Editing Capture effect (section)

== AM capture effect use ==
When [[AM broadcasting|AM]] (Amplitude Modulation) transmitter share the same center frequency, the weaker signal will introduce distortions in form of beat frequencies or both signals interfere completely with each other. If only a carrier devoid of modulation is received or added in a receiver, e.g. using a [[Beat frequency oscillator|BFO]] (Beat Frequency Oscillatior), a tone with the frequency offset between the two carrier frequencies will be heard. 

By introducing a frequency offset between the carrier frequencies in the magnitude of at least the sum of the highest modulation frequencies employed by both transmitter, will eliminate generation of beat frequencies, e.g. highest modulation frequency is 2.4 kHz therefore the min. frequency offset between both carrier is 4.8 kHz (=2 x 2.4 kHz). 

The capture effect is actively employed in Europe for aeronautical VHF-communication in the band 118 MHz to 137 MHz to provide coverage for aircraft flying under [[Air traffic control|ATC]] (Air Traffic Control) in large ATC sectors that cannot be covered by a single transmitter site. For ATC large sectors up to five transmitter sites are required to provide continuous coverage within the area of a large ATC sector. Transmitter are strategically placed therefore ensuring that not all transmitter will be received at any point in space simultaneously.

Operation of [[instrument landing system localizer]] (ILS-LOC) and [[Glide path|instrument landing system glide path]] (ILS-G/P) at airports is often impossible due to strong reflections, e.g. on terrain and buildings. The solution lead to the development of two-frequency ILS-LOC and ILS-GP.<ref>{{Cite book |title=FAA RD-69-12, Final Report, Study of two-frequency capture effect on ILS-receivers, Charles Manney, March.1969}}</ref> Two frequency ILS systems use an additional carrier frequencies with a frequency offset of at least 6 kHz (2 x 3kHz as the highest voice modulation frequency), two separate antenna pattern that overlapp only in part and a higher [[Effective radiated power|EIRP]] for the ILS-localizer course signal for a range of up to 25 NM and a lower EIRP for the ILS-Localizer clearance signal for a range between10 NM to 17 NM. For difficult terrain and weather impact 2f ILS-GP are employed.<ref>{{Cite book |title=U.S., FAA-R-6750.1, Final Report, Capture-Effect and Sideband-Reference Glide-Slope performance in the presence of deep snow, Kent Chamberlin, July.1978.}}</ref> Two frequency ILS-LOC and ILS-GP are an ICAO standard precision approach system and are standardized for worldwide use in ICAO Annex 10.<ref>{{Cite book |url=https://elibrary.icao.int/reader/299828/&returnUrl%3DaHR0cHM6Ly9lbGlicmFyeS5pY2FvLmludC9wcm9kdWN0LzI5OTgyOA%3D%3D |title=ICAO, Annex 10, Vol.I Radio Navigation Aids, Edition 8, Amendment 93,. In: ICAO, International Standards and Recommended Practices. Juli 2023}}</ref>

AM receiver will demodulate any carriers and their side bands that are strong enough for demodulation within the receiver passband resulting in an audio mix, if their carrier frequencies are not separated by the sum of the highest modulation frequencies of employed in both transmitter. If only a carrier devoid of modulation is received or added in a receiver, e.g. using a [[Beat frequency oscillator|BFO]] (Beat Frequency Oscillator) the resulting tone is the frequency offset between the two carrier frequencies.  {{efn|If AM signals are close but not exactly on the same frequency, the reception mix will not only have the audio from both carriers but depending on the carrier separation will include an audible [[heterodyne]] [[beat note]] equal to the difference between the carrier frequencies. For instance, if one carrier transmits at 1000.000&nbsp;kHz, and the other at 1000.150&nbsp;kHz, then a 150&nbsp;Hz beat note tone mix will result. This mix can also occur when a second AM carrier is received on an adjacent frequency if the receiver's ultimate bandwidth is wide enough to include reception of both signals. In ITU Region 2 locations, consisting of the Americas, for the AM broadcast band this occurs at 10 kHz; elsewhere it can occur at 9 kHz, the AM band frequency spacing commonly used in the rest of the world. Where such an overlap within the passband occurs, a high-pitched heterodyne whistle at precisely 9 or 10 kHz can be heard. This is particularly common at night when signals from adjacent frequencies travel long distances due to [[skywave]]. Modern SDR-based receivers can eliminate this by utilizing a [[brick-wall filter]] narrower than the channel spacing, which reduces signals outside the passband to inconsequential levels.}}