NEW MAST AT PAENGAROA

New Zealand Listener, Volume 20, Issue 517, 20 May 1949, Page 20

NEW MAST AT PAENGAROA

Key to 1YZ’s Coverage ANY a listener in an out-of-the-way spot in New Zealand has by now been surprised at the clear and strong reception of 1YZ, the new NZBS station at Rotorua. Although this station was designed to serve the Rotorua-Bay of Plenty area, there is nothing freakish about its good performance, which is due partly to the rapid advance of radio technique, but principally to the use of a new type of tower radiator, and to the excellence of the transmitter site. In earlier days it was thought that the best place for a transmitter.was on a promontory, but current practice is in favour of level ground with fairly damp soil for the earth-mat. This type of site gives the broadcast wave a good initial impetus, and the station better signal strength. At Paengaroa, where the Rotorua transmitter is situated, there is a good deal of water seepage, and this, coupled with the fact that the transmitter apparatus is more modern in design than that of the existing 10 k.w. stations, is responsible for Rotorua’s effective coverage. The present 10 k.w. stations (1YA, 3YA and 4YA) will be fitted with new 10 k.w. equipment, and the four ZB ‘stations will also be raised in power from 1 k.w. to 10 k.w. when new apparatus is ready. Station 2YA (60 kw.) will also have new 60 k.w. equipment installed. The present transmitting apparatus at the four YA stations is to be serviced, and these transmitters will then be used to broadcast the programmes of 1YC, 2¥C, 3YC and 4YC. To meet the power requirements for increased signal strengths the NZBS is installing diesel generating plants where necessary, so that there will be no extra demand on present electricity resources. Although the transmissions from the big steel radio mast at Paengaroa, 25 miles by land line from the 1YZ studios, are fast becoming part of the accepted pattern of the NZBS network, many listeners are probably unaware that the mast itself represents a new type of radiating system-the first of its kind to be erected in New Zealand. With the advance of radio engineering the design of efficient aerial systems for broadcasting stations has developed from the trial and error methods of the early days to an exact science, and it is now possible to design from paper calculations an aerial system giving any required radiation pattern. bz. The radiating element at Paengaroa is not an aerial, but the sectionalised 500foot mast itself, which is supported at its base on a heavy insulator. This is necessary to isolate the high tadio-fre-quency voltages in the mast from the ground. At a height of 370 feet the mast is sectionalised; that is, a second insulator has been introduced into the

structure, the remaining 130 feet of height being completely insulated from the first 370 feet. The two sections of the mast are electrically connected through a heavy copper inductance coil. Engineers of the NZBS explain that when considering the coverage required, it is necessary to take into account the mutual effect of the two primary’ radiations from the aerial-the ground wave and the sky wave. The sky wave is reflected from the upper atmosphere, and is received at points a considerable distance from the aerial where interference takes place with reception of the direct ground wave, causing fading of the signal at the receiver. It has been found that to produce the best coverage, by giving a maximum strength to the ground wave and a minimum strength to the sky wave (thus increasing the distance from the aerial at which fading starts to take place) the length of the radiating system should. be seventwelfths of the wave-length of the station. For the usual aerial system using a vertical mast, this would mean a physical mast height of about seven-twelfths of the wave-length of the station which, in the case of the Paengaroa transmitter, would be approximately 700 feet. The

cost of constructing a vertical steel mast increases very rapidly with height, and a 700 foot mast would be twice as high in cost as a 500 foot mast. By sectionalising the mast, and inserting the inductance coil between the two insulated sections, the "electrical length" ofthe system is maintained and the physical length is reduced by 200 feet, giving the same radiation performance at a substantially reduced cost.