ABSTRACT
RF measurements are needed to ensure that transmissions are within prescribed limits. The impact of out-of-specification transmissions can be interference to other users of the RF spectrum as well as degrading the broadcast service. Round the clock broadcasting gives little opportunity for off-line
maintenance of transmitter equipment. Evenwhen systems that have fault-tolerant designs it is very unlikely that full power can be maintained with part of the system isolated to carry out maintenance tests. The capital cost of a high-power transmitter normally makes it impossible to justify the cost of a full-power reserve arrangement. A typical high-power television transmitter system has par-
allel high-power amplifiers. It will normally be possible to isolate one amplifier and connect it to a high-power RF test load whilst the other amplifier maintains the programme transmission. However, the resulting reduction in radiated power of at least 3 dB and the likely need for short breaks in transmission whilst feeder switches are operated will always be unpopular with the programme makers. Consequently routine maintenance measurements need to be carried out, as far as possible using the broadcast transmissions. Fortunately for analogue TV transmissions, many routine performance measurements can be made using the ITS line in the vertical blanking interval. Measurements at high-power transmitter sites pose particular
problems because of the potential for RF interference to the test equipment. This is especially true if there are co-sited VHF or MF transmitters. When making measurements always check to see that the results look sensible, if not repeat the measurement. Check that moving leads or changing the physical position of equipment does not affect the readings. For example, RF power meters often comprise a measurement head connected by a flying lead to the meter. RF pick-up on the connecting lead can result in false readings on the meter. RF connectors should always be treated with care. Mating
surfaces must be kept clean. To avoid wearing out the mating surfaces never rotate the connector body, only the fixing collar. For example,when fittinganattenuatorwithN-typeconnectorshold the attenuator body still and only rotate the threaded collar. Similarly, RF leads should always be handled with care to prevent damage.
