Main transmitter
From Freeradio
The main transmitter refers to the transmitter which broadcasts a pirate station on the main broadcast band used - generally FM, but a few pirates have used medium wave or Short Wave for their main broadcasting frequency.
The term is used to differentiate the broadcast Transmitter from the link transmitter(s) used by some of the larger pirates. Links on non-broadcast frequencies involve a link transmitter in (or directly connected to) the studio, plus a link receiver connected to the main transmitter.
Reasons for using links
The use of links gives the studio greater protection from a DTI raid, protecting the DJs from prosecution and the equipment (records and CDs included) from confiscation.
In addition, linking allows the studio premises to be in a low-rise building, while the transmitter is at the top of a high-rise tower block, allowing maximum coverage. (For this reason, links have sometimes also been used by legal RSL stations, such as Palace FM in Tamworth, and Challenge FM in Wolverhampton).
In the case of legal broadcasting, "main transmitter" is used to denote a high power transmitter, as opposed to low power relays. A handful of pirates have used relays, to cover nearby towns, but they are few and far between. Examples of stations using relays include Dream fm Leeds.
Protecting the studio
Some of the larger pirates have operated high power transmitters for 24 hours every day, and as a result have been heavily targeted by DTI raids. To protect their studios, a simple link system may not be sufficient.
To increase security, some are understood to have used a chain of links, starting with a UHF or microwave link to link to the long-distance link transmitter. Other studio to "main link transmitter" link techniques have involved lasers, ultrasonic, and mains-loop transmission systems.
Other techniques to improve security of the link transmitter include using a modulation system not easily audible on a scanner, such as encryption, transmitting on sub-carriers, or using DSBSC (double side band suppressed carrier). The disadvantages are, this doesn't actually stop the DTI tracking the link transmitter's frequency, and it also requires a more complicated (and probably more expensive) link receiver at the main transmitter end.
In the 21st century, a few pirates have used internet streaming to connect the studio to the main transmitter (examples include Freeze FM of London and Nottingham). Such a technique makes the studio potentially more secure from a DTI raid, and also allows the use of transmitters and studios in different towns, possibly separated by hundreds of miles.
