|
Grounding Tower & Shack (Advice Needed)
Disconnecting coax, wire, etc. from the house with at least 10 ft spacing,
before storms arrive is a good idea in any case... |
-- Listen to Alternative News and Conversation You Won't Hear On Commercial Radio. Visit http://live365.com/stations/pascoradio YOU HAVE BEEN WARNED! First Time Users May Be asked To Do A 1 Time Setup. "Jack Painter" wrote in message news:r4_Rc.12079$Yf6.1279@lakeread03... "Harry Conover" wrote (Private) wrote in message . com... Hello, I am looking for some advice on if the ground system featured below is sufficent or should be upgraded. It consists of: - 3 ground rods 10' each around the tower (bonded together) - 2 ground plates (one outside, one in the shack, also bonded together) - lightning arrestors and/or feedthrough adapters - tower to mast ground - interior coax switch (not shown) I provided some pictures below: http://www.telusplanet.net/~homac/exteriorground01.JPG http://www.telusplanet.net/~homac/exteriorground02.JPG http://www.telusplanet.net/~homac/exteriorground02a.JPG http://www.telusplanet.net/~homac/exteriorground03.JPG http://www.telusplanet.net/~homac/exteriorground04.JPG http://www.telusplanet.net/~homac/interiorground01.JPG http://www.telusplanet.net/~homac/interiorground02.JPG I am looking for constructive feedback..... Thank-you.... Lloyd Hi Lloyd, Looks like a nice installation, although your grounding rods might be placed farther apart, or augmented by some heavy gauge radial wires (depending on your local ground condictivity). 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 Hello All. First let me comment that the lightning mitigation techniques used are better than many ham installations. That being said, I would increase the conductor size between the tower legs and the ground rods. When I saw these, I thought a "fast acting fuse." The amount of current that the tower can handle cannot be safely terminated to ground with smaller conductors. Also, the screw terminals need to be checked periodically because they will loosen themselve due to "cold flow." Why not repace them with crimp types and then solder the crimp with a torch as a back up? I am asuming that this device is covered by something to protect it from rain, etc? John-WA4JM, Dade City, FL, home to some of the most ferocious lightning activity in the western hemisphere. |
"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. |
Thank-you for the advice to this point. I think I will invest in the
rotator cable ground. Not sure what to do about the lightning choke. I don't mind replacing the antenna/mast if I receive a direct lightning hit, I just dont want to turn be fried to a crisp..... Lloyd (Harry Conover) wrote in message om... "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. |
"Private" wrote in message om... Hello, I am looking for some advice on if the ground system featured below is sufficent or should be upgraded. It consists of: - 3 ground rods 10' each around the tower (bonded together) Lightning likes to go straight. Try to have at least one ground rod connected to the tower base at the base; the rod should be directly under the base or as close to touching the base as possible. A 10' ground rod is good if in conductive earth. In sandy Florida, where I tood several hits, it took over a 20' length of ground rod (1/2" steel water pipe) to hit conductive "hard pan". I just washed it in until it hit some solid clay, and then washed it into the clay as far as I could. - 2 ground plates (one outside, one in the shack, also bonded together) - lightning arrestors and/or feedthrough adapters - tower to mast ground - interior coax switch (not shown) The coax switch should be a grounding switch. Floating elements on an antenna could actally attract a lightning hit. Also, for induced hits (not a direct strike, but with enough voltage to damage equipment due to a nearby strike), the grounding of the antenna lines gives the charge a nice safe path to follow. That's better than letting the current find its own path by arcing somewhere. I like to turn off the AC power to my entire equipment setup when not in use. So, with the power switch on the transceiver and the main "shack" ac power switches off, a lightning surge on the power line can only get me or the equipment when I am actually operating. One last suggestion: put a big (3 or 4' diameter) vertical loop (preferably near the ground) in all tower cables going into the house. (Right over your ground plate might be a good place.) The inductance of the loop, and the fact that lighting likes to ionize paths in a more or less straight lines, will keep the main current surge of a direct hit from entering your house and finding its "home" in your power line or telephone line. I survived operating for many years in the lightning belt of Florida and have the burn marks on my mast to prove it. The only known damage in Florida was some induced power that killed a couple of ICs connected to a printer ribbon cable. That was just from the current going down the tower on the outside side of the wall. Lightning protection is still as much art as science, Lloyd - but what you have done so far should fairly well protect folks in the house. If lightning is going to hit, just let it find a nice safe home - and try not to be operating when it does. HI HI In Florida, a house one block from us burned to the ground when lightning started a fire in their attic. That couldn't have happened at our house, as the lighting had a 70' tower to hit first, and a 23' ground rod to give the current a safe place to go. We were hit - several times. So, look at your well grounded tower as an asset for true lightning protection - not a liability. 73 ak |
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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 |
Thank-you so much for the feedback and information thus far.
Although I really appreciated the information, I do not want to get too hung up on the coiling of the coax. This weekend I plan to upgrade the gauge of the wire from #8 to #3. Also I have now bonded the cold water pipe entering in the basement floor to the exterior ground plate. Any other feedback on the pictures below are appreciated...... Lloyd - 3 ground rods 10' each around the tower (bonded together) - 2 ground plates (one outside, one in the shack, also bonded together) - lightning arrestors and/or feedthrough adapters - tower to mast ground - interior coax switch (not shown) I provided some pictures below: http://www.telusplanet.net/~homac/exteriorground01.JPG http://www.telusplanet.net/~homac/exteriorground02.JPG http://www.telusplanet.net/~homac/exteriorground02a.JPG http://www.telusplanet.net/~homac/exteriorground03.JPG http://www.telusplanet.net/~homac/exteriorground04.JPG http://www.telusplanet.net/~homac/interiorground01.JPG http://www.telusplanet.net/~homac/interiorground02.JPG "Jack Painter" wrote in message news:9WQSc.16277$Yf6.6584@lakeread03... 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 |
Richard Clark, KB7QHC wrote:
"(don`t fall for the monster under the bed stories of gazillion volts at a bajillion amps)." Good point! It`s akin to: "You can`t protect against a direct hit!" Oh yeah? How about 10,000 medium-wave broadcast stations struck by nearly every charged cloud passing overhead? Sometimes several times a minute for a long time period. The listener is often unaware of the instantaneous carrier drops to extinguish the arcs initiated by the lightning strikes. And, one of the most important lightning opponents is a large coil of large wire in each tower lighting wire at the base of the tower. It keeps lightning as well as R-F out of the electrical service to the station. If tower lighting chokes stepped up the lightning, they would all be replaced with Austin transformers or some other technique such as shunt feed of the radio towers to eliminate the base insulator. Truth is, lighting chokes are very effective at keeping lightning out of the power supply. Best regards, Richard Harrison, KB5WZI |
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