For the calculation of adjacent-channel FM translator interference and for calculating and determining the locations of HD IBOC host interference when separate antennas are used. 

The XField software package, is used by the broadcast engineer when re-locating FM translator stations to new channels that are 2nd or 3rd adjacent to full service stations or otherwise protected translator stations.  The new software program is expected to be useful to LPFM stations under the soon to be released relaxed FCC rules. For FM translator stations, FCC requires a second or third adjacent translator to have a signal strength of no more than 40 dB above the signal strength of the full service station. The problem for engineers is that it is a time consuming and laborious effort to calculate the field strength  of a translator, particularly one with an azimuth pattern, at all points where the U to D ratio could be above the required 40 dB. In order to properly consider the U to D ratio at all pertinent points, a translator antenna's vertical elevation field must be considered.  In many cases, where the proposed translator has a rather low antenna, without consideration of the vertical elevation field at a given angle from the antenna, it is very possible to exceed the 40 dB U to D ratio and for the applicant receive a dismissal from the Commission.

To consider the vertical elevation field of a given antenna the engineer must calculate the ERP of the translator antenna along the ground, or at a user selected height, at various azimuths and distances from the translator antenna where dwellings or major roads are located.  The XField software solves this problem by employing the translator's vertical elevation field and azimuth pattern to produce a graph or table of the points where the 40 dB limit is exceeded. A translator station can be brought into compliance by either reducing its ERP, using an antenna with more bays (keeping the higher signal strength away from the immediate tower location) or increasing the translator's antenna height.  The XField graph below shows a condition when a 2nd adjacent translator exceeds the 40 dBu limit (red portion of the graph) and for which an application filed with the FCC would be dismissed. While this graph shows the graph along the 90 degree radial, any azimuth can be used. 

The program's main interface is shown at the top of this page.  Once a study is performed all parameters can be saved as a work file and reloaded at a future date. The table of this graph from 2 to 50 meters is shown below:

If the user does not enter a specific azimuth to study the program will produce tables for each azimuth at 1 degree radials for a total of 360 radials. The output can be filtered to show only the delta dBu that is over the 40 dBu limit. 

The table Parameters for the table shown above has been set at 2 meters between points from zero to 250 meters from the translator. XField comes with numerous vertical elevation field digital graph files for single to multiple bayed antennas in full-wave and half-wave configuration.  The screen below shows a ten bay vertical field pattern for the SHPX10.VEP.

Azimuth patterns can also be loaded and considered in the calculations. The screen below shows an azimuth pattern that can be used with the program.  The pattern can be edited in effective field, dBk or kW and it can be rotated and then saved to the user's satisfaction. The user can work with 36 points or 360 pattern points if desired. 

IBOC Host interference Calculations:

The screen portion above shows an example of a 100 kW station and a 1 kW IBOC station located on the same tower structure. Using the "Browse" button, the Test Reference Station can be drawn from the provided FM and TV databases. 

The graphics above is a portion of the Main screen. The IBOC study assumes that you will be operating with a separate IBOC antenna on the same tower. You can select the vertical elevation field graphs of the host station and that of the proposed IBOC station antenna. Click the "Antenna #2, V-Field" button to select the IBOC antenna's vertical elevation field pattern. Enter the ERP you propose for the IBOC antenna. Then, enter the height above ground of the antenna. Note that the 70% button, puts the IBOC antenna at the maximum vertical distance allowed from the host antenna. In the study shown below the user has selected a ten bay half-wave antenna for the host station and a 2-bay full-wave for the IBOC antenna. 

The IBOC graph below shows the signal values of the host station and that of the IBOC station. Since IBOC operates using about 1/2 of the first adjacent channel, the graph uses the FCC's first adjacent D/U of -6 dB. When the IBOC signal rises within the host station's signal by -6 dB or more one can expect receiver interference to the host station's analog broadcasts. We have authored a paper which was published in RW's Engineering Extra showing that the locations of the interference predicted on the graph are highly associated with the locations from listeners who call the station to complain about interference. 

The portion of the graph that turns red shows that the points where the predicted signal strength of the IBOC station is less than 6 dB below the signal of the host station. In this case, assuming a non-directional host antenna, there will be a ring of interference around the antenna between 0 and ~260 meters from the tower. Additional rings will be found at ~400 meters, ~600 meters to 830 meters and at ~1200 to 1770 meters. This study is for a -1 dBc IBOC power. Imagine the host interference for this setup using -10 dBc. Directional azimuth patterns can be loaded for both the host and the IBOC antennas. 

Xfield can be used with any of the V-Soft terrain elevation databases, (provided as an option.)