A unique international prefix

Beginning at the left of the call sign block, the country chooses one, two or three characters from within the range assigned by the ITU, enough to distinguish its call signs from other jurisdictions.

A "letter range" always first refers to the first letter of a block, meaning that in the letter range "AAA–ALZ", the "A" is the letter range-designator.

Factors for a country to consider when choosing within its assigned range:

• When a one-letter prefix is enough – This is possible only when the whole letter range is assigned to one jurisdiction. For example, KAA–KZZ is assigned to the United States. The U.S. Federal Communications Commission (FCC) (the American national body) can assign a single letter 'K' prefix as the US is assigned the whole 'K' block. As numbers warrant the US can also assign prefixes 'KA', 'KB', 'KC' and so forth as needed. A single-letter prefix is also possible for block ranges beginning with B (China), F (France), G (United Kingdom), I (Italy), K (USA), M (UK), N (USA), R (Russia) or W (USA) as each are also assigned by the ITU to single jurisdictions
  • When assigning from the KAA–KZZ block, the FCC could assign three-letter prefixes (e.g. KAA, KAB, KAC, etc.) but this produces cumbersome call signs, and one- and two-letter prefixes produce more than enough than needed as it is.
  • None of the B, F, G, I, K, M, N, R, and W ranges are issued with a numeral as a second character, so the first digit in a call in those ranges is always the separating numeral.
• When two characters are needed – Two character prefixes are needed when the letter range is divided among two or more jurisdictions. For instance, AAA–ALZ is assigned to the USA; but Spain, Pakistan, India and Australia are assigned other portions of the 'A' block, so at least two characters from the left need to be assigned by each country to produce unique call signs. Letter-number prefixes 'A2' through 'A9' are also assigned to eight other jurisdictions, so a callsign prefix with a single 'A' does not uniquely distinguish any of them.
  • Korea has issued a special event callsign of D9K.^[7] The 'D9' is the ITU prefix for South Korea, so they have issued a call with no separating numeral.
  • Bahamas issues call signs without a separating numeral. They are assigned the C6A–C6Z block, and the '6' is part of the 2-character prefix. Examples are as found on QRZ.COM (C6AFO, C6AGB, etc.).
  • Cyprus has issued H2T^[8] as a special event call sign. Whereas Cyprus is assigned the H2A–H2Z block, there is then no numeral separator, just a one-letter suffix.
• When a three-character prefix is needed – This is an unusual situation and occurs with callsigns in the 3DA–3DZ block range. Fiji and Eswatini are assigned 3DN–3DZ and 3DA–3DM respectively, so they should choose also the third character from the left to produce unique call signs, but in practise do not.
  • Fiji has issued many call signs with a '3D' prefix and a '2' numeral separator. In theory this does not distinguish their call signs from Eswatini which is issued the 3DA–3DM block.
• This situation also could arise with the SSA-SSZ range as it is shared between Egypt (SSA-SSM) & Sudan (SSN-SSZ). However, both countries use other prefixes for amateur stations.
• Jurisdictions frequently adopt one or only a few of the prefixes allowed to them within a block-range, reserving the others for other occasions. Canada has 24 possible, two-letter prefixes from its assigned ranges, but assigns only CY, VA, VE, VO, and VY for normal operation.

A unique internal numeral and suffix

The jurisdiction then assigns a single digit (a numeral to separate prefix from suffix) as well as a suffix of from one to four characters (the last being a letter) and appends them in that order to their assigned prefixes. The resulting call sign must uniquely identify a ham radio operator within that jurisdiction.

Sometimes the prefix plus separating numeral is together referred to as the prefix.

This produces internationally recognized, unique call signs to identify licensed operators.

Suffix assignment

Since suffixes can also contain digits, some countries issue suffixes (usually temporarily) beginning with enough digits to produce a number, usually associated with the special event (for example the number of years, see New Zealand below).

In normal call sign assignment, if a call sign has two digits (e.g. S59DSC or 2S4LGR – PPNSSS), the first digit is almost always a prefix character (e.g. S5 indicating Slovenia, or 2M indicating Intermediate License holder in Scotland).

