# Reference distance > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Reference_distance > Markdown URL: https://mediated.wiki/source/Reference_distance.md > Source: https://en.wikipedia.org/wiki/Reference_distance > Source revision: 1343388034 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) In [broadcast engineering](/source/Broadcast_engineering), the **reference distance** is the distance at which a [radio station](/source/Radio_station) signal is predicted to reach a [service contour](/source/Service_contour) under standard propagation conditions. For [FM broadcasting](/source/FM_broadcasting), this typically corresponds to the 1 mV/m (60 dBµ) contour under average terrain and [normal atmospheric conditions](/source/International_standard_atmosphere).[1] This reference distance primarily depends on the station’s [effective radiated power](/source/Effective_radiated_power) (ERP) and [height above average terrain](/source/Height_above_average_terrain) (HAAT), with adjustments applied according to [Federal Communications Commission](/source/Federal_Communications_Commission) (FCC) propagation charts ensuring that the station does not exceed licensed limits.[2] The actual distance a station's signal travels depends highly on [weather](/source/Weather): factors like [temperature inversions](/source/Temperature_inversion) and heavy [precipitation](/source/Precipitation_(meteorology)) have a strong and highly variable influence on [radio propagation](/source/Radio_propagation). However, for purposes of [broadcast law](/source/Broadcast_law) such as [construction permits](/source/Construction_permit) and [broadcast licenses](/source/Broadcast_license), fixed calculations called [propagation curves](/source/Propagation_curve) are applied to determine the reference distances for all existing and proposed stations. These also take into account [beam tilt](/source/Beam_tilt), [carrier frequency](/source/Carrier_frequency), and even the [Earth's curvature](/source/Earth's_curvature) at longer distances.[citation needed] The reference distance is in turn used to define most [broadcast classes](/source/Broadcast_class) for [FM stations](/source/FM_broadcasting) in [North America](/source/North_America). Each class (except D) is defined as having a maximum ERP and HAAT. If the HAAT of a station's [radio antenna](/source/Radio_antenna) exceeds that specified for the class, it must reduce ERP so that its signal does not exceed the reference distance. The signal strength used differs by class, but generally the value is 1.0 mV/m (millivolt per meter) or 60 dBu (decibels over one microvolt per meter) for most of the [United States](/source/United_States), and 0.5 mV/m or 54 dBu in [Canada](/source/Canada), and for some U.S. stations in parts of certain areas including [California](/source/California), the [Great Lakes region](/source/Great_Lakes_region), and the [Northeast](/source/Northeast_(US)). This is considered the [service contour](/source/Service_contour) of a station by the [Federal Communications Commission](/source/Federal_Communications_Commission) (FCC) and the [Canadian Radio-television and Telecommunications Commission](/source/Canadian_Radio-television_and_Telecommunications_Commission) (CRTC), and by [COFETEL](/source/COFETEL) in [Mexico](/source/Mexico), according to [NARBA](/source/NARBA) and other international agreements among the three. The reference distances are in turn used to create mandatory minimum spacing distances among [co-channel](/source/Co-channel) stations, and certain [adjacent channels](/source/Adjacent_channel) as well.[citation needed] Real-world calculations can also be done by including data from digital [topographical maps](/source/Topographical_map), typically along 12 or 16 [radials](/source/Radial_(radio)), and including the specifications from the [radiation pattern](/source/Radiation_pattern) for a [directional antenna](/source/Directional_antenna). However, this does not fit the definition of a [reference](/source/Reference) in this sense, even though it determines the actual [broadcast range](/source/Broadcast_range) of a station. This actual range is used in the [reserved band](/source/Reserved_band) in the U.S., while the reference distance is used for [commercial radio](/source/Commercial_radio) stations.[citation needed] ## References 1. ["Power and antenna height requirements."](https://www.ecfr.gov/current/title-47/chapter-I/subchapter-C/part-73/subpart-B/section-73.211) (regulation). *[National Archives and Records Administration](/source/National_Archives_and_Records_Administration) (NARA)*. [U.S. Government Publishing Office](/source/U.S._Government_Publishing_Office) (GPO). 47 CFR § 73.211. Retrieved 2026-03-12. 1. ["Engineering charts"](https://www.ecfr.gov/current/title-47/chapter-I/subchapter-C/part-73/subpart-B/section-73.333) (regulation). *[National Archives and Records Administration](/source/National_Archives_and_Records_Administration) (NARA)*. [U.S. Government Publishing Office](/source/U.S._Government_Publishing_Office) (GPO). 47 CFR § 73.333. Retrieved 2026-03-12. This section consists of engineering propagation charts (Figures 1, 1a, 2, etc.) used in broadcast coverage and contour prediction under Part 73. --- Adapted from the Wikipedia article [Reference distance](https://en.wikipedia.org/wiki/Reference_distance) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Reference_distance?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.