Alexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson

Ernst Alexanderson

Ernst Frederick Werner Alexanderson was a Swedish-American electrical engineer, who was a pioneer in radio and television development.-Background:...

 and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2nd of July , is the day of the open house at the Swedish government VLF transmitter Grimeton, call sign SAQ, located near Varberg

Varberg

Varberg is a locality and the seat of Varberg Municipality, Halland County, Sweden with 26,041 inhabitants in 2005.Varberg and all of Halland are well known for their 'typical west coast' sandy beaches. In Varberg the coast changes from wide sandy beaches to rocky terrain that continues north into...

. On Alexanderson Day, Christmas Eve, and at other times during the year, the only workable Alexanderson alternator

Alexanderson alternator

An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...

 transmitter in the world is used to transmit short Morse

Morse code

Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

 messages on 17.2 kHz, which should be easily receivable in all of Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

. The transmitter is preserved as a historical remnant of early radio

Wireless telegraphy

Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....

 technology and as an example of VLF (Very Low Frequency) equipment.

The transmissions can't be received by ordinary radios because of their low frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

. Modern enthusiasts often monitor the transmissions using a PC with a coil connected to the soundcard input and FFT analysis software, or a short-wave receiver with an upconverter, or any of several receivers designed to receive VLF transmissions directly. In recent years, Software-defined radio

Software-defined radio

A software-defined radio system, or SDR, is a radio communication system where components that have been typically implemented in hardware are instead implemented by means of software on a personal computer or embedded computing devices...

receivers have proven to be very useful and effective in receiving transmissions from SAQ around the world.