Call signs with more than one digit

Call signs with two (or more) digits in them can arise a number of ways. When the digits abut one another, it is important to distinguish which digit belongs to the prefix, which is the separating numeral, and which may belong to the suffix.

In every case (Bahamas being an exception), a jurisdiction assigned a letter-digit prefix by the ITU will have a second digit as their internally assigned prefix/suffix separator. An example is A33A, a Tongan call sign; the first '3' is the second character of the prefix and the second '3' is the numeral separating 'A3' from the single-letter suffix 'A'. There are no single letter prefixes allocated by the ITU with an 'A', so the first '3' must be part of the prefix.

Neither New Zealand's nor the Republic of Ireland's prefixes have numerals as prefix-characters. However, both allow a second numeral as the leading character of the suffix and is not to be confused with the sign's separating numeral. As the first character of the suffix, the two digits can be taken together; for instance, to represent a two-digit number of significance to the operator.

A New Zealand amateur who has been active for 30 years and currently is assigned call sign ZL1xxx can operate as ZL30xxx for up to three months.^[9] Technically, the '3' is the separating numeral and the '0' is the first character of the suffix.

Similarly a club with call ZL4xxx which has been established for 23 years can operate as ZL23xxx for up to three months.

The New Zealand operator substitutes their identifying separating numeral with another, so long as a second digit is added to the beginning of their normal suffix. This may result in call sign confusion in the rare case of two amateurs in differing numeral-areas also both having the same number of years of operation and suffix.

Ireland also takes advantage of the ITU standard to allow digits as suffix-characters. The Irish Radio Transmitters Society operates as EI75IRTS celebrating 75 Years of incorporation – 1932–2007.^[10]

Ofcom in Great Britain also allows numerals in special event call signs. For instance GB75RD was a special event sign for the 75th anniversary of the Reading and District Amateur radio club.^[11]

Numerous other cases of multiple numeral prefixes exist. An example occurred in 1987 when the "200" was used in place of district numbers for the many stations that celebrated the bicentennial of the U.S. Constitution.

Exclusions

The 26 letters of the English alphabet and ten digits may be used to form call signs, accented letters excluded.

Letter combinations which can be confused with distress calls or which are reserved as abbreviations for radiocommunications services are excluded (e.g. Q codes).^[12] The ITU Article 19 exclusions are those found in ITU-R M.1172.^[13] In practice, no prefix begins with the letter 'Q', but 'Q' can be the second letter (e.g. Malawi assigned the 7QA–7QZ block).

Double- or single-digit prefixes are excluded. A callsign with a leading digit in the prefix always has a second character which is a letter and in rare cases a third character which is also a letter.

Currently, no allocated prefix has 0 (zero) or 1 (one) as one of its characters as they can be confused with the letters O (Oscar) and I (India).

All ten digits from 0 to 9 are allowed to be used as a separating numeral at the discretion of national allocating bodies.

Secondary prefix or suffix types

Ancillary prefixes or suffixes further identify the location and/or operating condition of an amateur operator.

According to the Canada/United States Operating Agreement treaty^[14] amateurs from one country operating in the other sign with their home call sign, but attach the call area prefix where they are operating to their call. For instance, an amateur from British Columbia (VE7 in Canada) operating in Washington State (K7 in the USA) would amend their home-call with a trailing /K7 (e.g. VE7xxx becomes VE7xxx/K7).

Radio amateurs from countries that apply the CEPT recommendation T/R 61-01^[15] operating as a visitor in countries that apply the same recommendation are required to use the appropriate host country's prefix before their home call sign^[16] (in the case of Peru, after the home call sign) and may use the appropriate operating suffix (see below). For instance a British (English) amateur holding a call of G3xxx operating in France would sign as F/G3xxx, a Swiss amateur operating from a car in Germany as DL/HB9xxx/m. Similar rules apply in cases when bilateral agreements on visitors licenses exist, or a visitor is permitted to operate without being assigned a local call sign.

When a country's separating numeral denotes a geographic area within, an operator from one region operating in another region can affix a secondary suffix indicating so. For instance an amateur from Queensland, Australia, operating in Tasmania can sign as VK4xxx/7 or VK4xxx/VK7.

