Editing Numbers station (section)

== Transmission technology ==
[[High-frequency]] radio signals transmitted at relatively low power can travel around the world under ideal [[Radio propagation|propagation]] conditions – which are affected by local [[electromagnetic interference|RF noise]] levels, weather, season, and [[sunspot]]s – and can then be best received with a properly tuned antenna (of adequate, possibly conspicuous size) and a good receiver.<ref name="MNT" />

Although few numbers stations have been tracked down by location, the technology used to transmit the numbers has historically been clear—stock shortwave [[transmitter]]s using powers from 10{{nbsp}}kW to 100{{nbsp}}kW.{{citation needed|date=December 2015}}

[[Amplitude modulation|Amplitude modulated]] (AM) transmitters with optionally–variable frequency, using [[Class-C amplifier|class-C]] power output stages with [[radio transmitter design#Plate AM modulators|plate modulation]], are the workhorses of international shortwave broadcasting, including numbers stations.{{citation needed|date=December 2015}}

Application of [[Spectrum analyzer|spectrum analysis]] to numbers station signals has revealed the presence of data bursts, [[radioteletype]]-modulated [[subcarrier]]s, [[Phase-shift keying|phase-shifted carriers]], and other unusual transmitter modulations like [[Multiple frequency-shift keying|polytones]].<ref>{{cite book |last=Schimmel |first=Donald W. |title=The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications |edition=3 |location=Solana Beach, California |publisher=High Text Publications, Inc. |year=1994 |isbn=1-878707-17-5 |pages=27–28}}</ref> (RTTY-modulated subcarriers were also present on some U.S. commercial radio transmissions during the Cold War.<ref>{{cite journal |last=Collins |first=Barry W. |title=The day the U.S. Army invaded W4TLV |journal=QST |volume=81 |pages=48–49 |date=July 1997 |issn=0033-4812}}</ref>)
[[File:Sprach-Morse-Generator.jpg|thumb|The Speech/Morse generator (pictured here) is a machine that has been used for many well-known numbers stations]]

The frequently reported use of high-tech modulations like [[burst transmission|data bursts]], in combination or in sequence with spoken numbers, suggests varying transmissions for differing intelligence operations.<ref>{{cite web |url=http://www.cvni.net/radio/nsnl/nsnl015/nsnl15vs.html |title=NSNL 15: Voice stations |publisher=Cvni.net |date=3 July 1999 |access-date=26 August 2010 |archive-date=26 January 2021 |archive-url=https://web.archive.org/web/20210126030846/http://www.cvni.net/radio/nsnl/nsnl015/nsnl15vs.html |url-status=dead }}</ref>

Those receiving the signals often have to work only with available hand-held receivers, sometimes under difficult local conditions, and in all reception conditions (such as sunspot cycles and seasonal static).<ref name="MNT" /> However, in the field low-tech spoken number transmissions continue to have advantages even in the 21st century. High-tech data-receiving equipment can be difficult to obtain and even a non-standard civilian shortwave radio can be difficult to obtain in a totalitarian state.<ref>{{cite web |last1=Wagner |first1=Thomas |title=If it had not been for 15 Minutes, Chapter 6 |url=http://radio-weblogs.com/0101986/stories/2002/04/24/ifItHadNotBeenFor15MinutesChapter6.html |website=Radio Weblogs |access-date=18 March 2022 |archive-url=https://web.archive.org/web/20141101065629/http://radio-weblogs.com/0101986/stories/2002/04/24/ifItHadNotBeenFor15MinutesChapter6.html |archive-date=1 November 2014 |date=24 April 2002 |url-status=live}}</ref> Being caught with just a shortwave radio has a degree of [[plausible deniability]], for example, that no spying is being conducted.{{citation needed|date=December 2015}}