You will see IO91 in the header of this site, on my QSL card and in my logbook. It is my Maidenhead locator – a short code that describes where a station is without giving away a street address. Radio amateurs all over the world use it, especially on VHF and UHF, in contests and on digital modes such as FT8, where it is sent automatically with every CQ.
How the grid works
The system divides the whole Earth into a grid and then subdivides each piece. Every pair of characters zooms in further:
| Characters | Name | Size | Example |
|---|---|---|---|
| 1st pair (letters A–R) | Field | 20° longitude × 10° latitude | IO |
| 2nd pair (digits 0–9) | Square | 2° × 1° (roughly 140 × 110 km in southern England) | IO91 |
| 3rd pair (letters a–x) | Subsquare | 5′ × 2.5′ (roughly 6 × 4.6 km) | IO91cb |
Longitude always comes first, then latitude. Fields are counted from 180° W and 90° S, so the letters increase as you go east and north.
Decoding IO91
- I – the 9th field going east from 180° W, which covers 20° W to 0°.
- O – the 15th field going north from 90° S, which covers 50° N to 60° N.
- 9 – the last 2° slice of that field: 2° W to 0°.
- 1 – the second 1° slice: 51° N to 52° N.
So IO91 is the box between 2° W and the Greenwich meridian, and between 51° N and 52° N. It covers Salisbury, but also Winchester, Swindon, Reading, Oxford and a good part of London – which is why IO91 is one of the busiest squares in the UK.
Why four characters are usually enough
For most HF contacts the four-character square is plenty. On VHF, UHF and microwave, contest scoring is based on distance, so stations exchange the full six-character locator. Six characters also matter for aiming beams and for satellite and EME work.
Finding your own locator
- Find your latitude and longitude (any map app will show them).
- Use an online locator calculator, or let your logging software work it out.
- Use the six-character version in contests and on VHF, and the four-character version for everyday FT8.
Some awards are based on locators too – collecting different grid squares is a popular challenge on 6 m and 2 m, and my logbook page counts how many squares I have worked so far.
73 de M6OER, IO91