Ham Radio/Ham Radio CD-ROM (Emerald Software) (1995).ISO/tech/smitty/smitty.doc

Text File | 1988-02-14 | 13KB | 299 lines

SMITTY.DOC -- Page 1


          SMITTY Documentation -- Transmission-Line Calculations
              By Jerry Hall, K1TD     Rev. 3.01, February 1988

         SMITTY is a unique program for doing transmission line
    calculations, as you would do them on a Smith Chart.  But no knowledge
    of a Smith Chart is required to use the program.  SMITTY lets you
    calculate input or terminating impedances, as most other Smith Chart
    programs do.  In addition, SMITTY lets you calculate line Zo,
    electrical length, attenuation and velocity factor from measurements
    with open- and short-circuit terminations.  SMITTY also allows you to
    design stub matches and series section matches, make admittance-
    impedance conversions, series-parallel conversions, and SWR
    calculations.

         SMITTY.COM is written in Turbo Pascal and compiled for the IBM PC
    and compatibles.  It uses no graphics, nor is a math coprocessor
    required.  Three files make up the complete program, SMITTY.COM (the
    program file), SMITTY.DOC (this file), and SMITTY.DEF.  SMITTY.DEF
    contains values defining the number of decimal digits SMITTY.COM
    displays during calculations.  More about this file later.

         SMITTY.COM and the accompanying files are offered as copyrighted
    software, but permission is hereby granted to copy the program and pass
    it along to others at no personal gain.  Be sure to include all three
    files.


                               REGISTRATION

         No registration fee for SMITTY is requested or expected.  I wrote
    this program to satisfy my own needs, but I am pleased to share it with
    any other user who finds it to be of value.

         If you find the program useful, I do request that you register
    your copy.  Just send me your name and mailing address, and mention
    what version of SMITTY you are using.  Registered users will be advised
    of any later program releases.


                            GENERAL INFORMATION

         There are no restrictions in SMITTY for data that is practical.
    For example, you may use 0 + j0 for an impedance value (a short
    circuit) or 1/4-wavelength for a line length, etc.  (HINT: Should you
    wish to specify an open-circuit termination, use 1E+09 + j0.)  However,
    impractical data such as zero for a frequency or for a line impedance
    may create divide by zero errors, causing the program to abort.

         SMITTY is menu driven.  All menu options are selected merely by
    pressing a letter key; upper or lower case is accepted.  The letter-key
    selections are itemized in the menu, as well as being highlighted in
    the menu description of the function.  From the main menu, the screen
    will clear and the opening screen for the selection you've made will be










    SMITTY.DOC -- Page 2


    displayed.  Some menu items cover two procedures, such as converting an
    impedance to an admittance, or vice versa.  At the opening screen of
    these procedures you'll be prompted for which calculation you wish to
    perform.  Again, and throughout the program, responses to all prompts
    other than for data entry are made by pressing a single key.

         During program execution you'll be queried with prompts for
    various program options.  One example is taking line attenuation into
    account, with a Y or N response expected.  Default options are shown
    for each of these prompts, an N in this case.  The default may be
    selected by pressing the ENTER key (as well as by pressing the letter
    key for the default).  Pressing any other keys (in this case, anything
    but Y, N, or ENTER) does nothing.

         In some procedures you will be asked to enter reactance or
    susceptance values at the keyboard.  Be sure to enter negative values
    for capacitive reactance and for inductive susceptance.

         You may exit the program gracefully from the main menu by pressing
    Q (quit).  You may also abort the program at any time by pressing Ctrl-
    C.


                              SMITTY FEATURES

         Probably the most common use of the Smith Chart is to determine
    the load impedance on a transmission line based on a known input
    impedance, and vice versa.  SMITTY does indeed support such
    calculations.  These may be made with actual R + jX values or with
    normalized values.  A similar calculation is to determine the load
    admittance, G + jB, based on a known input admittance, or vice versa.
    SMITTY supports these calculations as well.  The choice to work with
    impedance or admittance data is made after you select "toward load" or
    "toward generator" at the main menu.  Once you've made that choice, say
    for impedance, you will continue to work with impedance values until
    you return to the main menu.


    Frequency Correction

         During the execution of some procedures, Smitty requires reactance
    data in ohms.  The data to be entered at the keyboard may be the result
    of impedance bridge measurements.  Many bridges, such as a General
    Radio 1606-A, give a reactance dial reading that must be divided by the
    measurement frequency in megahertz to get the ohmic reactance value. In
    these procedures you may enter the dial values from such a bridge, and
    allow the program to make the frequency correction. A message similar
    to the beginning lines of this paragraph will first be displayed, and
    after the data is entered you'll be prompted about correcting
    reactances for frequency.  Unless you enter bridge dial readings,
    you'll want to choose the default response, N(o).












    SMITTY.DOC -- Page 3


    Line Length and Attenuation

         In SMITTY there are several options for entering line lengths.
    You may enter electrical lengths directly as either degrees, radians,
    or wavelengths.  Or you may enter a physical line length in feet and
    SMITTY will calculate and display the electrical length.  In this case,
    additional data SMITTY requires for the calculation are the velocity
    factor of the line and the frequency.  Velocity factors are always
    entered as a decimal fraction, and frequencies in megahertz.  (These
    are clearly indicated at the data prompts.)

