Audioclinic (Q and A) (Nov. 1972)

Lightning Protection

Q. I have some unrelated questions concerning lightning protection for the antenna mast.

I live in a rural area, dotted with villages and towns. Throughout this area, servicemen, who make antenna installations, seem to have no interest in grounding the antenna system for lightning arrest. The general consensus seems to be that it is not necessary to do so.

They will perform the task upon insistence. I do not ask that you understand the psyche of the installers. I am curious, however, about the old tradition of grounding. Is it still timely and necessary? Is grounding the mast needed if only to avoid stealing “dB's" from the antenna? All lightning rods I have ever seen have been pointed at the top. Is the point functional in meeting the bolt?

-James D. Boltz, Fredonia, N.Y.

A. If you are located in an open area, you are definitely a candidate for a lightning stroke. Grounding, therefore, is the only protection you have. When the mast is mounted at ground level, all that is required is to sink the mast a few feet into the earth and the job is done. I would locate the antenna away from the house. Further, I would make provisions at the bottom of the mast to disconnect the lead-in. By disconnecting this lead-in cable at the approach of a thunderstorm you will add an additional safeguard in terms of preventing lightning from entering your house, possibly damaging your audio gear or the house itself.

When the antenna is mounted on the roof, however, you must make your ground connections to the mast, using heavy gauge wire. If you should receive a direct hit from a lightning bolt, the grounding won't be adequate. You had better try prayer. The instantaneous peak current involved is more than any reasonable wire gauge can withstand.

If there is some signal lose introduced by the proximity of the mast to the antenna, that same loss will occur whether the mast is grounded or not.

Remember that at the FM wave lengths, effective grounding cannot be established. There is so much inductance in the length of the mast, plus ground lines, that there is no real way to ground the system for r.f.

Lightning rods have points because it is well known that lightning is attracted to high, sharp objects. (This is why chimneys are struck more often than the rest of the house. If chimneys were a lot larger, the bolt would not jump to them.) Lightning rods, being of small diameter and higher than any other point on the structure to which they are mounted, will be a better attractant to lightning than any other portion of your home, including the chimney and the antenna system.

Lightning rods offer the best protection possible. In order to take maximum advantage of this fact, the rods must be installed by a professional in the field of lightning protection.

Signal-to-Noise Ratio

Q. Please define signal-to-noise ratio. I always thought the higher this ratio was, the better. Some companies claim that they have recorders with low signal-to-noise ratios, made even lower with the use of Dolby. Please clear this up for me.

- Preston Schaffer, Brooklyn, N.Y.

A. In addition to the desired signal information, any recording or reproducing system always contains a certain amount of background noise. The amount of desired information present, divided by the unwanted background noise yields the signal-to-noise ratio. By arithmetic, we can express this ratio in terms of dB. The greater the number of dB of signal-to-noise, the better is the device being considered, at least in this regard.

A manufacturer would strive for the greatest possible difference between the desired signal and the inherent background noise present in his device.

He would, therefore, probably not advertise a low signal-to-noise ratio; he might, however, advertise his extremely low background noise or his high signal-to-noise ratio.

The use of the Dolby system would decrease unwanted background noise as compared to the wanted program, thereby increasing the signal-to-noise ratio.

(Audio magazine, Nov. 1972)

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