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Print PageThe Effects of Icing on UHF Yagi Antennas

By Joseph H. Reisert

Icing is one of the most serious problems for antenna installations. Ice buildup not only increases antenna wind load and weight but often detunes an antenna to the point where it's no longer usable.

When ice first forms on an antenna, it is usually wet and conductive. This is the most destructive condition for electrically detuning an antenna. Afterwards, ice build-up increases and eventually will freeze solid. Detuning may not be as severe after the ice dries but now the wind load has greatly increased and the antenna may be stressed to the breaking point.

When ice melts, it may do so in an asymmetrical fashion so one side of the antenna may be more affected than the other. Antennas can also be damaged by flying ice from other nearby antennas often found on a tower installation. This can often cause catastrophic failures since ice is heavy and large ice sheets often break loose with wind or melting.

The best way to handle ice is to not let it form on the antenna in the first place. Over the years many attempts have been made to protect against ice build up by the use of various antenna coatings. Hydrophobic agents are recommended for coating antennas but they are expensive and must be periodically reapplied. Other coatings such as teflon (RTM) or PVC type materials have been tried but while they may delay icing, they seldom prevent ice buildup altogether.

Another method of protection against ice buildup is to enclose the antenna in a radome. Fiberglass tubes or radomes are often used on vertical omni directional antennas and are quite effective. However, vertical omni antennas are not as vulnerable to detuning and hence are less affected by ice. However, icing will still increase windload significantly.

Yagi antennas have a special problem when coated with ice. When the director elements become fat (due to the ice), they electrically lengthen and start to perform like reflectors. In extreme cases of ice buildup, the maximum gain may be higher off the rear of the antenna!

Some manufacturers have resorted to placing a radome completely around a Yagi antenna or at least over some of the elements (e.g. the driven element and first few directors). While this may prevent ice buildup on that part of the antenna, it significantly increases cost and windload and may not completely prevent performance degradation from ice on unprotected elements.

One way to decrease the affects of icing is to use larger (fatter) diameter elements. Fatter elements have a lower Q and the percentage change with ice buildup is decreased. These are available in our heavy duty Yagi series.

Another technique is to tune Yagi antennas slightly higher in frequency thus increasing the cutoff frequency. If this technique is properly employed, the gain drop over the band of interest is insignificant, typically only tenths of a dB. However, when ice begins to form, the cutoff frequency slowly decreases and the gain gracefully degrades instead of completely reversing direction. We feel that this approach is more cost effective and apply this technique to all our Yagi antennas whether they are the regular or heavy duty type.

Still another possibility to decrease the effects of icing is to use a stack of antennas. This not only increases performance but when icing occurs, if it is not identical on both antennas, the performance may not be degraded completely.

Suffice it is to say that Astron Wireless Technologies, Inc. is working on many aspects of icing and has recommendations for your individual installation. Let us know your problem and how we can best help you.

Astron Wireless Technologies, Inc. and the author retain the rights to all intellectual property contained herein.
This information should be used as a guideline only to help you in the appropriate selection of an antenna.

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