






                    ----- EZNEC Example Antenna File Notes -----

                                                     
        Several example antenna description files are included on your
        EZNEC disk. Following are some notes about each one. All files have
        the extension ".EZ" added to the name shown here.

        Some of the files have been saved with 3D (three dimensional) Plot
        Type specified; others have 2D Azimuth or Elevation type selected.
        To change, use 'PT' in the Main Menu. Do the "On The Race Course"
        portion of the Test Drive in the EZNEC manual to learn more about
        using 2D and 3D plot features.


                                      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. EZNEC reports an accurate forward gain and
        front/back ratio. If you've been using ELNEC or other MININEC-based
        program, you've seen that they give an inaccurate front/back ratio
        for this antenna. The ELNEC segment length tapered model 15MQUADT
        compares very favorably to EZNEC's analysis of this quad.

                                      20M5ELYA

        This is a high-performance, real-life 20 meter Yagi design. It's
        from the ARRL Antenna Book, 17th Edition, p. 11-19, model 520-
        40.YAG. EZNEC's stepped-diameter correction comes into play with
        this one to give very good results. The antenna was created by
        describing one element. Then the element of 11 wires was copied
        five times. Using the Group Edit feature, the X coordinate of each
        11-wire element was changed with one command to get the proper
        spacing. Finally, the end "wires" of each element were adjusted to
        the proper lengths.

                                      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,


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        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 and later editions. 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.

        This model is fed simply with current sources, to simulate a
        correctly-fed array.

                                       4SQTL

        This model uses EZNEC's transmission line models for the feed
        system. Note that the lengths aren't anything like you'd expect.
        This is because the delay in a transmission line isn't equal to its
        electrical length except in special circumstances -- circumstances
        which don't occur in most phased arrays. To see where these
        feedline lengths came from, see "The Simplest Phased Array Feed
        System . . That Works" in the ARRL Antenna Compendium, Vol. 2.

        If you do a comparison with the 4SQUARE model, you'll find a little
        difference. This is because the elements in the 4SQUARE model
        aren't quite self-resonant and they're not spaced quite exactly a
        quarter wave apart. (The feed system was designed for those
        conditions.) If you delete three of the elements and adjust the
        length of the fourth to make it resonant (zero feedpoint
        reactance), then change the lengths of all elements to this new
        length, and you adjust the spacing to exactly 1/4 wavelength,
        you'll find very good agreement with the 4SQUARE model. This
        illustrates how touchy this array is.

        As it stands, the model isn't good for testing the feed system over
        a range of frequencies. This is because the feedline lengths are
        specified in degrees, rather than feet. This makes the lines magic,
        because they keep the same electrical length regardless of
        frequency. To make a realistic frequency-dependent model, you'd
        have to specify the transmission line lengths in feet.



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                                      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. The current sources simulate a perfect feed
        system.

                                       CARDTL

        This is the same array as above but with a transmission line feed
        system. It comes from the same source as the feed system used with
        the 4SQTL model, and the same comments apply. Notice the good
        agreement between this model and the CARDIOID model, and the
        unusual transmission line lengths.

                                      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.

                                       DIPTL

        This illustrates modeling of a coaxial cable by using a combination
        of a transmission line model (for the inside of the coax) and a
        wire (for the outside). This is described in the manual; see
        "Coaxial cable" in the index. The transmission line is about 1/2
        physical wavelength long. After you run the program, go to View
        Antenna and use <CTRL>+ to magnify the current. Note the current on
        the feedline. If you go to the Options Menu and enter a power
        level, then go to the Main Menu and type 'CU', you can see how
        large the current is for a given power input. At 100 watts, the
        current is greater than 100 mA at the center of the feedline. This
        may or may not cause trouble with RFI if the feedline is close to a
        TV transmission line, phone line, etc. Note that the coax is fully


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        symmetrical with respect to the antenna. For an explanation of what
        causes the current, see "Some Aspects of the Balun Problem", by
        Walter Maxwell, W2DU, QST, March 1983, or "Baluns: What They Do and
        How They Do It" in the ARRL Antenna Compendium, Vol. 1.

                                      ELEVRAD1

        This illustrates a vertical antenna with elevated radial system. At
        0.000109 wavelength, the height is about as low as EZNEC can model
        accurately. This file won't give accurate results, however. Because
        of the low height, segment tapering must be used, and the source
        placement changed, as described in the EZNEC manual. (See "Elevated
        Radial Systems.)

                                      ELEVRAD2

        ELEVRAD2 is ELEVRAD1 modified as described in the EZNEC manual for
        improved accuracy. If you run ELEVRAD1 and ELEVRAD2 you'll see that
        the results are quite different.

                                        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 EZNEC, 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.

        It would be difficult to model this using transmission lines
        because the elements are both transmission lines and radiators.
        Another problem is that the insulation makes the radiating portion
        appear about 3% longer.



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                                        K5RP

        This was described in the ARRL Antenna Compendium, Vol. 2. A single
        loop constructed with these dimensions is about resonant and has a
        feedpoint resistance of around 12 ohms. By putting a second turn on
        the loop, the resistance is quadrupled to near 50 ohms to ease
        matching. The pattern is essentially that of two verticals, since
        the radiation from the horizontal portion cancels.

        This antenna makes an excellent illustration of the three "real"
        ground models in EZNEC. Run it with MININEC, Fast, and High
        Accuracy grounds, save each plot in a file, then recall and
        superimpose them.

                                       LOGPER

        This is an example of a three-band 17m-10m log periodic antenna
        from the ARRL ANTENNA BOOK, 16th Edition, p. 10-6. Recall LOGPER
        and look at it with Antenna View, using the zoom feature to see the
        details. This method of construction, with one transmission line
        conductor above the other, frequently is used for VHF and UHF log
        periodic antennas. Note the feedpoint. If the source is just
        connected to a wire across the transmission line section, the wire
        is much shorter than the 0.02 wavelength minimum recommended for
        wires containing a source. So two out-of-phase sources were created
        and put on longer wires beyond the normal feedpoint.

                                      LOGPERTL

        The same antenna but using transmission line models. It has quite a
        bit more gain than LOGPER at the example frequency. This is
        probably because the elements are effectively longer, having no gap
        in the center.

                                      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 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


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        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.

        This is also an illustration of the use of split sources to allow
        placement at wire junctions.

                                      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.

                                       VERT1

        A "plain-vanilla" resonant vertical over average ground.

                                       VHFGP

        This is a pretty ordinary VHF ground plane. For an interesting
        exercise, add a wire extending downward from the junction of the
        radials to simulate the outside of a coax feedline. With the wire a
        half wavelength long, you'll notice significant pattern distortion
        caused by current induced on the new wire. Various lengths, open or
        connected to ground, will cause different results. You can simulate
        addition of "current" (or "choke") baluns by adding loads of about
        500-1000 ohms to the new wire.

                                        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


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        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 much
        greater frequency range than the Yagi.





































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