CD Shareware Magazine 1997 January/CD_shareware_1-97.iso/DOS/COMUNICA/VHFPRP11.ZIP/VHFPROP.DOC

Text File | 1996-06-21 | 11.0 KB | 204 lines

                             Documentation for
              VHFProp -- An interactive Signal Analysis Program
                 for the 6 Meters through 23 CM Amateur Bands

VHFProp is a computer program to implement the procedures of the QST article
'A Method for Determining V.H.F. Station Capabilities' published in November
1961 by D. W. Bray, K2LMG, and reprinted in most ARRL VHF and Antenna Handbooks
since that date.  K2LMG is also the author of VHFProp.

The program allows the user to estimate received signal strength from a distant
VHF/UHF amateur radio station in the 6 meter to 23 cm bands.  The estimate is
based on distance between stations, transmitted power, antenna gain of both the
transmitting and receiving stations, line losses, antenna heights, station site
heights above average terrain, and other factors.

The program is 'window oriented' and as such need little explanation about its
operation.  Some help about the technical aspects are a given below, however.

There is one addition to this program which does not appear in the QST article.
This is determining the station capabilities when using a repeater for one or
both of the stations.  There is no difference between a base station and a
repeater in their propagation characteristics except a repeater is usually
located at a site which is far above the average terrain in the local area.  It
is this situation which has been added to the calculation.

PROGRAM USAGE

In order for two station to communicate it is necessary that the Signal-to-
Noise Ratio (S/N) of the received signal is greater than 0db.  The greater the
S/N ratio the better the signal. The desired S/N is usually considerably
greater than 0db.  Typical program usage is to enter values for both stations
and then decide if the S/N is great enough. If the program indicates a S/N
which is not satisfactory, then you can explore changes to your station by
adjusting the transmitted power, antenna gain, antenna height, etc. until a
satisfactory value is obtained.

When the predicted S/N value is 0db or greater the received signal should be
readable under the assumed conditions. As you know propagation conditions vary
from hour to hour.  If you have set the Mode Reliability value to '50%', then
50% of the time (averaged over many days) the signal should be readable.  Also
if the Mode Fading is set to 'No' then at 0db S/N you should hear the signal
fade in and out over a period of minutes. If you have a station setup, you can
check out the program and the values you have entered by observing the
receiver's S meter.  S meters usually read about 6db per S unit, so you can get
a rough check of the S/N ratio by dividing the predicted S/N value by 6 and
then checking it out on your receiver's S meter.

When the program starts up you will find that most of the parameters are set to
zero and the cursor is located at the parameter for distance between the
stations.  The distance value is pre-set to 10 miles.  First change the
distance for the value between the two stations of interest.  You will see the
resulting Signal-to-Noise value, Path Loss, and Station Gain for the initial
values.  As you continue to change the parameters new results are calculated.

The values are changed by entering the desired numbers and then pressing the
Enter (or Return) key, or pressing an arrow key.  If Enter is pressed the value
is entered and the cursor remains at the same parameter. If an arrow key is
pressed the value is entered and the cursor is moved to the parameter in the
direction of the arrow.

Please refer to the sample VHFProp display screen:

              VHFProp -- An interactive Signal Analysis Program

    Transmitting Station Parameters    |    Receiving Station Parameters
---------------------------------------+-------------------------------------
 Power (watts):                 50.0   | Noise Figure (db):             1.0
 Line Loss (db):                 3.0   | Line Loss (db):                2.0
 Antenna Gain (db):             11.0   | Antenna Gain (db):            14.0
 Antenna Height (feet):         80.0   | Antenna Height (feet):        30.0
 Site Height (feet):          1500.0   | Site Height (feet):            0.0
 Horizon Angle (degrees):        0.0   | Horizon Angle (degrees):       0.0
                                       | Bandwidth (KHz):               5.0

 Distance (miles):             100.0       Modes - Select with Enter Key
                                         ----------------------------------
 Line of sight distance:        47.4     Frequency Band:                 2m
 Eff. scatter distance:         52.6     Modulation:                     fm
                                         Reliability:                   99%
             Results                     Include Fading:                Yes
 -----------------------------------
 Signal to Noise Ratio (db):    10.5
 Path Loss (db):               185.6
 Receiver Sensitivity (db):   -163.4
 Station Gain (db):            196.1
                                                                Use ESC to Quit

The display is divided into three areas: Parameters, Modes and Results. The
Parameter area is that part of the display where the parameters are shown and
may be edited. This area is basically the top part of the screen down to and
including the Distance parameter. The arrow keys allow you to move over the
various parameters and change them by entering numbers as described above.  The
Modes area is the four lines below the title: 'Modes - Select with Enter Key'.
The arrow keys allow you to move into this area also, but these parameters are
changed by pressing the Enter key repeatedly until the desired value is shown.
The possible values of the modes are:

Frequency Band:   6m, 2m, 1.25m, 70cm, 33cm, 23cm
Modulation:       fm, ssb, cw, am
Reliability:      50%, 99%
Include Fading:   No, Yes

The Results area displays the four results of the calculations.