         Various options also exist for taking line attenuation into
    account during calculations.  You may indicate a lossless line (no
    attenuation), or you may enter attenuation either in decibels or
    nepers.


    Other Capabilities of SMITTY

         SMITTY lets you calculate line Zo, electrical length, attenuation
    and velocity factor from measurements with open- and short-circuit
    terminations.  The basis for this procedure is contained in Chapter 28
    of The ARRL Antenna Book, 15th Edition, 1988.  If you are not familiar
    with this technique and want to try the program feature, use these data
    values.  These values are the results of actual measurements on a
    length of foam-dielectric cable.

              Open-circuit readings:  85 + j1280/freq
              Short-circuit readings:  4.8 - j80/freq
              Frequency: 7.14 MHz
              Line length: 100 feet

         SMITTY allows you to design stub matches.  You may specify a line
    impedance for the stub that is different from the main transmission
    line.  Note that for some loads, such as 40 +/- j20 with a 50-ohm line,
    one of the two possible solutions occurs with the stub placed 0.0
    wavelength from the load.  This is not a program error, but is a valid
    solution meaning to shunt the load itself with a matching stub.  Such a
    situation exists any time the normalized conductance of the load is 1.0
    siemen.  Determining stub positions is an iterative procedure, and
    calculations for some loads take more time than for others . . .
    typically one second or less for most loads.  Calculations taking the
    longest, up to a few seconds, are for cases when the unmatched load
    creates a high SWR on the main line.

         You may also design series section matches with SMITTY.  Details
    for this match are contained in the chapter on matching the line to the
    load in any recent edition of The ARRL Antenna Book.  This form of
    matching is not widely used, but is more efficient than a stub match.
    A 1/4-wavelength transformer is a special case of the series section
    match.  Just as the 1/4-wavelength section of line having an
    intermediate impedance can match two unlike impedances, a series
    section of the proper length and almost any impedance, at the proper










    SMITTY.DOC -- Page 4


    position in the main line, can match two unlike impedances.  The
    advantage of the series section is that it may be a standard type of
    line, and the load may contain reactance as well as resistance.

         As presently written, no provisions are made in SMITTY for taking
    line losses into account for stub matching or for series section
    matching.  I have investigated this and find it does make a slight
    difference in the outcome of calculations when line attenuation is
    considered.  A later version of the program may include this
    capability.

         With SMITTY, admittances may be calculated from impedances, and
    vice versa.  Similarly, series equivalent impedances may be converted
    to parallel equivalents, and vice versa.  These are direct conversion
    processes, with no data required for line length, attenuation, etc.  If
    the data you enter is normalized, the results will be normalized, and
    if the data you enter is unnormalized, the results will be
    unnormalized.  (Never mind that these procedures may ask if the data is
    normalized; your response is used only to determine how many decimal
    places to display.)

         SMITTY also lets you calculate standing wave ratios or SWR values.
    This is a simple procedure that can be done on a 4-function calculator.
    But I find that doing it quickly in the computer often beats digging
    out the equation (which I can never seem to remember), and then doing a
    couple of dozen keystrokes on a calculator.


                                SMITTY.DEF

         SMITTY.DEF is an ASCII text file containing values that define how
    many decimal places SMITTY displays during program calculations.  You
    can change these values to meet your requirements (any value in the
    range from 0 to 9) with EDLIN.COM or your favorite word processor.  The
    results are rounded (not truncated) to the number of digits displayed.
    Just be sure to treat SMITTY.DEF as an ASCII text file, and do not
    change the order in which the values appear.  The values included in
    SMITTY.DEF as originally distributed are as follows.

              Resistance and reactance -- 3
              Resistance and reactance, normalized -- 5
              Conductance and susceptance -- 6
              Degrees -- 3
              Wavelengths -- 5
              Decibels (attenuation) -- 2
              Standing wave ratio (SWR) -- 3
              Feet (line lengths) -- 2
              Velocity factor -- 2
              Voltage reflection coefficient -- 4














    SMITTY.DOC -- Page 5


                              PROGRAM ERRORS

         Although I think the program is completely debugged, it is
    possible that undetected errors exist.  Please let me know if you
    experience a problem.  But do not confuse program errors with data
    input errors.  As mentioned earlier, impractical data will create
    divide by zero errors.  The program will abort and a message such as
    this one will be displayed:

    Run-time error 02, PC=50E7

    The 02 run-time error code signifies a divide by zero error.

         If you run into an actual program error causing it to abort,
    please immediately jot down from the screen display the exact entries
    you made for data and the complete error code line.  (Or better yet,
    immediately do a print-screen operation to a printer.)  Do this before
    you clear the screen or start SMITTY again.  Send me this information,
    and I'll get back to you.

         During initial development of the program I carefully checked the
    results of all procedures against those from manual Smith Chart
    calculations, or against manual calculations if the Smith Chart was not
    required.  I believe you will not encounter calculation errors.

         I hope you find SMITTY to be a useful tool for solving your
    transmission-line problems.  I'd appreciate comments and suggestions
    for program improvements.  With many thanks,

      Jerry Hall, K1TD, 181 Brimfield Rd., Wethersfield, CT 06109
        February 14, 1988