Ham Radio 2000 #2

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----- ELNEC Example Antenna File Notes ----- 1 February 1991 Several example antenna description files are included on your ELNEC disk. Following are some notes about each one. All files have the extension ".EN" added to the name shown here. Rather than having the examples all show ELNEC in its best light, some were chosen specifically to illustrate weaknesses or limitations you need to be aware of when using ELNEC or any MININEC-based program. DEMO USERS NOTE: The antennas marked '*' are too complex for the DEMO program due to its 15-pulse limit, so they're not on the DEMO disks. Also, some of the other antennas differ slightly from the descriptions given here because the total number of segments was reduced. * 15MQUAD The dimensions for this 15 meter quad, modeled in free space, come from the ARRL ANTENNA BOOK, 15th ed., p. 12-2. The design is attributed to W7ZQ. As it does on most quads, ELNEC (and MININEC) reports a poor front/back ratio -- only 2 dB for this quad. This is a result of MININEC's "cutting corners" where wires are connected. A much more accurate analysis is obtained by tapering the segment length using ELNEC's automated segment tapering capability (see the manual for information). The result of tapering is file 15MQUADT, described next. If you delete the parasitic loop and look at the pattern of the driven element by itself, you'll find that very few segments are necessary to obtain an accurate pattern. * 15MQUADT If you take the 15 meter quad, above, and taper the segments using ELNEC's automated segment tapering with default values, the result is this model. The indicated front/back has improved to 8.3 dB from 2 dB and the forward gain has increased about 0.8 dB. I believe that this is an accurate representation of the operation of this quad. You probably can get sufficiently accurate results with fewer total segments by experimenting with the maximum and minimum segment length when doing the tapering. Note that very many segments per side are necessary to approach the accuracy of the results achievable by using the segment tapering technique. 4SQUARE This popular phased array was invented by Dana Atchley, W1CF. It has several desirable properties. Because of its symmetry, it's easy to switch in four directions. The forward lobe is broad enough that four-direction switching gives good coverage to all directions. Good rejection of signals occurs over a broad region to the rear. The small rear nulls can be eliminated and the forward gain increased slightly by increasing the element spacing. However, this may make the physical spacing too far to conveniently use some feed methods. Feeding of the four-square array is covered in detail in Chapter 8 of the ARRL ANTENNA BOOK, 15th Edition. One interesting feature of this array is that one element has a negative feedpoint resistance if loss is low enough. This is a valid result; the element is absorbing power from the others by mutual coupling and feeding power back INTO the feed system. There is still some lingering belief that the fields from the elements of an array are proportional to the powers delivered to them. Element 1 of this array has a field equal to that from the other elements, and it's FURNISHING power, showing the error in this belief. Observing the currents in the elements reveals the truth: The element CURRENTS determine the field strength, and they're equal in all four elements. CARDIOID A popular phased array, the 90-degree phased, quarter-wavelength spaced, two-element array is effective and forgiving, and gives an honest 3 dB gain over a single element. It's modeled here over perfect ground to point out the small reverse lobe which doesn't usually show up in the textbooks. This is due to the change in current distribution on the elements from mutual coupling effects. See July 1990 QST, page 39, for more information on this phenomenon and its consequences. For information on how to design a feed system for this array see the ARRL ANTENNA BOOK, 15th Ed., Chapter 8, and "The Simplest Phased Array Feed System -- That Works" in the second ARRL ANTENNA COMPENDIUM. DIPOLE1 This is a plain dipole in free space, about the simplest antenna you're likely to model. The frequency is 299.7+ MHz (selected by entering '0' for the frequency), at which a wavelength is one meter. The antenna dimensions, in meters, are therefore also the dimensions in wavelengths. Note that this antenna, exactly a half- wavelength long, is greater than a resonant length, as indicated by the positive feedpoint reactance (seen by selecting 'SD' in the Main Menu). A resonant "half-wave" antenna is somewhat shorter than 0.5 wavelength, the amount depending on its diameter. FDSP A personal favorite, the "Field Day Special" has been built on several bands and accounts well for itself from the home QTH as well as on Field Day. It was described in June, 1984 QST. The elements are folded dipoles made from twinlead, connected by a half-twisted twinlead "phasing line". Folded dipoles are difficult to model with ELNEC, so the elements are modeled as ordinary dipoles with a diameter equivalent to the effective diameter of the two-conductor twinlead. This is valid since the radiation properties of ordinary and folded dipoles are identical -- only the feedpoint impedance is affected by the "folding" process. The program used to design the original antenna wasn't entirely accurate so the element currents reported in QST weren't quite correct. The source currents in the model are the currents actually measured on the elements of a Field Day Special built to the dimensions shown in the QST article. This antenna has a respectable gain at low angles, a good f/b ratio, and a broad forward lobe. It's also quite forgiving. Constructed from twinlead, the input SWR is near unity. * N4PCLOOP This multiband horizontal loop antenna was created by Paul Carr, N4PC, and described in December, 1990 CQ Magazine. A unique feature is that it's driven at opposite corners by out-of-phase signals as in the W8JK antenna. This results in an overhead null on all bands. If you look at the Wires Menu description, you'll see two sources shown IN phase. This is necessary due to a combination of the way MININEC assigns pulses to wires and the direction current is assumed to flow in the wires. When multiple sources are placed in connected wires as done here, you must check the resulting currents to make sure they're flowing in the directions you thought. If not, you must reverse one or more sources. Note also the relatively large number of segments for each wire. On 20 meters, the 51-foot sides are approaching a full wavelength long, so an appropriately large number of segments must be used. See the manual for more information on multiband antennas and on using multiple sources in an antenna. NBSYAGI This is a good test of program operation. The National Bureau of Standards carefully built and measured several Yagi antennas. This is a 50 MHz one, with dimensions from the ARRL ANTENNA BOOK, p. 18- 7. VERT1 A "plain-vanilla" resonant vertical over average ground. VERT2 Place VERT1 at the top of a 45-degree sloping hillside, and this is what you get. Note the large improvement in low-angle radiation. This is an example of use of the multiple ground media capability of ELNEC. A linear boundary is used in this case. W8JK Originally designed by John Kraus, W8JK in about 1940, this antenna has some interesting properties. It's characterized by two closely-spaced elements driven out of phase. Although the fields from the elements don't fully reinforce in any direction, gain is nonetheless achieved because of lowering of the radiation resistance due to mutual coupling. And lower it is -- note the feedpoint impedance of only 3.73 - j24.48 ohms -- compare this to a single element. The lower resistance results in heavier current, hence greater field strength, for a given power input. The difficulty is that system losses can quickly eat up the gain. Making this antenna from #12 copper wire (try it -- and include wire loss) drops the gain about 0.65 dB, not too bad. But great attention must be paid to losses in matching networks. And losses rapidly increase in significance as the spacing is made closer than the 0.1 wavelength of the example. When mounted low (0.25 wavelength for the example), W8JK-type antennas have a lower radiation angle than many other horizontal antennas due to the inherent lack of high-angle radiation. As an interesting exercise, save the pattern for later comparison. Then delete the second source, making the antenna into a Yagi. Note the increased gain. Even though the takeoff angle is higher, the Yagi gain is as good or better even at lower angles. In addition, the feedpoint impedance has increased to a much more manageable value. On the other hand, the W8JK will retain its performance over a wide frequency range, while the Yagi won't.