If as you enter data the computer beeps the data entered is out of range. The
distance must not be greater than 500 miles. The transmitter power must not
be less than 0.01 watts.  The antenna and site height must be positive.  The
horizon angle must be such that the effective distance is greater than 0 and
not greater than 500 miles.


TECHNICAL HELP and HINTS

Each of the parameters and modes is discussed next.

TRANSMITTER POWER.  This is simply the transmitter output power in watts.

NOISE FIGURE.  This parameter is a measure of the noise introduced by the front
end of your receiver.  Most receiver manuals will give you this figure.  If
not, these days 1db is a good value unless you have a special preamp.  In that
case the preamp manual will tell you its noise figure.  If you have an old or
inexpensive receiver you might want to use 2 to 3 db for the noise figure.

LINE LOSS.  Refer to almost any ARRL handbook.  Measure the length of your coax
and multiply by the loss per foot for your type of coax.

ANTENNA GAIN.  The antenna gain parameter is more than just the gain of your
antenna. In addition to the gain of the antenna itself you should add about 4db
for earth reflection. If your antenna is more than a wavelength above the
ground, and the soil is a good conductor (that is, has a fair amount of
moisture etc.) the earth acts as a mirror, providing a reflection of your
antenna "under the ground".  Under perfect conditions this would give you a 6db
additional antenna gain.  4db is probably a good value unless the soil is dry
or rocky; in that case reduce the value below 4db.

If you don't know the gain of your antenna you can estimate it as follows:

A 1/4 wave Ground Plane: 0 db
A 1/2 wave dipole: 1.2 db
A yagi beam: 10 * log(L * F * N / 98) where: log is the base 10 logarithm, L
     is the length in feet, F is the frequency in MHz, and N is the number of
     yagi's stacked widely apart.

ANTENNA HEIGHT:  This is simply the height of the antenna above the base of the
mast, in feet.

SITE HEIGHT:  This is to be used when one of the stations is located high above
the surrounding terrain.  It should be set to the height of the ground (the
base of the antenna mast) at the station above the average terrain; in
particular the height above the average terrain in the direction of the other
station.

HORIZONTAL ANGLE:  This parameter is related to the Site Height. If one of the
stations is located either high on a hill, or down in a valley, such that the
angle to the horizon is not near to zero, then the angle to the horizon in the
direction of the other station should be used.  The elevation angle may be
either positive or negative.  Negative if the station is on a hill overlooking
the terrain, and positive if the station is in a valley. This is to adjust be
large elevation differences in the stations.

     NOTE: When one station is on a high hill only one of the parameters: Site
Height, OR a negative Horizon Angle should be used, NOT both.  You will see
that you get approximately the same value using one or another of these
parameters. In case one of the stations is in a valley, and the other is on a
high hill, then it is permissible to use Site Height for one and a positive
Elevation Angle for the other.

     Estimating the angle to the horizon is difficult.  The disk of the sun is
about 1/2 a degree.  So if the sun rises or sets near the desired direction the
angle can be estimated fairly well.  There is an optical illusion when the sun
is near the horizon making it look a little larger than it really is, so you
might want to compensate for that.  Also the sun moves at a rate of 1/4 degree
per minute.  Sometimes that fact can help.

RECEIVER BANDWIDTH.  Your receiver's instruction book will probably tell you
its effective bandwidth for the modulation mode and filter you indent to use.

FREQUENCY BAND.  The amateur band you are using.

MODULATION.  The modulation mode you are using.

RELIABILITY.  This mode parameter compensates for long term (day to day)
tropospheric scatter variations.  If you are going to use the path for
communications you should choose 99% reliability.  If you are using the path
for experimentation you probably want to use 50% reliability.

INCLUDE FADING. This mode parameter compensates for short term (minute to
minute) fading. Set this according to the path's intended use, as discussed
above.

MORE INFORMATION

Refer to almost any ARRL VHF or Antenna Handbook to find more information.  You
will find an explanation for making the calculations using charts (nonmgrams),
along with an example.

--------------------------------------------------------------------------------
This program was written by D. W. Bray -- K2LMG.  It has been placed in the
Public Domain by the author.  The program is written in the C language, and
compiled under Turbo-C. (Not all Turbo-C compilers can compile the program
because of cursor control.) The distribution compressed archive which you
received should have included: VHFPROP.EXE, VHFPROP.DOC, VHFPROP.C and READ.ME
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