A subspace transporter is a transporter that uses subspace to transport an object.
Unlike a normal transporter, which sends its signal through normal space and thus limited to a much shorter range, this transporter uses subspace. Compared to a normal Federation transporter, which had a range of about 40,000 kilometers, it has a range of several light years. A probe was once transported from a distance of approximately 300 billion kilometers this way. For comparison, a Galaxy-class starship would take twenty minutes at warp 9 to bridge this distance. A transport in progress would not be detected by sensors because of the use of subspace.
Although transporting through subspace increased the effective transporter range and made it possible to beam through deflector shields, the technology behind it was unreliable and energy intensive. Because it uses subspace to transport matter, this matter needed to be in a state of quantum flux, which is highly unstable.
If the signature of a subspace transporter is known, one can detect it when a particular subspace carrier wave is approaching. This makes it also possible to determine where the end coordinates of the transporter beam will be. Because the transporter operates via subspace the subspace field coils needed to be tied into the transporter system. Depending on the type of transporter the subspace signature might decay faster than normal, making it difficult but not impossible to determine the origin of the transporter beam.
Someone who used a subspace transporter would leave a distinct subspace signature on anything he or she came in contact with, although this contact needed to be more than brief. To prevent a beam-out with such a transporter a pattern lock needs to be prevented.
Federation transporters could be modified to transport via subspace (although this was not advisable) by aligning the field coils and synchronizing the phase dampers, after which the transport pattern was modulated. The transport sequence itself was longer than normal because the pattern needed to be shunted through the subspace field coils. There was no guarantee that the person in question could be beamed back. Geordi La Forge and Data used this technique to beam Jean-Luc Picard to the ship of DaiMon Bok.
The Federation researched this technology but abandoned it because of its unreliability and energy requirements. The only known people to use a subspace transporter after that were DaiMon Bok when he tried to get even with Captain Picard for killing his son, and then the crew of the Enterprise to transport Picard to Bok's ship shortly afterwards. (TNG: "Bloodlines")