Other secondary operating suffixes can be attached such as /P (for portable operation), /M (for mobile operation), /AM (aeronautical mobile), /MM (maritime mobile), or /A (for operation from an alternative location that is registered with the licensing authorities). Depending on the jurisdiction, the use of these five suffixes may be required for these types of operation. Occasionally self-assigned operating suffixes such as /QRP for operation at low power are heard.

Some repeaters have automatic call sign transmission at regular intervals and use the secondary suffix /R at the call sign's end. Some jurisdictions discourage this practice on the grounds that it could be confused with an amateur from the repeater's location working portable in Russia. Similarly, some beacon stations use the self-assigned secondary suffix /B.

General issuing practices

Each national authority has some options in relation to the form of the prefix, as long as enough characters are selected starting from the left of their assigned block to produce a prefix unique to its jurisdiction.

Each country has authority over which numeral separates the prefix and suffix. The prohibition of the use of the digits 0 and 1 in land mobile stations does not apply to amateur stations. The ITU however does not issue prefixes with either a 0 or 1 as one of the characters.

Bahamas issues call signs without a separating numeral. They are assigned the C6A–C6Z block, and the '6' is part of the prefix. Examples are as found on QRZ.COM (C6AFO, C6AGB, etc.)

INDIA In India the International Telecommunication Union (ITU) has divided all countries into three regions; India is located in ITU Region 3. These regions are further divided into two competing zones, the ITU and the CQ. Mainland India and the Lakshadweep Islands come under ITU Zone 41 and CQ Zone 22, and the Andaman and Nicobar Islands under ITU Zone 49 and CQ Zone 26. The ITU has assigned to India call-sign blocks 8TA to 8YZ, VUA to VWZ, and ATA to AWZ. VU call-signs are listed according to licence grade: for General (formerly the Advanced Grade and Grade–I) licence holders, the call-sign prefix is VU2; for Restricted (formerly Grade–II and Grade–II Restricted) licence holders, the prefix is VU3. The VU3 prefix has also been granted to foreigners operating in India. As of 2011, call-signs consist of only letters, not numerals, and can be either two or three characters long. Examples of Indian amateur radio call-signs are "VU2XY" and "VU3EGH".

The suffix can be from one to four characters subject to ITU exclusions (above).^[12] On special occasions, for temporary use, administrations may authorize use of call signs with more than four suffix-characters.

Allocation options within a country

Whereas for ITU purposes the prefix does not include the separating numeral, for country purposes often the separating numeral is included when the prefix is referred to. Thus for Canada VE6 or VA6 are the prefixes for Alberta, while VE2 or VA2 are the prefixes denoting Quebec.

• The most common suffix has three characters. The ITU requires only that the last suffix-character be a letter, although with XE21 Mexico broke this rule in 1995, as did Spain in 2014 when it issued EF6 to commemorate the ascension to the throne of King Felipe VI, and Canada in 2017 with VX7150 to commemorate its 150th anniversary (the "7" indicated the operator was in British Columbia). In practice, suffixes are frequently composed of one to three letters. Portugal uses four-character suffixes for repeater stations. Long ones have also been used for commemorative events, such as Canada's VE9COAL.
• Most countries select permanent or renewable calls from a narrow, specific range of possible assigned prefix alternatives. For instance, the Philippines is allocated the DUZ–DZZ and 4DA–4IZ blocks – 12 possible two-character prefixes in all – , but almost all Philippine amateur callsigns are issued with the DU prefix.
• Some countries add or reserve an allowable second-character letter to the prefix to indicate the internal region of the operator – (e.g. G3xxx in England becomes GD3xx for the Isle of Man; or for KL6xxx, the L indicates Alaska).
• Some countries reserve their separating numeral to indicate the internal region in which the individual operator resides (e.g. the United States: 6 – California, 4 – The Southeast, 3 – Pennsylvania, Maryland, and Delaware, etc. although in the U.S. licensees can change districts and retain their original calls). Callsigns may also indicate where within a country a person is operating; thus a VE3 (Ontario) operator would add "/VE4" to the callsign when transmitting from Manitoba.
• Some countries reserve the first letter of the suffix to indicate the internal region in which the individual operator resides (e.g. Argentina assigns LU#N to Santiago del Estero where # is any numeral, or Russia where the separating numeral plus the first letter of the suffix denotes the Oblast of the call sign).
• Some countries issue call signs meant for use in Antarctica (e.g. South Korea assigns HL8 to stations at its Antarctic base).
• Some countries reserve allowable prefixes for use in space. (e.g. China reserved BJ1 prefix to all Chinese stations orbiting the Earth.)
• Canada is the only jurisdiction which issues a call sign prefix for use in international waters – VE0, although Panama has allowed "HP/" without a district number to be temporarily used by persons on cruise ships registered in that country. Similarly, Germany reserves the prefix DP for German stations that operate from outside German territory, such as DP0GVN for the German research station in Antarctica and DP0POL for the Polarstern research vessel.
• Some countries reserve allowable prefixes to indicate the operating class of the amateur (e.g. China used to issue the BA prefix to 1st class operators).
• Some countries limit two- or one-letter suffixes to operators with advanced privileges (e.g. the Republic of Georgia limits one-letter suffixes to "extra" class licenses). As noted above, beginning in 1977 this was generally the practice in the United States, and many operators holding the highest class license took 2×1 or 1×2 calls.
• Some countries reserve allowable prefixes for foreigners licensed in their jurisdiction (e.g. Japan reserves the 7J prefix for foreigners).
• Malta reserves the 9H5 prefix for VHF-use and up.
• Some countries reserve amateur radio prefixes for shortwave listeners (e.g. Germany officially does not issue DE callsigns,^[17] but the DARC association unofficially issues the DE prefix for SWLers.^[18] 'DE' is also a Morse code abbreviation amateurs use meaning "from" when one station contacts another – e.g. VE7xxx de WB4xxx means, "calling station VE7xxx from my station of WB4xxx". Thus Germany has eliminated any potential confusion in the use of DE as a prefix, as opposed to it being used as an abbreviation).
• Belarus reserves some of its prefixes for WWII veterans.
• Belarus reserves suffixes YAA–YZZ for female operators.

Rare ITU prefixes/DXCC Entities

A country can consist of many DXCC entities depending on its geographical make-up. Some islands which are separate DXCC entities are uninhabited and can only be worked when a DXpedition travels there. The following are countries and/or entities which appear perennially on various listings of rare countries:^[19]

Countries which are rarely heard, roughly in this order:

• North Korea – The ITU-issued P5 prefix is rare, as North Korea does not issue amateur radio licenses to its citizens, and very rarely to foreign nationals.
• Yemen – The ITU-issued 7O (note: letter O) prefix is rare as Yemen does not issue amateur radio licenses to its citizens, and rarely to foreign nationals.
• Eritrea – E3 prefix.
• Myanmar – XZ prefix.
• Bangladesh – S2 prefix.
• Iran – EP prefix.
• Nauru – C2 prefix, an island nation in Micronesia in the South Pacific.
• Equatorial Guinea – 3C prefix.
• Somalia – T5 prefix.
• Nepal – 9N prefix.
• Bhutan – A5 prefix.

Islands which are rarely heard DXCC entities:

• Bouvet Island – 3Y/B, uninhabited Antarctic volcanic island in the South Atlantic Ocean, Norwegian dependency.
• Navassa Island – KP1, uninhabited island in the Caribbean Sea, claimed as an unorganized unincorporated territory of the United States, Haiti also claims the island
• Prince Edward Islands – ZS8, Antarctic islands of South Africa. The only human inhabitants of the islands are the staff of a meteorological and biological research station run by the South African National Antarctic Programme on Marion Island.
• Heard Island or Macquarie Island – VK0, Australian possession, among the most remote places on Earth between Australia and South Africa, but closer to Antarctica.
• Île Saint-Paul – FT5Z, part of the French Southern and Antarctic Lands in the Indian Ocean.
• Timor – 4W prefix, belonging to the state of East Timor.