Editing Radio wave (section)

== Measurement ==
Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit ''volt per meter'' (V/m) is used for the electric component, and the unit ''ampere per meter'' (A/m) is used for the magnetic component. One can speak of an [[electromagnetic field]], and these units are used to provide information about the levels of electric and magnetic [[field strength]] at a measurement location.

Another commonly used unit for characterizing an RF electromagnetic field is ''power density''. Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the [[far field]] zone of the radiation pattern.<ref>{{cite book |last=National Association of Broadcasters |url=https://books.google.com/books?id=5u_iAAAAMAAJ&q=power+density+commonly+used+unit+for+characterizing+an+RF+electromagnetic+field+is |title=Antenna & Tower Regulation Handbook |date=1996 |publisher=[[National Association of Broadcasters|NAB]] |isbn=9780893242367 |pages=186 |archive-url=https://web.archive.org/web/20180501015942/https://books.google.com/books?id=5u_iAAAAMAAJ&q=power+density+commonly+used+unit+for+characterizing+an+RF+electromagnetic+field+is&dq=power+density+commonly+used+unit+for+characterizing+an+RF+electromagnetic+field+is&hl=en&sa=X&ved=0ahUKEwjj967xl6fVAhVG0FQKHTgTDh0Q6AEILDAB |archive-date=1 May 2018 |url-status=live |department=Science and Technology Department}}</ref> In closer proximity to the transmitter, i.e., in the "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, with the unit milliwatt per square centimeter (mW/cm<sup>2</sup>). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in the far field zone.