The AM Forum

   At a recent hamfest, I picked up a pile of PolyPhasers really cheap; some of them brand-new. I got curious when I noted that they all had a frequency range marked on them: some 125 to 1000 megahertz; others 50 to 700 megahertz, and still others 1.5 to 400 megahertz.
   I had always assumed that they utilized a gas discharge tube, GDT, from the center conductor to ground, but why, then, would different models be restricted to specific frequency ranges? The arrestors I'm using now use GDT's, and have no such restrictions.
   Seeing that I now had several of them, I figured I'd sacrifice one, an obviously used one, in the interest of learning.
   The one I chose to disassemble was a 125 to 1000 megahertz model. Removing two of the N-connector mounting screws from each end allowed the body to be separated into two halves. Inside was, as I figured, a GDT connected from the center conductor to the body, but also, as you can see in the picture, a ceramic chip capacitor (which measured 286 pf) in series. The GDT is soldered directly to the center pin on the antenna side, with its other side apparently press-fit into a milled recess in the aluminum body, while the chip capacitor is soldered to a thin metal strip, which in turn connects to the opposite N-connector, which is marked as the output side. The intent, apparently, is for an incoming surge to be shunted directly to ground through the GDT, with the DC blocking action of the capacitor protecting against any remaining DC component.
   Well, that answers my question about the frequency restrictions: the capacitor would have to be chosen to provide a low impedance at your operating frequency while still providing a high impedance to the lower frequency lightning pulse. I'm thinking that the capacitor value used in the 1.5 megahertz version would probably be somewhere around 0.1 microfarads rather than the 286 picofarads in this one.
   My other motivation behind autopsying this one was to find out whether the GDT can be replaced, since they degrade every time they're forced into conduction. My first thought was that the GDT used was the type with a threaded stud on each end; the lower end being screwed into a tapped hole in the body of the PolyPhaser, and the upper end being clipped off and soldered instead. That would have made it easy to replace: simply remove the solder from the upper end and then rotate the GDT capsule to back the other end out of the threaded hole. But, no, they didn't make it that simple; probably, I suspect, to force you to buy a new one. The lower end appears to be press-fit firmly into the recess in the body. Firmly enough, actually, that I fractured the capsule trying to pull it out. Looking at the picture, notice the smaller hole drilled into the perimeter of the larger one. I suspect that the idea was that the small amount of displaced metal from that smaller hole, intruding into the large hole, makes for a tighter and more secure press-fit. It looks as if there might have been a very slight trace of some sort of conductive adhesive, as well.
   Overall, the PolyPhaser looks like a well-engineered and manufactured product, but considering that they're not cheap, it would have been nice if they'd allowed for replacement of the GDT.

I bought an lighting arrestor from Morgan Systems which is similar in design to the Polyphaser but less expensive. Seems to be a quality product. In these units the GDT can be replaced easily if needed.

I went Nextech for my setup here in the virgin islands.

7 16 din out to antennas, N on input from shack.

7 of them.

Nextek PTRONF7AF99S Lightning Surge Arrestor.

4.5 kw carrier over 9kw pep rated.  Replaceable GDTs in them.

9 where not cheap.  Neither was the 2 x 3 stainless enclosure or solid copper sheeting to mount it all to.

I also have an F connector GDT as well as a few for the AIS, ADSB and APRS receivers.  External wifi antenna gets one as well.

The thing nobody concerns themselves with is the ground system.   You have to have a decent ground bond to shunt the energy to as well.  8 rods here, cad welded, and a ring of copper around the house, 4 foot out from the foundation.

You only build a station once.

