Contours are a standardized method used in broadcasting to predict a station’s coverage area and identify potential interference zones. They are based on whether a signal can be received at a specific field strength on 50% of receivers, 50% of the time. Two common contour types are:
- F(50, 50) Curve:
- The F(50, 50) curve is used to determine the coverage area of FM radio and analog television stations.
- It calculates the field strength at which the signal can be received by 50% of receivers for 50% of the time.
- This curve helps broadcasters estimate their service area.
- F(50, 10) Curve:
- The F(50, 10) curve is employed to assess potential interference between stations.
- It determines the field strength at which the signal can be received by 50% of receivers for 10% of the time.
- This curve aids in identifying areas where one station may interfere with another.
In addition to these curves, digital television uses an F(50, 90) curve, which considers the more stringent signal strength requirements for TV broadcasting.
These curves are presented on charts, with engineers historically needing to determine values at specific X and Y coordinates on the chart. These coordinates are based on factors like height above average terrain, field strength at 1 kW Effective Radiated Power (ERP), and distance from the transmitter. The curves serve as valuable tools for broadcasters and regulators to understand signal coverage and potential interference zones in the broadcast spectrum.
In the modern era of computing, the need for manual charts in broadcasting has diminished, as computer programs now handle the complex calculations involved. The FCC’s contour charts are also adopted in Canada and Mexico, while the International Telecommunications Union established a separate set of charts as a global standard.
The concept of contour overlap employs the F(50, 10) interfering contour of an undesired station and ensures it does not overlap the F(50, 50) protected contour of a desired station. This is achieved using the Desired to Undesired (D/U) ratio, which varies based on the station relationship. For instance:
- Co-channel: D/U ratio of -20
- First-adjacent (+/- 200 kHz): D/U ratio of -4
- Second/Third adjacent (+/- 400 or 600 kHz): D/U ratio of 40
These ratios determine the acceptable overlap between contours for different station classes and relationships. The F(50, 50) contour, also known as the protected or service contour, outlines a station’s coverage area, while the (50, 10) contour represents interference limits.
In the United States:
- For reserved band (88.1~91.9) FM and certain commercial FM stations, the interfering contour of a proposed station must not overlap the protected contour of an incumbent station, and vice versa.
- FM translators can have their service contour inside an incumbent station’s interfering contour, acknowledging potential interference.
Full-service FM and LPFM stations have specific distance separation minimums to avoid overlap between interfering and protected contours. Engineers use computer software to calculate contours, based on factors like Height Above Average Terrain (HAAT). Contour measurements involve HAAT calculations at multiple points along radial lines from the transmitter. However, contours are primarily used for allocation purposes and do not necessarily represent precise signal coverage. Contours may not reflect actual coverage for antennas located under 30 meters HAAT, as these are treated as 30 meters HAAT for allocation purposes. Thus, contours may not always accurately predict usable signal areas.