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August 10th 04, 11:17 PM
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"Jack Painter" wrote in message news:r4_Rc.12079$Yf6.1279@lakeread03...
"Harry Conover" wrote (Private) wrote in message . com... What I did note missing was the mention of 'lightning chokes' wound in the coax lines decending from the tower. These are basic to lightning protection for broadcasting towers, but I've rarely seen them implemented by hams. The idea of a lightning choke is to add a small amount of inductance to the coax so that if a direct lighting strike happens, the instantaneous current flowing though the outer jacked of the coax into your lightning arresters will at least have some amount of impedance limiting the current magnitude, thus reducing the probability of destruction of both the coax and the the arrester itself. These chokes are more often than not implemented by winding a dozen or more turns of coax around a form (say a 4" diameter phenolic tube) prior to the arrester or spark gap. Harry C. Harry, that winding of coax may be useful as an RF choke, but it is most certainly not a lightning choke, and will act more like an air-wound transformer than anything else. Not only is this not specified for any lightning protection systems, it is specifically warned against in many. Jack Painter Virginia Beach VA Jack, while I'm sure that you believe this, I really can't guess where you obtained such IMHO such massive misinformation. First of all, it is the purpose the the coiled coax to act as an inductor (r.f. choke) because this is how it resists the very rapid di/dt common mode characteristic associated with a lightning hit, thus limiting the peak discharge energy that the following spark-gaps and lightning suppessors must absorb and reducing the overall peak impulse power damage potential. It's basically a brute-force version of the common computer technique of placing a ferrite torroid around cables to attenuate their common mode impulse transmission/conduction ability. Realize that with sufficiently low SWR, currents through both the inner conductor and outer shield of a coax are equal, summing to zero, hence there is no net electromagnetic field produced. The same is not true for a common mode impulse traversing the transmission line. The bottom line is that only the common mode impulse resulting from a lightning strike will experience the results of the "choke". I'm not sure why you would believe that a coil of coax would act like a 'transformer' of any type, unless the SWR is truly enormous, a probem in itself. A case in point is that in both proton synchrotrons (typically operating in a swept frequency range of roughly 3-30 Mhz), and in large phased array radars (some of which operate in the UHF range) employ multiple feeder coax transmission lines that are cut the the same electrical length, with the cable lengths in excess of the required physical run length being coiled up somewhere in the system. There is no net significant electrical effect on the cable's transmission characterists in either case. Prior to completing my degree work at Drexel, I spent 8-years installing and maintaining broadcast transmission systems ranging from 5-Kw to 50-Kw. This included 5 years as chief engineer in one station (WBUD in Trenton, NJ) plus part-time work for WFIL, WCAU, and KYW in Philly. (Additional work in 4 or 5 smaller stations.) Quite honestly, I can't remember a single one of these that did not protect their very costly antenna installations, transmitters, and on-air reliability without lightning chokes employed in their transmission lines. Still, these broadcasters use equipment that, in general, nothing more sophisticated than your average ham station, but on steroids! True, the average AM broadcasting tower usually exceeds 200-feet, but in an intense lightning storm, a 40-foot ham tower is fully capable of experiencing the same energy lightning hit! I purchased my first ham receiver, an SX-71 from a ham named Bob Rutkowski (sp?) in Trenton, NJ. A day or so before I picked it up from him he took an evidently direct lightning hit on his 40-some foot crank up tower holding a 2M beam that was attached to his house. He had grounded the outer shield of the RG-8U coax at the based of the tower, but without a lightning choke in the coax, the hit simply vaporized the majority of his RG-8U, his grounding connection, and most of the final tank circuit in his rig in the basement! Not a pretty sight! Realize that a commercial radio broadcaster has to survive episodes like this without disruption of their operations. Hams don't. Still the emulation of the broadcaster's time proven protection techniques involves only a small additional cost to an otherwise excellent installation. For more information, see: Edmund LaPort, "Radio Antenna Engineering", McGraw-Hill Book Company, New York. (My issue carries a 1952 copyright, still it's an 'oldie but a goodie' with many subsequent editions -- and AFAIK is still the bible of the broadcasting industry.) IIRC, early editions of the ARRL Handbook also described this protection technique (likely pre-1970) in the days when most hams built their own rigs. Harry C. p.s., Jack, I'd love to hear a citation where "it is specifically warned against", and why. |
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