--Shane
WP2ASS / ex KD6VXI

   I suppose the only real sure-fire way to protect your equipment is to disconnect it when not in use, and, of course, not connect it when lightning is expected. If your station gets belted with a huge direct strike, your shack may burn down around your gear, anyway, but at least it will survive up until that point.
   GDT's, probably, provide the most practical solution, but they aren't perfect. There's a very small latency, on the order of a few microseconds, before they conduct, since the gas inside needs time to ionize. A simple air spark-gap would have a similar latency, I'd imagine, since the air between the electrodes would also need time to ionize, but the performance would be less predictable, since humidity, temperature, and atmospheric pressure would affect the voltage at which the spark would jump.
   The problem with GDT's is that they can't be practically tested, since multiple discharge events will likely increase the voltage required to cause conduction. If they failed shorted, it would be obvious. My coaxial lightning arrestors use GDT's, but I have no idea at all how many times, if any, they've been driven to conduction. I do know that, before I was using them, very close lightning strikes would sometimes result in an audible arc across the disconnected coax connector. Yow! That's some voltage!
   The degree of surge protection you'd get from a GDT, I suppose, depends on how much power you're typically running. If you're running the typical 100 watt SSB plastic rig, the voltage across the antenna's 50 ohm load, assuming a perfect match, would be somewhere around 70 volts, so you could get away with a GDT rated to conduct at as low as 100 volts, although you'd probably want a bit more headroom. On the other hand, if you're a real AM tall ship, running 750 pre pee-e-pee-limit watts, you'd probably want to go with a 500 volt GDT, and the potential surge voltage delivered to your rig goes up accordingly.
   All my cabling coming into the shack is GDT protected, including the rotor cable, but really, I'm doing this not so much to protect the equipment, which is always disconnected when not in use, but rather to lessen the chance of big sparks flashing around looking for trouble in my shack, should the antennas or tower take a big hit.
   A current project is to install a "curtain ground" array of lightning rods running from the south side of the house, where the electrical service enters, to the north side, where the tower and most antenna cabling is grounded. Soil has a resistance, and a large nearby strike could result in huge potential differences between ground rods separated by some distance, in my case 35 feet. I've got multiple ground rods at the base of the tower, and multiple ground rods where the service enters, but in the event of a close strike, I can't count on them both to be equally "groundy."

A long time ago when I was a member of LARC in Lynchburg, VA, we had a member who started out working for GE/Ericsson. His name and call sign escape me now, but he became interested in lightning and its effects, to the point that he was tapped-out anytime one of their systems failed due to a lightning strike. He became so proficient in analyzing on the technical aspects that he taught a course at Mary Washington College about it. One meeting night he was our spot-light speaker and gave the 'condensed version' of his course to our club. The biggest take-away I remember was that you can put as many ground rods as you want, you can spend tens of thousands of dollars on lightning rods, you can put GDTs all over the place BUT there is always that granddaddy strike lurking out there that will render your efforts useless!
Now, he did say that would mitigate most of the strikes that occur but don't ever think you have 'Mother Nature' beat!

   Like seat belts and airbags, both excellent precautions, but in a head-on impact at highway speeds with a tractor-trailer, you probably shouldn't count on them to save you.
   People need to be aware of the limitations of the protection they're employing. I worked with a guy, many years ago, just a young kid, who came to work one day covered with red blotches and lumps around his face and neck. Seems that, over the weekend, he'd discovered a huge white-faced hornet nest in a shrub in his yard, and came up with a great idea: he sprayed himself all over with insect repellent, which he figured would keep the hornets off him, and proceeded to wade into the shrub and rip the nest apart with his bare hands. He was lucky they didn't kill him.
   Employ common sense protection, but know its limitations.

with regard to antennas and lightning protection especially when it comes to home owners insurance is to either have lightning protection system installed or inspected by a qualified specialist and they sign-off on it. If one wants to avoid that then simply hard ground outside and disconnect rather going the ham method no matter how good we amateurs think we are and risk a non-payout by insurance if ones home is burnt to the ground. Insurance is very picky these days and prefer not to pay out with the slightest excuse. keep the legalities in ones favor.

I learned my lesson about 40 years ago. Thankfully there was no damage by the event but at the time I figured I would simply disconnect the feedline to my 75m dipole for an impending T-storm and just lay it on the basement floor and not worry.  Well, imagine my surprise by a nearby lightning strike. The flash-bang outside was almost instantaneous so the strike was fairly close. I had just disconnected the feedline and walked away from it and heard a loud snap and saw this arc about 18" inches long hit the side of the boiler. Glad I wasn't holding it.  Today, I simply disconnect outside and ground outside when storms are in the future.  

Not radio related but I have been hit by lightning on 2 occasions indirectly. One incident landed me on the garage floor just after cutting the lawn. I was hot and sweaty and a storm was rolling in. And I like to watch turbulent weather. Not thinking, I was leaning up against the metal rail for the garage door and a nearby strike induced a current in the rail. It was hot jolt and I fell to the ground but got up shaking and closed the door. Another time, I was on the landline with my future wife during a storm of course and another nearby strike sent a jolt right into my chin. Came right out of the handset. What a surprise.  

