special valve. In armoured fighting vehicles an Electronic Automatic Noise Reducing System was provided. Clansman accessories IBRU (Interconnecting Box Radio-rebroadcast Unit) Used in conjunction with the "Harness" point on VRC 353 and VRC 321 units, to operate 2 or more sets as a ReBroadcast ("rebro"). This allows two or more networks, on different frequencies, to be combined, effectively increasing the ground size of the net. For instance, a vehicle using whip antennas may only be able to communicate 30 km; though placing a rebro vehicle 30 km away could extend this to 60 km by re-broadcasting the signal on a second net. DMU A Digital Master Unit or DMU is used to provide digital communications and to provide secure encrypted communications on a VHF network, when working to similarly equipped radios. This required additional equipment such as the BID 250 and associated Crypto key material. SURF A SURF (Selector Unit Radio Frequency) is used to prevent interference to operation of the radio sets. The SURF is an electrical filter, designed to reject unwanted interference from other radios when operating in close proximity to each other. The SURF is tuned using a manual control on the front of the unit, so that only the frequency in use, and those close to it, are accepted by the set. A Range of SURFs were provided for attachment to the VHF PRC350, PRC351/2 (SURF 4 Watt) and also the HF PRC320 (SURF 12 Watt) and VRC 321 (SURF 25 Watt). Antenna tuning and matching TURF A TURF (Tuning Unit Radio Frequency) is an Antenna Tuning Unit or ATU. It is used to artificially lengthen the antenna (or tune it) Often at HF wavelengths for example an antenna would be physically too long to attach to a radio without affecting portability and manoeuvrability. A shorter vehicle whip was more practicable and also field antennas were constructed with for example, vertical radiators constructed using the Racal 8 or 12 metre masts. Because these were an electrical compromise, they had to be tuned by use of a TURF (or electrically matched to the set). The TURF 25 Watt (a separate unit to the radio) is considered mandatory when using the VRC321: it tunes both unbalanced, end fed wires and whips and also balanced antennas such as the dipole. An antenna tuning unit is built into the manpack PRC 320. The TURF requires users to manually tune the