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Reasons not to create a coiled-coax "choke"
http://www.comm-omni.com/polyweb/hamradio5.htm Even though inductive properties of the coax cable appear to be beneficial, and some extra inductance can be created by adding a few turns to the coax; don' t do it. The added turns can also act like an air wound transformer that can couple more energy into the line. This is just the opposite of the desired effect. Instead, make sure that coax lines leaving the tower remain at right angles to the magnetic field surrounding the tower. http://www.wrblock.com/Papers/Amatue.../APARS_P09.htm Neatness counts - cables (transmission lines, power (ac and dc), speaker, microphone, computer, control) should be cut to length and routed neatly and cleanly between boxes using the most direct practical route. The coiling of excess cable length on the protected side should be avoided since it could act as an air-wound transformer coupling magnetic energy from a nearby lightning strike back into the protected equipment. http://www.marcspages.co.uk/tech/2100.htm Some in the RF industry would have seen coils used as static drains. The theory is the coil is high impedance at RF and so looks open circuit, whilst still presenting a short to the DC and draining it to deck. The problem with them is they too can start reacting ('scuse the pun!) with the capacitance present on the system, especially at the lower ends of the band. 24hr ops FULL COVERAGE PROTECTION (no "chokes") http://users.erols.com/n3rr/lightningprotection/ http://www.alvarion.com/RunTime/Mate...arch31_R41.pdf ALVIRON SUBSCRIBER SYSTEMS TOWER LIGHTNING PROTECTION (no "chokes") And one major US communications company which drawings and specs are confidential and proprietary information - but they do NOT use any kind of coiled-coax and prohibit same from all systems. Here's one exception - from an Amplifier and relay company - they are NOT in the lightning protecton business and this is very outdated advice, but shown anyway becasue we're honest! http://www.ameritron.com/ameritron/man/pdf/RCS-4.pdf We strongly recommend the use of lightning retarding loops in the coaxial cables near the relay box (see illustration). Remember that lightning travels through the path of least resistance. Station ground leads should be solid, large surface area conductors. Do not use braided or stranded wire for the ground leads. Avoid sharp bends in the ground leads. Use multiple ground rods and/or radials to provide the earth termination. --- Recommend you ignore this and maintain direct paths - JP Finally, Richard Clark's mention of "code" is pretty important. Reference the NEC-70 and NFPA-780 for US installations. Jack Painter Virginia Beach VA http://members.cox.net/pc-usa/station/grounding.htm |
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