Hyperfix Antennas
Hyperfix, with lane identification working and the capacity to support more transmitter stations, could cover a much larger geographical area than Hi-Fix, its predecessor. To provide the extra coverage, the transmitter was uprated to 50 Watts output, and better antennas provided. The standard antenna for fixed chain stations was the quarter-wave vertical. This was a guyed lattice tower 30 metres tall, manufactured by Francis & Lewis; their model number WD25. The tower was triangular in section, manufactured from galvanised steel and standing on a concrete pad. The tower itself was supported on three ceramic insulators. The feedpoint was simply a wire attached to a screw in one leg of the tower. The guys were manufactured from stainless steel and had insulators in them 'break them up' to radio frequencies. The guy anchor points had a small concrete block set in the ground.
The required length for a quarter-wave at 1900kHz allowing for velocity factor is about 35 meters so the match would have been close to 50 ohms, requiring a small inductive reactance to tune.
The alternative to a alttice tower is the 10m tubular pole which was used by the Ministry of Defence at their sites and also for temporary sites. It was hinged at the base, and permitted very easy erection using a simple method. The hinged base has an extension to which a pole can be attached. One set of guys are attached to the end of this pole, and the others attached to their proper anchors. The mast is then pulled upright- using a vehicle if necessary. The guys at each side keep it from falling to one side, and the guys attached to the erecting pole keep it from sagging at the end.
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In addition to the antennas described above, other types were in use. There is evidence that some of the East Coast chain stations were converted from Hi-Fix, which used a 9.5 metre high antenna, by removing the top set of guys and therefore the capacity hat from the antenna. This would reduce its capacitance dramatically- the impedance of the Hi-Fix antenna was supposed to be 10 ohms +300pF. In theory at least, this modification would permit the antenna to be inductively base loaded by the Hyperfix ATU. The resulting antenna would have looked like this:
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Last Updates: 5/9/06, 1/5/05 AC