Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/mininec/mn3docs/mntheory.doc

Text File | 1996-06-30 | 6KB | 146 lines

     ELEMENTS OF MININEC THEORY

     MININEC solves a set of complex relations involving a number of
     aspects of electromagnetic propagation. Detail understanding of these
     is not necessary to make practical use of the program. However, a
     basic understanding of the principles is helpful in setting up the
     data input and run conditions for the program. Planning can reduce the
     time needed for a run, can give the best accuracy, and lead to proper
     interpretation of results.

     The following is not a complete exposition of the theory. For one
     thing, it has been kept free of mathematics, only concepts being
     covered. The actual program is based on the mathematics of
     electromagnetic propagation. For full analysis, the following
     (unclassified, general distribution documents) should be consulted:

     Mininec: a MINI-NUMERICAL ELECTROMAGNETICS CODE, Alfredo J Julian,
     James C Logan and John W Rockway, Naval Ocean System Center Technical
     Document 516, 6 September, 1982,

     The New MININEC (Version 3): A Mini-Numerical Electromagnetic Code, J.
     C. Logan and J. W. Rockway, Naval Ocean System Center, Technical
     Document 938. September 1986.

     Both of these are available from The National Technical Information
     Service, 5285 Port Royal Road, Springfield, VA, 22161.

     The Basic Concepts

     Mininec deals with wires, or elements which are thin enough to be
     treated as wires. For simplicity, wires are assumed to be straight.
     Each wire is assumed to be divided into segments, such as -----, but
     with the segments long enough to connect to each other. Note that
     Mininec treats wire ends as being half-segments, so this wire would be
     a 4-segment dipole.

     Somewhere in the assembly of wire segments which make up the antenna,
     there must be at least a single segment which is fed by a current from
     an external source. For example clarity, assume that there is a single
     driven segment, and that this is the center segment of the lower of a
     pair of wires, as =====.

     The radio frequency current flowing in this driven segment induces an
     electric field in the surrounding space. The intensity of this is a
     function of the distance and angle from the driven wire. A result of
     the difference of intensity is that each of the segments of the top
     wire sees a different intensity, maximum for its center segment and
     decreasing towardd the ends. The field difference causes a difference
     in the electric charge along the wire, that is, a potential difference
     between one place to another.

     As a consequence of this potential difference, current will flow in
     the segments of the top wire. Equally, there will be a potential













     difference for the non-driven segments of the bottom wire, and current
     flowing in each segment. Since current is now flowing in all segments,
     each will produce a field component, and in turn, a current component
     in all other segments. Solution of the antenna performance requires
     that all of these currents be evaluated.

     Since the process relates to voltages and currents, it is convenient
     to express their inter-relation in accord with the alternating current
     version of Ohm's law, as:

          Current=Voltage/Impedance

     Where each of the three quantities has many components, one for each
     wire segment for current and voltage, and the square of this number
     for impedance.

     Mininec solves these relations from the antenna geometry, plus the
     supplied driving current. In the process, it also provides data for
     other important items, the major ones being:

          Driving Impedance
          Input Power
          Mutual Impedance

     Since the current in each segment is now known, and the geometry, the
     radiation field can be calculated by array theory. This gives the
     radiation pattern. Front/back and front/side ratios can be
     calculated.

     Refinements

     A number of refinements are available. Two or more wires can be
     connected at a point, to form antennas such as the "T". Impedances can
     be introduced at any segment, as in loaded antennas or for impedance
     matching.

     One of the important added faactors is the presence or absence of the
     earth. Mininec assumes either that the earth is remote, or that the
     part directly under the antenna is perfect. This corresponds to adding
     a image antenna identical to the source, and underground by the
     antenna height. However, the radiation pattern is  calculated either
     for a remote earth, or for a near earth using provided earth
     constants. An option for these is a perfect earth.

     Practical Computer Limits

     Storing the arrays used by Mininec, especially the impedance array,
     takes memory. As a result, a given computer will have a limit to the
     antenna complexity it can handle. Similalary, because of the enormous
     number of computations necessary, Mininec will be slow when dealing
     with antennas with a large number of wire segments.

     This matter of run time should not discourage use of Mininec on large













     problems. The solution is simply to accept the time necessary, and to
     do something else while the computer is running. For example, for a
     full scan of pattern versus frequency over an entire band, it is
     convenient to start the run in the evening, and have the results the
     next morning. (Just be certain that there is a surge protector on the
     power line. An unninterruptable power supply is nice also, but a
     re-run is just as good, and costs much less).

     See the file MNinput for hints on getting good accuracy with
     reasonable time.