Plotting
Interference Area
Here the main components of
the interference area can be defined:
Threshold
for reception – This is the
minimum field strength that should be considered to be valid coverage.
Difference
Threshold – This is the
maximum difference in field strength between any two stations.
If the field strength difference is greater than what is defined here, it
is assumed that the receiver will lock to one signal or the other and that no
interference exists.
Maximum
Allowed Signal Time Delay
– This is the maximum
allowed difference in signal arrival time. If
the arrival time difference is greater than what is specified here, then
interference exists at that point.
To generate a report of the
population and area where interference exists, select “Time Delay Population
Report” from the “Report” menu. The
following options window appears when generating this report:
The selected transmitter in
the list will be used in determining which stations are considered.
The options match those found in the plotting options (and these will
default to the current specified plot options).
To facilitate the process
of determining what time delay settings are best, a map tool for examining time
delay has been added to the program. To
access it, select the “Map Information Tool” (this is the one that looks
like a crosshair), then right click on the map.
The following menu will appear:
Choose “Show Time Delay
Info” to show the time delay information window.
This window looks like this:
As the map information tool
is moved around the map, the numbers on this window will update to those
matching the current location of the cursor.
A built-in time delay for each transmitter is considered in the final
“Time Difference” that is reported. Use
the “Set” button to change the time delay for a given transmitter (if both
delays are set to zero then it is considered that both signals leave the
transmitters at exactly the same time). Note
that that time delay for a specific transmitter can also be set on the
“Info” tab of the transmitter properties window (however the map will not
automatically redraw after changing the setting there).
For the purposes of
generating an example, the FM on-channel booster situation of WFUV (the main
channel) and WFUVF2 (the on-channel booster) has been used.
For the first example, the
FCC propagation methodology has been used (in an attempt to simply things).
The first map shows all of the area where the signals are within 3 dB of
each other.
Note that the WFUV main
station is shown to the north-west of the booster WFUVF2.
This case is not considering any time delay, it is simply showing the
area where the signals differ by less than 3 dB.
The next map shows the case
in which areas where the signals that arrive within 4 uS of each other are NOT
considered to be interference. Also
not that for this map, the signals are assumed to be leaving both transmitters
at exactly the same time.
In this case you will note that the interference area is the same as before. Because of the geography of the stations and directional pattern of WFUVF2, there are no points in the 3 dB difference zone where the arrival time is a less than 4 uS different.
The following map shows the
situation where the WFUVF2 signal is delayed by 27 uS (so that its content is
broadcast 27 uS after the main transmitter).
In this case you will note
that the size of the interference is greatly reduced because signals are now
synchronized. By changing the
relative time offsets of the stations in the study, the location of the
interference area can be moved around within the “3 dB” threshold zone.
The shape of the possible
interference area becomes more complicated when the Longley/Rice propagation
model is used because the signal is no longer a decreasing function along each
radial (it can go up and down). The
next three maps are the same as those shown before, but using Longley/Rice
instead of the FCC model.
All of the area where the
signals are within 3 dB of each other:
Please contact us at (800)743-3684 or info@v-soft.com
with any questions you may have.