Steve, This is a topic that has many proponents of certain brands or grounding methods. I liken it to the way some people (usually guys) talk about motor oil. You know the type: "Well, I never use anything but Castrol GTX in MY car!" The guys who say this have no special laboratory equipment or technical background to justify their position, but they crow about its benefits and offer the fact that their engines haven't blown up in 20,000 miles as "proof" of its superiority.
Okay, I know a little about lightning protection, having worked as an EMC engineer on the Space Shuttle program. NASA did a huge amount of research on lightning at Kennedy Space Center in Florida, sending up small rockets that trailed a thin copper wire that actually triggered a lightning strike. There are hundreds of test reports in the NASA archives that document how lightning affects electronic equipment on the ground as well as manned space vehicles. I read most of those reports, and had the honor of meeting Dr. Philip Krider, one of the world's experts on lightning. I know that Polyphaser is a popular name in "lightning arrestors" but I wondered why I never saw that brand being used at a cellular telephone site. The brand I did see at most cellular and commercial sites was Huber + Suhner. So, I bought one of each brand, both of them specified as a bulkhead mount for 150 MHz and 100 watts. The Polyphaser unit was about $70 and the Huber + Suhner unit was about $125 with the proper gas capsule. The physical differences were considerable: The H+S unit was waterproof while the P unit was not; the H+S unit was completely machined out of stainless steel while the P unit was made of two diecast aluminum pieces; and the H+S unit had an easily replaceable gas capsule while the P unit had a soldered-in-place capsule. The real difference was in RF performance: the H+S unit had no impedance bump when tested on a network analyzer while the P unit had a prominent bump. And I can see why- the P unit has an asymmetrical jumper inside the case. Since cellular telephone systems operate generally in either the 880 MHz band or the 1900 MHz band, any impedance discontinuity can have significant and detrimental effects. But, let's get back to the lightning protection issue. Most high-quality antennas used with repeaters are DC grounded, so it is likely that most of a lightning stroke's current can go straight down to Mother Earth, instead of taking a side trip into the repeater equipment. Aluminum dipole antennas are favored for mountaintop sites simply because the mast itself- which is a good conductor- projects above the active elements and acts like an air terminal. However, even if the bulk of the stroke current goes into the ground system, there will be a high voltage spike going through the feedline. Ideally, the lightning arrestor should trigger at a voltage just above the voltage caused by the transmitter. A 100 watt signal in a 50 ohm circuit will have a peak amplitude of about 100 volts, so we want the gas tube to conduct when the voltage reaches 125 volts or so. If the gas capsule doesn't trigger until 500 volts is reached, a lot of damage can occur. What is nice about the Huber + Suhner arrestors is that the gas capsule can be selected to closely match the power level of the transmitter, thus affording maximum protection to the equipment. Go here for more info about Huber + Suhner products: http://www.hubersuhner.com/products/hs-p-rf.htm One thing to consider is that, regardless of the brand you choose, when the gas capsule triggers, it places a dead short across the feedline. If your transmitter is keyed when this short occurs, the power will be reflected back to the PA and may cause damage. It is always a good idea to have an isolator at the PA output to handle this momentary power reversal. Finally, I completely agree with several other posters who mentioned that a lightning arrestor on the feedline is only a small part of a properly-engineered lightning protection system. The tower itself should have a very robust grounding system, which should be solidly bonded to the building electrical and communication system grounds. Both the electrical supply and the communication (telephone) circuits should have surge protection. One of the most authoritative references for the best practices in this regard is the Motorola R56 Manual, also known as "Standards and Guidelines for Communication Sites." The printed version, which comes in a nice binder, is 6881089E50 for about $85. The CD-ROM version is 9880384V83 for about $64. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Stephen Rice Sent: Thursday, March 02, 2006 6:51 AM To: [email protected] Subject: [Repeater-Builder] lightning arrestors Hi all, I had a question that I believe some of you can answer! I have a repeater with an antenna only about 20 feet off the ground here in Florida and am now looking for a budget lightning arrester. I have seen so many different ads I figured I would go to the horses mouth and ask you all if you have to do it on a budget what did you use? Did you mount the arrestor outside with an appropriate ground or inside? By the way the antenna is low because of the community regs. and it does cover the community for GMRS emergency work at that height. Thanks Steve N4YZA Yahoo! Groups Links Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
