Probe 4 Optional Booster Time Delay Module
The new Probe “Time Delay” Module is designed to be used in determining interference areas in systems containing on-channel boosters and single channel networks. The “Time Delay” module allows the user to examine the effect that different time delays in signal origination have on the predicted interference area. With the input of “acceptable” time delay in microseconds Probe’s mapping displays will define areas where the arriving signals will be far enough out of sync to cause appreciable signal distortion. Such areas can be confined to within user defined U/D rations. Both graphically population reporting is available.
Plotting Interference Area
To plot the area where the main transmitter and on-channel boosters signal are close, perform a standard Probe “Coverage” study with the transmitter and booster defined as two separate stations within the study. After the signal calculations are complete, use the “Display Options” button to change the draw mode. Set the draw mode to “Time Delay Interference”. Using the “Change” button next to the selector, the drawing properties can be defined. The properties window looks like this:
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).
The Time Delay Map Tool
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:
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.
Using the FCC Propagation Method
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 microseconds 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 microseconds different.
The following map shows the situation where the WFUVF2 signal is delayed by 27 microseconds (so that its content is broadcast 27 microseconds 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.
Using Longley/Rice Signal Prediction
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:
The areas where the signals that arrive within 4 microseconds of each other are NOT considered to be interference (once again it is the same as the first map, but that will not always be the case depending on the situation):