Bluewave Antenna

ICING

Icing and Antenna Performance

Icing is one of the most serious problems for antenna installations. Ice build-up 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.

Some solutions

Over the years, some attempts have been made to protect antennas against ice build up by the use of various protective coverings.

A Polytetrafluoroethylene (PTFE) coating may be applied during the manufacturing process. While it does delay icing significantly, these coatings seldom prevent ice build-up altogether. However, in combination with a carbon-black additive (to reduce the effects of UV), the coating does tend to absorb any available radiant energy, which promotes melting much more quickly than a silver (reflective) surface.

The Bluewave antenna stands out, preventing ice build-up.

The best way to handle ice is to not let it form on the antenna in the first place.

A sure method of protection against ice build-up is to enclose the antenna in a radome. Fibreglass tubes or radomes have always been 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. Nonetheless, icing will still increase the wind load significantly.
Some manufacturers have resorted to placing a radome completely around a Yagi antenna. This eliminates the chance of signal degradation of the antenna, but introduces considerably greater cost into the overall antenna price.

Alternatively, a partial radome which covers some of the elements, particularly the driven element and first few directors, is an option. While this does prevent ice build-up on that part of the antenna, it may not completely prevent performance degradation from ice on unprotected elements. In either case, the addition of either a full or partial radome significantly increases the wind load of the antenna and may necessitate the use of larger, more robust masts or towers, thereby directly affecting overall costs.

In order to prevent damage from falling ice, particularly on high towers, ice shields are installed that are intended to deflect any pieces that may slough off during melting. As you all know, this does not always work.
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 build-up, the maximum gain may actually be higher off the rear of the antenna!

One way to decrease the affects of icing is to use larger (fatter) diameter elements. Fatter elements have a lower Q (high Q restricts antenna bandwidth) and the percentage change with ice build-up is decreased.

Another technique is to tune Yagi antennas slightly higher in frequency, thus increasing the cut-off 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 does begin to form, the cut-off frequency slowly decreases and the gain gracefully degrades instead of completely reversing direction.

A final suggestion 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.

Due to a combination of the broad banded design of the Bluewave yagi antenna, (at least 70 MHz in the ISM band), and the use of larger, solid elements (low Q), the operating characteristics of the antennas are demonstrably superior when compared against competitive models in similar conditions, and often are not noticeably affected by the effects of ice and hoarfrost. In areas that are regularly subjected to excessive icing during the year, Bluewave offers PTFE coating as an option.

ADAPTED FROM A PUBLISHED ARTICLE BY
Joe Reisert