On May 12, 2010, at 12:22 AM, Paul Plack wrote: > > Guys, > > I'm considering the relative value of various coax protection schemes for a > planned UHF ham repeater. I know PolyPhaser is the industry standard, but I > also know they don't last forever, and can cause problems with spurious > emissions in some failure modes.
I can only speak from experience here... - Tower grounds done correctly - Building entrance "panel" made of copper - EVERYTHING entering the building goes through the panel and through a Polyphaser - Overhead "halo" ground for all equipment indoors, and/or if you must, copper strap on a concrete floor - Cabinets grounded to the "halo" or floor system - Everything in the cabinets grounded to the cabinets - Other brand protection on any Power-over-Ethernet stuff going up the tower, tied to the entrance panel - Everything has an isolator and filtering on it for when the Polyphaser goes "pop", so the PA's don't die - All antennas are DC-grounded NON-fiberglass, folded-dipole variety (Etc. Probably more.) After becoming a Repeater-Builder fan, over time I've learned that these best practices are common of a Motorola site building document, others have mentioned in the past. And I've since SEEN some of these Moto buildings and sites that were built in the 70's and 80's around here, and the only better grounding I've seen was in AT&T telco/microwave sites... both are top notch. If you can find one or the other to go look at, and maybe imagine what they looked like in their "prime" since many have been hacked up by newer (clueless) owners after they were sold on the open market... they're a great reference in most cases. (Just don't look at the crappy screw-in tower anchors at most of the Moto sites... those things are way more stupid and dangerous than a traditional "dead-man" or better... Moto often put up really beefy towers for their day and design, and then cheesed out on the guy wire anchors... big time.) We've taken NUMEROUS direct lightning strikes at multiple sites over the years and suffered little to no damage to any gear done "right" like this with the Polyphasers. We've had to replace blown/dead Polyphasers, but never had any spurrious behavior out of them (knock on wood) after they're blown. We know we've taken the strikes, because we've found arc welds on antennas, tower, etc. Including antennas that mysteriously stop radiating, but look okay for return-loss, that when you replace them... replace the Polyphaser... hmm, still sucks... go back to site, take spare antenna along, mount it in some awkward temporary location on the tower, hook it up with whatever crap feedline you have handy... voila, repeater sounds normal again! Crap... now order a new antenna, new feedline (unless you trust the old stuff and test it end-to-end), and replace. "The repeater's working great again!"... you might hear on-air if you're lucky. Maybe one e-mail of thanks to the tech crew/club out of 400 members. LOL! The VAST majority of damage has come from the AC input power to the sites. Power strips make good "sacrificial lambs" as well as MOV's on/across AC lines. One club in the area does a nice mod to GE MASTR II power supplies and puts the MOV's behind the little panel in the front where the AC plugs are. Ingenious. I need to find time to do that to ours. I have SEEN on the wall, but don't have any at our sites... AC input isolation transformers, supposedly to stop such silliness from coming indoors, but at all of our sites, we're on someone else's AC power distribution setup (other than a breaker panel we installed for ourselves at one site), and doing AC work they usually don't want the hassle of allowing us to pull permits, find a licensed electrician, etc... so we kinda live with "whatever comes out of the plug"... and deal with it after that point. It would be nice if sites would do more to protect the AC coming in. Many sites DO at least provide generator backup, something we might not have funded on our own, or been able to get permission to put a large propane or other style tank outside, in the woods... Damage since 1981: - Power strip explosion, gear was fine. - Ball lightning inside the cabinet jumped from AC power line to mounting screw in a 19" rack where a Kendecom was not properly grounded to the rack rails by nature of their desire to cover all the metal in blue paint. Repeater was okay other than a capacitor or two that had to be hunted down that popped in the audio circuitry and a couple more that failed years later, but paint was unceremoniously scraped off around all the screw mounts IMMEDIATELY, and no problems since. - Antennas internally destroyed (not-radiating) but all other gear fine. That's really about it. I think maybe "before my time" one controller had something go "pop" in it, but I don't have the details. It wasn't major damage that I know of. If there was more damage at any time, I'm unaware of it. This is impressive, considering that Colorado last I checked, was the 2nd highest lightning strike state in the country, behind Florida. So am I a believer in Polyphasers? Yep... but only as a part of a well-engineered overall grounding system plan for the whole site. The real key is like it's always been... give the high voltages of lightning plenty of ways to dissipate to ground that are low impedance (big flat multiple INCH wide copper strap) and make it all rise and fall together... tower grounded to site underground grounds (cad-welded to it, actually), site gound tied to building "halo" system and entrance panel, polyphasers at entrance panel to stop stuff coming down the center conductor from getting indoors, phone lines with gas-discharge devices (can be Polyphaser, but there's lots of options cheap there), MOV's on AC lines as "sacrificial" high spike devices with slower gas-discharge devices backing them up (both usually blown to smithereens, but they save the far more expensive items plugged into the AC), as much as possible everything in the cabinet running from DC power behind a big fat GE MASTR II power supply...) It all works together to create a lightning protection SYSTEM that usually works or at least sacrifices cheaply replaceable components, before allowing the voltages present in a lightning strike to "hit" the more expensive gear. One thing I would like, but haven't seen, is a gas-discharge device on the AC breaker/entrance panel. Problem there might be, if it popped, everyone/everything is down until someone gets there with a replacement. But the amount of damage we've taken from the AC input vs. down the tower, is easily a 5:1 or more ratio, so far. Many houses out here have the gas-discharge devices in little boxes on the side of their entrance panels in new neighborhoods/new developments. They're not required by code anywhere yet here that I'm aware of, but after my dad took a lightning strike to a tree in the yard that traveled into the consumer Dish receiver, and blew it up so badly it sounded like a rattle-toy, and blew every surge protector in the house, he had one installed as part of the rather large insurance claim that included having an electrician come and use a "megger" to make sure all the house wiring wasn't fried behind the drywall in the walls... I see no good coming of the shorted tee, or shorted cavity. Nothing you could short them with could withstand the voltages present in a direct strike. The Polyphaser is more "sacrificial" than it is "reusable protection" in a direct hit, but they're *relatively* cheap devices to replace. The stub would probably just explode, while some of the voltage present would just pass through the tee and kill the rest of your gear too... in a non-direct strike, it'd probably work, as do the Polyphasers, but a direct one... stuff gets vaporized. You want it to be cheap stuff, OUTSIDE the building if at all possible at the entrance panel. Just my $0.02... a Polyphaser by itself is a small, maybe useless gesture in a direct strike, but coupled to a large copper entrance panel, grounded with big flat wide copper strap... that's gonna stop most hits and blow things up there, not indoors on the antenna system's various connection points to your expensive gear. Nate WY0X
