Ed (and all), This may stir up a hornet's nest but the answer to your question is yes, if a ground wire is run from the leg of the tower through the concrete pad to a separate ground rod several feet away, that can be a problem. But the answer is also no, IF you use the Ufer method of grounding everything. If the rebar is tightly double wrapped, or tack welded (if you have the time and ability) and adequately bonded to the rest of the ground system, you won't have any problems. If the installation is sloppy and not enough attention is given to the bonding of the rebar inside the base pier, the "spark gaps" will light up and the intense discharges will indeed cause damage -- perhaps a few cracks, perhaps blow the thing completely apart. But when the system is installed properly, the concrete becomes part of the ground system.
The following is a clip from http://www.scott-inc.com/html/ufer.htm: During World War II, a retired Vice President of Underwriters Laboratories, Herbert G. Ufer, developed it [the Ufer Grounding System] for the U.S. Army. Igloo shaped bomb storage vaults were being built, and possible static and lightning induced detonation problems were of concern. Ground conductivity was poor, and to be effective enough, ground rods would have to be driven several hundred feet. After much research and testing Mr. Ufer advised the Army to make connection to the steel bar that would internally reinforce the concrete foundation. He had determined that concrete was more conductive than all but the best soil, and that this improved semiconducting characteristic would enhance surface area contact with the surrounding soil. The wire ties normally used would be extra secure, and attention would be given to bonding or welding the lattice-type network together. The Army adopted the idea, and built the vaults as specified. After construction, ground resistance tests were made. No measurement exceeded five ohms. This value was considered extremely low for the local soil conductivity. Later tests confirmed stability. Mr. Ufer went on to develop the concept of concrete encased grounding electrodes. Many of his findings are detailed in IEEE Transactions paper # 63-1505. His system has since been used by the military, utility companies, Lake Tahoe lifts, and industry throughout the country. This site is only a brief introduction the concept. Google (or your favorite search engine) "UFER" and you'll come up with a whole lot more on it. I can almost guarantee that, if you read and understand about it, the next time you install a tower you'll want to do this. I installed an STL (Studio Transmitter Link) tower this way for a radio station I used to work for. I was the chief engineer for KGNC when the AM array had to be rebuilt and the STL tower got redone at the same time. I had never been too keen on "wasting" a section of tower by setting it into the ground. After all, that's what base plates were for. All to say, I learned that some tower installations are better served by a means for the tower to "move" and others not necessarily so. Short towers, a relative description I suppose, such as for most ham uses, are well served by the rigidity afforded by incorporating a section of the tower into the base support structure and some taut guy wires. This affords you the opportunity to bond the tower, rebar, AND several copper clad ground rods together for the best possible DC ground for your tower. If you want an RF ground return for your installation, install a heavy duty copper ring and bond it to the ground rods. Back to my STL tower; I never had another lightning or static discharge problem after putting up the 70' Rohn 25 tower this way. To address the original question posed I offer the same advice that Rohn used to publish in all their catalogs and literature. No tower is self-supporting at any height. (Unless it is manufactured to be a self-supporting tower.) Carefully consider how high you want to go, how much antenna will be catching the wind, and, can you sustain the loss if it was to occur? Thanks to those who stayed with me all the way through the explanation. I usually try to be more brief. (Hi Bob, you knew it was just a matter of time before I joined this list!) Best Regards, Steve White, W5SAW SW Commercial Electronics -----Original Message----- From: Ed Swynar [mailto:[EMAIL PROTECTED] Sent: Monday, June 12, 2006 6:14 AM To: Stevan A. White; 'Discussion of AM Radio'; [EMAIL PROTECTED] Subject: Re: [AMRadio] Tower Construction Hi Steve, By running the ground through the concrete itself, do you not risk the potential of the block being damaged, should your tower suffer from a direct lightning strike...? Or is this merely another "...urban legend" I've heard about tower installation, that has clung to the grey cells lo these many years...? Hi Hi ~73~ Eddy VE3CUI - VE3XZ ----- Original Message ----- From: "Stevan A. White" <[EMAIL PROTECTED]> To: "'Discussion of AM Radio'" <[email protected]>; <[EMAIL PROTECTED]> Sent: Sunday, June 11, 2006 11:32 PM Subject: RE: [AMRadio] Tower Construction > DO RUN THE GROUND THROUGH THE CONCRETE! Take a look at the > information on this site first though. You may be glad you did. > > http://www.scott-inc.com/html/ufer.htm > > Best Regards, > Steve White, W5SAW > SW Commercial Electronics > > > -----Original Message----- > From: Ed Swynar [mailto:[EMAIL PROTECTED] > Sent: Saturday, June 10, 2006 2:16 PM > To: Discussion of AM Radio; [EMAIL PROTECTED] > Subject: Re: [AMRadio] Tower Construction > > > Hi Dave, > > I have a 48' tall, tapered, self-supporting "Delhi"-brand tower --- 6 > sections at 8' long each. > > The prescribed / manufacturer's recommendation is to bolt a 3' > straight formed extention at the base of each leg (total of 3), & to > "suspend" these > (a temporary wooden "cradle" will do admirably, as the cement sets) in > a hole dug 4' square, & 4-1/2' deep --- the cement is to come but a > few inches > below the bottom legs of the actual tower section. > > Oh yes --- the bottom 1' of the square hole is to be "belled" outward > a foot, or so. > > The documentation says this is good for heights of up to 64', or > so...I've never gone beyond 48', & have never, EVER had an ounce of > trouble in the two > locations that I've had my tower up. > > BTW, the top of the tower as an old Cornell-Dubelier AR-44 rotator, & > a 3-element Hy-Gain TH3 MkIII triband yagi... > > Use "industrial"-grade coarse cement, & do NOT run any ground leads through > the block itself! > > ~73~ Eddy VE3CUI - VE3XZ > > > > ----- Original Message ----- > From: "david knepper" <[EMAIL PROTECTED]> > To: <[email protected]>; <[EMAIL PROTECTED]> > Sent: Saturday, June 10, 2006 12:44 PM > Subject: [AMRadio] Tower Construction > > > > I am thinking about buying a stick of Rohn 55G in cement with about > > 3 or 4 feet sticking out of the ground. I wonder how many sections > > of 55G I > could > > mount without guys. I do have a strong base plate for the 55G but > cementing > > a section seems to be sturdier. Any thoughts? > > > > Thanks > > > > Dave, W3ST > > Publisher of the Collins Journal > > Secretary to the Collins Radio Association www.collinsra.com - the > > CRA Website Now with PayPal > > CRA Nets: 3805 Khz every Monday at 8 PM EST > > and 14255 every Saturday at 12 Noon EST > > > > ______________________________________________________________ > > AMRadio mailing list > > Home: http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:[email protected] > > AMfone Website: http://www.amfone.net > > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb > > > > > > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb >
