A MATTER OF HEIGHT

New Zealand Listener, Volume 17, Issue 430, 19 September 1947, Page 6

A MATTER OF HEIGHT

"HERE nigy be among Listener readers some who will want to know why the masts for transmitting different stations’ radio programmes differ in height. And they may wonder why, in some cases, aerial wires are now dispensed with, leaving the mast itself to act as the radiator. The Listener asked an NZBS engineer to explain these things. — as far as possible in non-technical terms. Masts, he said, differ in height according to the varying wavelengths used ‘for transmission. The longer the wavelength, the longer is the mast required -710ft. in the case of 2YA’s mast at Titahi Bay. Smaller stations have shorter wavelengths and therefore shorter masts; some of these are as low as 200ft. But the wavelength is not related to the power of a broadcasting station, although it is a vital factor in coverage. The longer wavelength is used wherever possible for the more important stations because the coverage obtained is greater, with a minimum of fading and distortion. The Fading Ring There is a certain distance from every broadcasting station where the maximum degree of fading and distortion occurs; this is known technically as the fading "ring," and it is further away from the station when a longer wavelength is used. Through gradual development in broadcasting. methods, the engineers have found that masts of certain heights

are more effective than others in pushing the fading ring as far away from a station as possible. They have discovered that masts whose height is half the wavelength are the most economical. For further economy the masts of to-day are built in sections, one section being insulated from another and inductance loading (or a tuning coil) inserted between the sections. This arrangement gives the effect of increased height without the cost of building very high masts. The use of the suspended copper wires in the past was part of an attempt to obtain the equivalent of a high aerial mast. At Titahi Bay there is a semiumbrella type of aerial which has been effective, but the sectionalised mast is even more useful. The engineer explained that half pee wavelength of 2YA in an unsectionalised mast would require a height of, 1,000ft. The whole idea of doing away with the suspended wires and using the masts themselves as radiators has been /to achieve the most effective height m economically. 7 More than one station can broadcast a programme from the one mast aerial. Introduced between the two transmitters is a tuning unit containing components which cause the aerial to radiate, simultaneously, the frequencies of the two or more transmitters. In effect, the tuning unit separates the frequencies from the transmitters and allows the aerial to radiate them independently.