Mathematics
University of Surrey
Guildford GU2 7XH
Click Here for the Authors' Biography
~ antenneX ~ December 2002 Online Issue #68 ~
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