Ham Radio 2000/Ham Radio 2000.iso/ham2000/antenna/elnecdem/antnotes.doc

Text File | 1991-02-01 | 11KB | 265 lines

                   ----- 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.