Antennas for UHF & VHF TV reception

When I retired and moved to Heatherbrae (NSW) some 11 years ago, it was stated that all local TV channels telecasting from Mount Sugarloaf would move to the UHF band. Due to the stupidity of a previous government, or their advisers, Channels 3, 4 and 5 were allocated bandwidth in the international FM band. The first to move out was the ABC which moved from Channel 5 to the then new Channel 5A. Then very shortly afterwards they established a new UHF channel. Station NBN also was to vacate their spot in the FM band and move to UHF.

On the strength of all this I built your bow-tie reflector UHF antenna which gave perfect results on UHF. I have been waiting ever since for NBN to close down their VHF transmitter and re-establish in the UHF band. The main reason is that there is some interference with their VHF signal here. My wife "discovered" that if the coax lead was removed from the wall socket and a pencil placed so that the pencil "lead" made contact with the inner socket, an excellent picture could be obtained if the setup was placed on the floor. The mind boggles!

However, the nuisance of moving the coax lead back and forth from the socket to the pencil proved too much and we just put up with the poor picture and waited (apparently in vain) for the change to UHF. Then along came your article on the FM antenna in the March 1998 issue. As this would cover the bandwidth of Channel 3 it seemed an ideal solution. I constructed it as per your instructions and erected it with a diplexer above the bow-tie. Low and behold we get a perfect, interference-free reception of Channel 3 and, with a splitter, every FM station within 100km roars in.

J. Lowe,
Heatherbrae, NSW.

Radio interference from power lines

I must make a clarification of the Vintage Radio article in the November 1998 issue. I am referring specifically to the comment regarding interference from high voltage power lines.

It is the mistaken belief by the majority of people within the trade that interference on high voltage power lines is caused by arcing. Whilst I was employed in the Department of Transport and Communications and its other names over a period of 23 years I did quite a bit of research on interference to radio, TV and radio communications reception, and methods of improving reception.

The only time that an arc creates interference is at the instance the arc is established and when it is extinguished, not during the time that the arc is present. High voltage power line interference is caused by sparking, much the same as the spark coil in a car causes sparks. Arcs may well be produced as well but because of the oscillatory nature of the waveforms, there are many sparks.

On HV power lines, there are both capacitive and resistive currents flowing over and through insulations, pins, cross arms, braces, nuts and bolts, etc between phases. If this current is constant, no interference is produced. However, interference is produced if within this leakage path there is a spot/area that has a high resistance but a low dielectric strength. Sparks will be produced across or through this area once the breakdown voltage is exceeded, towards the peak on each half wave.

Interference will be produced where there is any abrupt change in the current flowing. The harmonics of the basic 100Hz mains interference (every half cycle) can extend to hundreds of megahertz in many cases.

Generally, the majority of interference occurs where wooden high voltage transmission poles are used. As the wood dries out it shrinks and the hardware becomes loose, which often gives a discontinuous leakage path between phases. Instances like the high insulation resistance but weak dielectric strength areas as mentioned in the previous paragraph occur. These sparks are often inside the cross arms and initially when dew or rain falls, the conductivity on the outside of the pole and its hardware structures increases, resulting in increased interference as the area with the weak dielectric has a greater amount of sparking across it. This will continue until the area with the weak dielectric is completely wet and then all the discontinuities that cause the interference will be removed.

It is usually found that the interference increases as the dew settles then suddenly ceases late in the evening once these discontinuities are wet. To overcome these discontinuities the hardware on the high voltage section of the wooden pole is tightened and the problem usually disappears. The interference is more likely in the summer and autumn periods where the pole hardware is dry and has shrunk. With sustained rain, interference is rare except where cracked insulators or faulty high voltage surge diverters are evident.

With salt-encrusted HV hardware and insulators, the resistive current is greater than what it would be on inland installations. There can be a substantial loss of power in some locations due to this high leakage.

R. Champness,
Benalla, Vic.