I had to deal with white faced hornets and in ground yellow jackets about 10 feet apart from each other last week. I was surprised by the hornets when cutting the lawn last week. I didn't get stung thankfully but they were unhappy with the  lawn tractor.  The hornets nest was well hidden in a holly bush almost the size of a basketball.  The yellow jackets were a bit more docile but had to be eliminated because the XYL was weeding when they were discovered.

Back to the lightning and its effects. One thing that the guru at our club said was the easiest protection for home devices was to use the power cord to tie 3 knots in the line. There are people who will 'pooh-pooh' that idea but I can attest that it worked. We had a very close hit near our house. We had 2 TVs. One didn't have the knots since we received it from the MIL, that got wiped out. The other was an old Sony that my son was using for his video games. That had the cord blown at the first knot but the after replacing the cord it was ok. Now-a-days electronics I think are too sensitive and are a goner just at a strike of possible close proximity.

   At first glance, the knot thing might seem like snake oil, but on further thought, it sounds reasonable. The knot would appear as a reactance bump to the very fast rise-time pulse of a lightning surge, following the same reasoning used by lightning protection experts when they say to run your ground wires as straight as possible, without any sharp bends or loops.
   A ham friend of mine recently lost several pieces of home entertainment gear to a lightning surge. My electronics career involved military, industrial, and medical electronic equipment, so I have no inside track on home entertainment equipment manufacturing, but I think it's safe to assume that lightning surge protection is far from a priority in its design. With consumer electronics, where production numbers run into the tens of thousands, cost is the dictator. A GDT on the incoming AC line, and a transorb or two on the DC side, would surely save a lot of grief.

   Wow, scary stuff! There was a woman in my old neighborhood, a friend of my mother's, who was so terrified of lightning that she would hide in the closet during a storm. I will not laugh at anyone who fears lightning. It's like randomly aimed artillery. Your individual odds of being hit are small, but it has the potential (literally) to kill you.
   Never had any sparks flying around the house, here, but I witnessed a big bolt blowing a tree into shards right outside my workplace window a few years ago. It's something you don't forget.

   Yes, not directly radio related, but as hams, who are often up on ladders or poking around in trees, we're possibly more likely than most to encounter these things. As a bit of an amateur entomologist, I sort of admire the insect order of Hymenoptera, which includes a lot of completely harmless and benign families, but within the family tree of Hymenoptera lurks the more obnoxious Vespidae family, which includes the yellow jackets and white faced hornets. They may seem more docile in the earlier months of summer, but as autumn nears, and they have more invested in the nest, they become more and more belligerent.
   When I'm out mowing the lawn in early summer, I take advantage of the opportunity to observe, watching for insects repeatedly following a  regular path, and noting where they go, which often leads me to the nest. Better to catch it early than find out about it when there may well be hundreds or even thousands in there. Unknowingly blundering into a large nest could well be fatal, even if you're not allergic. I was fortunate to discover a white faced hornet nest early this summer, attached to my front porch light, and preempt it while it was only the size of a tennis ball. The only resident at that time was the queen, but she doesn't need to wait for a male to start the colony, she has the genetic material from the last male she had dealings with last summer all stored away within her, so once she's got the nest underway, she starts popping out fully fertilized eggs, and things progress rapidly from there. Fascinating critters, but they're not welcome on my property.

I haven't seen too many white-faced hornets in a while but we do have a lot of European ones around here that like to  strip the bark off of the lilac bushes.
Lightning I am very careful about it. In the summer months I play a lot of golf and many times find myself out on the course when storms erupt. I use the WeatherBug app on my cell to keep an eye on lightning and any storms lurking about. I can't do the '400 yard dash' like I could in the past. Lee Trevino once said, "When lightning is around, I grab my 1-iron. Even God can't hit a 1-iron!" Fast forward 3 weeks later after saying that, he got hit by lightning. Since then he takes heed!

Back in the 80s I worked for an audio video service shop as a service tech. I got called out to a home that was hit by lightning. It was a direct hit on the transformer which was on a pole but the utility ran underground to the house. Many things in the home were damaged except for the Sony TV but the most interesting thing was the energy unleashed on the service to their home. The ground looked like a big mole had just come through their front yard.  There was a hump a few inches high and about a foot or so wide.  I think if the wires were not underground things could have been much worse for the home owner.