SIDs are changes in the ionosphere total power detected when solar flare hits the atmosphere. The power is detected from Very Low Frequency (VLF) radio signals travelling through the ionosphere. These signals are sent by transmitters sending messages to nuclear submarines.
The aerials used for SID detection are composed of roughly 150m of cable – good news is that this can be wrapped around a wooden cross as in the photo of the two SID monitor aerials below – the cross on the left is the one for the SuperSID monitor; and, the one on the right, the one for the UKRAA VLF SID Monitor:
UKRAA VLF SID Monitor:
SuperSID Monitor, from Stanford Solar Center: