and is still in working condition. Each year, on a day called Alexanderson Day, either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2 July, the site holds an open house during which the transmitter is started up and transmits test messages on 17.2 kHz using its call sign SAQ, which can be received all over Europe. Working principle The principle used is that of a generator (also called alternator) with an exact large number of poles driven to an exact speed matching the number of pole changes with the desired output frequency (f = poles/2 * revolutions). The switching on and off of the transmitter with the signalling morse key makes the driver motor change frequency just a bit, so the frequency is outside the narrow band of the antenna and thus transmitted with much less power. In fact this forms an early and clever form of frequency shift keying (FSK). In the days of construction this was the only way known to output very high power at radio frequencies. Like turning the dynamo of a bike: a dynamo has for instance four poles and by turning this at 100 revolutions (turns) per second the output will be 200 Hertz. If the number of poles in the dynamo is increased to 80 the same driving speed will produce a 4 kHz signal. Increasing both the number of poles and the turning speed makes higher frequencies possible. Schematic diagram of principle Between points "a" and "b" there will be a signal with frequency 15 times (because of 30 poles) the number of revolutions. The turning rod with electrical windings creating a coil will undergo a sequence of north and south magnetic poles. This induces an alternating current that presents itself on the points 'a' and 'b'. This presented model has fixed (stator) magnets and a rotating (rotor) coil. The places of the two can be exchanged so the magnets are turning in the centre and the coils are in a ring around it. This makes transfer of the coil signal to the next stages (tuning network and antenna) easier. Antenna system The antenna system, consisting of antenna wires supported by towers like those supporting high-tension power lines, has a very low efficiency due to the fact that the length of the antenna is still relatively small compared to the output wavelength.