How to Calculate a VHF Path Profile

Use a path profile as a tool to help engineer your VHF radio system.

A VHF radio path profile illustrates the terrain between two antennas arranged in a line-of-sight (LOS) pattern where one antenna "talks" directly to another. Engineers make profiles to study wave propagation between the two ends and estimate what may happen to a signal as it bounces off objects within the path, with the ground elevation as the primary statistic. The bottom line of a path profile represents the distance between the two locations and the vertical dimensions represent the height of the terrain or structures between the two points.

Things You'll Need

• Geographical coordinates of the two sites
• Relief map with detailed ground elevation statistics
• Altimeter (optional)
• Binoculars (optional)
• Global positioning system (GPS) (optional)

Instructions

1. Make a spreadsheet with the two antennas at opposites sides. Calibrate the horizontal distance with appropriate measurements such as miles, kilometers or thousands of feet. Calibrate the vertical dimensions in meters, feet or tens or hundreds of feet. Use dimensions that give a clear picture of your path. Mark the elevations of the antennas at each end of the spreadsheet.

2. Mark the two sites on the relief map using their geographical coordinates, and draw a line between them. Enter dots on your spreadsheet at periodic distances along the line to illustrate the altitudes, as read from the map. The more dots you enter, the more accurate your profile will be. Try to at least enter a dot for every peak and valley.

3. Draw a rounded line to connect the dots to make a profile of the terrain between the two antennas.

4. Travel along the path as closely as you can. Note buildings, trees or other structures or towers along the way. Mark them on the spreadsheet at the appropriate distances along the profile. If necessary, use a compass, binoculars and GPS to collect more accurate information.

Tips & Warnings

• Aviation maps used by pilots have detailed information of altitudes and structures on the ground.

• Sometimes engineers travel along the path in a helicopter with binoculars, a compass and a GPS to compile more accurate information.

References

• TAPR: VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters

Resources

• Sky Vector: Flight Plan