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Basics of Painting, Covering and Camouflaging a Passive Repeater.

Hundreds of passive repeaters exist on microwae communications systems in various countries throughout the world, and particularly across the width and breadth of the North American continent. Most of these passive repeaters are standard models consisting of galvanized steel for the basic support structure and aluminum-alloy panels for the reflecting surface. Problems occur with only a very small percentage of passive repeater applications due to the particulatr environment of the installation. Some cold-weather environments cause ice accumulation on the reflecting face with a resulting signal degradation not accounted for by system design. Also, some orientation or large passive repeaters used at the higher microwave frequencies cause problems by absorption of the sun's radiation. Occasionally there are concerns about the impact of a passive repeater on the environment. Anxiety about appearances can occur. The effects of their appearances on the environment usually are greatly exaggerated, and the proposed environmental problems seldom are weighed properly with total environmental effects, benefits, alternatives and costs.

One or more of the following questions are involved in proposed plans to paint and/or cover a passive repeater:

* Will the passive repeater appearances be acceptable in the planned location?

* Can light reflections from the aluminum surfaces cause undue attention to the passive repeater?

* Will the orientation of the passive likely allow absorption of the sun's radiation, in combination with resulting temperature differentials on the panels, to cause face distortion beyond accepted tolerances?

* Will large accumulations of ice likely occur repeatedly on the reflecting face of the passive repeater? Blend into Surroundings

A flat-finish painted surface can be used to produce uniformity of coloring and to avoid light reflections. (Anodizing of the aluminum panels is sometimes considered, but the costs usually outweight the advantages. There also are too many chances that the surfaces will get scratched before the installation is complete.) For appearances, it may be desirable to paint the entire unit a green or tan color to match trees, grass or the general terrain.

However, painting can caused problems by increasing the amount of the sun's radiation absorbed over that amount absorbed by the natural surface of the aluminum alloy.

If painting aggravates the heat-absorption problem, the standard de-icing covers can be applied to shield the panels from the direct radiation of the sum. The covers may be acquired in a variety of colors or painted.

When painting is desired to improve appearances, but shielding of the face skin is advisable, the following steps could be specified:

* Paint the aluminum face skin with a special white paint to reduce heat absorption.

* Paint the white covers a color that generally blends with the surroundings. (The covers could be shop or field-painted.)

When a passive repeater is oriented toward the southeast, the panel assembly may be frosted in near-sub-zero temperatures very early in the morning. By mid-morning, the face skin may be heated by the sun's radiation causing extreme temperature differentials between the front and back of the panel assembly. This may cause distortion of the face beyond accepted tolerances at high system frequencies (11 GHz), resulting in signal degradation ordinarily not considered in the system design. The distortion of the face may take the form of a gradual warping of the entire panel assembly, along with a moderate ripple about the individual face skin stiffeners. No permanent deformation ocurs.

It must be kept in mind that this type of problem is not ordinary. However, the condition can be aggravated by the addition of dark, absorbing colors used for camouflage.

The chart illustrates the range of passive repeater orientation that causes the greatest susceptibility to temperature-differential problems. Conclusions that passives are restricted to the orientation not indicated by the figure must be avoided. In general, large passive repeaters used for 11-GHz systems should be painted white when oriented in the direction shown by the figure.

The best solution to the problem of temperature differentials, caused by the sun's radiation, is to paint the face skin with a special white paint to minimize the effects of the sun's radiation. In fact, this should be regarded as the only practical and economical solution of this problem for most installations. This technique is used frequently for large earth-station antennas. These antenna would not operate within specifications without the application of the white paint. Possible adverse appearances can be greatly exaggerated. The importance of proper operation and benefits in configuring microwave paths with passive repeaters should be emphasized to those concerned with appearances.

Turning to covers to reduce the effects of icing, Microflect, for example, has developed de-icing kits for all of its passive repeater models. These are desinged for either a shop-installed addition or for field installation without alterations in the shop processes. The covers are made from 18-ounce type ATV plastic-coated nylon fabric, and are fitted with a PVC pipe frame to space the fabric out from the reflecting surface. Black covers are supplied unless other colors or painting of covers is specified. The black covers are standard on arctic models.

The fabric cover does not prevent the buildup of ice, but rather promotes the shedding of ice after a certain accumulation has occurred. The effectiveness of the covers depends upon the type and rapidity of the ice buildup, the action of the wind and the radiation absorbed from the sun. It is not possible to predict the results in advance. However, these covers have been used with success, and the chances are good that their use will prevent serious outages due to ice accumulation. If only half the passive repeater is clear of ice, the received signal level will be down by 6 dB, which is usually tolerable under the usual stable path conditions on winter. The installation of the covers can be expected to reduce the passive repeater gain by about 0.5 dB for 6 GHz, and by about 1.0 dB for 11 GHz.

Camouflaging with variance of colors may seem effective when viewed from a particular range and position, but the distant view generally only yields a vague silhouette of the panel assembly without distinction in coloring. Minimizing the removal of small brush and trees in the vicinity of the passive probably causes the best camouflage effect. Adding covering to the support steel or adding irregular shapes adjacent to the passive generally adds to the size of the silhouette viewed and causes the close-in view to be worsened in appearance.

When anxieties arise about possible objections to a passive repeater's appearance, the first step is to evaluate just how serious the matter may be. This statement may seem trivial, but consider how the description of the unit in wording alone may be the only problem or the beginning of the anxieties. Therefore, avoid the use of "billboard" reflector. Use "radio mirror" or radio passive repeater" or anything that more accurately describes its function without being likened to advertising billboards. Meet with those who may object to appearances and describe the appearance and function, along with the appearances and physical disruption of alternate methods of establishing the communications facilities.

If painting is judged necessary, select a single color of flat paint. Be cautious of the possible effects of the sun's radiation with the application of dark colors. Elaborate multi-color camouflage schemes seldom yield the results desired for the effort and cost involved.

When painting is required to reduce the effects of the sun's radiation, paint the passive a flat white. In general, specify shop painting rather than field painting, and contact the manufacturer before the final decision to paint is made, to discuss alternatives and extent of the painting.

Where objection is made to the appearance of the white paint, try to convince those objecting of its necessity for performance.

Where attenuation due to ice accumulation is expected, specify black deicing covers. Go for Lightest Color

When covers are needed to protect the surface from the sun's radiation, specify the aluminum surface painted white, and select the lighest-color cover available and acceptable for appearance. In some cases it may be desirable to specify a white cover that will be painted in the field.

In general, avoid camouflaging by attaching covering material to the supporting steel. The steel supporting the reflector is least visible in its standard galvanized seldom enhance aesthetics.

Landscaping with trees or brush that remain below the passive reflecting surface in the rear can minimize the awareness of the reflector to the casual eye. However, landscaping can be very costly and have dubious value with regard to the real environmental effect of the passive repeater.

In summary, passive repeaters offer a number of solutions for overcoming ecological concerns. Among their advantages are no noise or harmful radiation, land requirements are small and need not be level, no aircraft obstruction lighting is required, the aluminum face refects surrounding terrain for natural camouflaging helicopters; they allow a wider selection of sites during planning stages; razing of land for access roads and power lines is not required.
COPYRIGHT 1984 Nelson Publishing
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Author:Kreitzberg, J.
Publication:Communications News
Date:Jun 1, 1984
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