Last year, the division investigated more than 2,400 cases of radio interference, M. H. Au, assistant director of Operations at OFTA, said.

''Most of them are truck and taxi drivers. I think they are well aware of the law, but they want to communicate directly on a mobile-to-mobile basis, which the illegal frequencies allow them to do.'' Mobile interference can be difficult to track down. The mobile units start by using direction finders to determine the general source of interference, then narrow down the target area with field strength meters.

Police licence checks often turn up unlicensed radio equipment, which they refer to OFTA units.

Road blocks are sometimes set up, once the general source area of interference has been pinpointed. Often, the source is an unlicensed portable radio in a car or truck.

Last year, OFTA set up 104 road blocks. A total of 209 raids were carried out on illegal ''stations'' (an unlicensed signal source), and dealers. A total of 391 pieces of equipment were seized.

Last month, there were eight road blocks and seven raids, resulting in the seizure of 29 pieces of equipment.

The new Telecommunication Ordinance is expected to streamline and simplify the licensing procedure for radio equipment, which could lead to more legal equipment being used.

Fixed sources of interference were easier to detect, Mr Au said.

''For example, one common form of interference is between the police beat radio, which operates near the 470-megaHertz band, and the television broadcast band at 470 MHz to 490 MHz.

''Sometimes the boosters for television reception can cause interference if they are not maintained.'' The operations department is also responsible for assigning the radio waves to ensure there are no clashes.

It follows international standards but needs to tailor much of its bandwidth allocation to meet Hong Kong's specific needs.

Because of the popularity of telecommunications devices, such as mobile phones and radios, and Hong Kong's cramped geography, the logistics of radio-wave assignment were more difficult than many other jurisdictions, Mr Au said.

''Demand in Hong Kong is higher and it is more closely packed here. So, the density, the intensity of radio usage, tends to be higher than elsewhere. It requires very detailed planning,'' he said.

Far more is involved in assigning new frequencies than simply looking at the chart of those already allotted.

Mr Au explained the phenomenon of intermodulation, whereby several frequencies combined to create a new one. OFTA used computers to identify these unintended results.

Co-operation with China is a big part of the section's work. It often meets with the Chinese authorities under procedures outlined in a frequency co-ordination agreement signed in 1992.

''Because Hong Kong is so close to south China, particularly Shenzhen where there has been such strong growth in the past 15 years, there is the opportunity for cross-border interference,'' Mr Au said.

''Therefore, there is detailed and frequent co-ordination between the two sides.'' In radio bands, OFTA deals with the Guangdong Wireless Management Committee and, in broadcast, with the Guangdong Broadcast and Television Bureau.

Assignments of most VHF and UFH bands outside sheltered areas involved exchanging data, he said.