Base Station Antenna Systems 2008 Product Selection Guide - AVW

Applications/Engineering NotesBase Station Antenna Materials and Mechanical CharacteristicsMaterialsThe selection of materials that will be used inan antenna or array breaks down to materialsfor the component parts: The radiating elements andsupport members Radomes Feed harness and connectors Hardware and mountingRadiating Elements andSupport MembersIn base station antennas, where size andweight must be considered, aluminum alloysthat combine high strength, low weight, goodresistance to corrosion, and good conductivityare a natural choice in metals.Pressure cast aluminum is very well suitedto certain parts such as bases, sockets,mounts and clamps. It has higher resistanceto corrosion than the high strength aluminumalloys while its hardness prevents metal“creep,” undesirable in clamps.Copper and brass also are frequently usedwhere size and weight are not factors. Principaladvantages are ease of plating and similarmetal contacts with the feed cable.RadomesRadomes are typically fabricated from highstrength, low RF loss materials such asfiberglass or ABS. Materials must be ultraviolet(UV) resistant to avoid deterioration after longexposure to sunlight.Mechanical FailureThis generally occurs when wind andice loads exceed the yield strength of thematerial or where metal fatigue occurs afterlong-term cycling back and forth of a memberdue to wind vibrational forces. The materialshape and size should be selected so that themaximum forces imposed on it—includingfatigue—will be less than the yield point ofthe material. Experience is the best guide forproper safety factor.CorrosionThis is an important consideration inmetallic members and where dissimilar metalsare brought into physical contact; care must begiven to the materials used in order to avoidsevere galvanic corrosion. Galvanic corrosionoccurs as a speed-up of corrosion wheremoisture is present between dissimilar metalscausing electrical current flow between themsimilar to a battery or electroplating action.Galvanic corrosion can be eliminated by theuse of similar materials or by passivating thematerials in contact by plating or chemicalconversion treatment (aladine or iridite). Wheredissimilar metals must be brought into contactunder stress conditions where the surface ofchemical conversion would be scratched orimpaired, the metals should be close to eachother in the galvanic series (see Table of GalvanicSeries) so that galvanic action is very slow.For example, copper or brass lugs shouldbe zinc plated (not silver) for connection toaluminum, and steel clamps or mounts toaluminum should be hot dip galvanized (moltenzinc). Copper or brass should never be placedin contact with aluminum without passivating orplating the metal surfaces in contact.Table of Galvanic SeriesRelative Position of Metals and PlatingsCommonly Used(1) Magnesium(2) Zinc(3) Aluminum(4) Aluminum Alloys(5) Cadmium(6) Steel or Iron(7) Stainless Steel (active)(8) Lead-Tin Solders(9) Lead(10) Tin(11) Nickel (active)(12) Brass(13) Copper(14) Monel(15) Silver(16) Gold(17) PlatinumNote: Low number is anode and high numbercathode. Metal flows from low number to highnumber in galvanic action. Water accumulationin hollow members can be avoided withdrainage holes near the low point. Not onlydoes this reduce corrosion but it protectsagainst freeze bursting in cold weather.InsulatorsThese include radiator support insulators,insulated element spacers or insulatedstiffeners. In general, insulators should beavoided wherever possible since they aresubject to breakage or damage and candeteriorate performance. At the higherfrequencies, they can introduce dielectriccapacitance that produces higher antennaVSWR. Desirable qualities in such insulatorsare: low dielectric constant, low power factor(low loss) at the operating frequencies, lowwater absorption, ability to operate well withinthe temperature range without130Andrew Wireless Solutions www.andrew.com