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Richard,
Thank you for the exhaustive reply! [...] this works for RF but for lightning that is another matter (and at different frequencies simultaneously). LOL! Your need for ground when you are using a dipole is unwarranted in terms of RF. Yes and no. A "stand alone" antenna such as a dipole is not worked against a RF ground. (BTW, I remember seeing Hertz's original dipole in a museum in Wuerzburg... & I swear it wasn't grounded). But a ground is a good pulse sink if it is very permeable across the spectrum. In that sense, a pulse sink should be a decent RF ground too. Make sure the ends stand off from the supports by a good amount - I can only guess, and that would be 10 feet. Absolutely! Insofar as safety ground goes, it is hard to believe that is not supplied in your apartment - somewhere. Further, with regard to lightning, you are probably already living inside a faraday shield (the building's steel, skeletal structure), but make sure you ground the coax coming into the apartment at the window or where it penetrates a wall. To answer my earlier question, connect the coax shield to the building frame before it begins its descent to your apartment. I wish... In this part of the world steel structures are unheard of 'cept in industrial buildings. The tallest building in the country is only 30 floors, and its structure is reinforced concrete. Also, walls are always some form of brick & mortar ot at least gypsum. This explain why fire safety & property insurance are almost an afterthought here. We do have disasters, but very few in post 1930's high rises - mostly it's cheap built 19th century buildings collapsing after a gas explosion. Otherwise, one flat may burn out but others won't be damaged. Also, a power grounding is very effective for low frequency events. If you get zapped by a pulse, all bets are off if your ground is high impedance - and an end-fed thin ground wire is. In my case, the shortest route to the building ground connection on my floor is almost 100 ft with lots of zigzagging. Think of a Gamma or Delta match. Although these connect to the metal of the structure they drive, they are inductive loops. You could as easily build the loop and place it in close proximity to the structure to accomplish the same thing. This gives you isolation except for the frequency the loop is tuned to (and would limit you to a narrow band of operation unless you have some means to remotely tune it). I had never thought of that. So a broadband / omni antenna puts a bit more punch into the coax than a narrowband / mispointed beam. But I'd still rather put up a simple, forgiving, uncritical broadband, even if known inferior. Keeping leads apart and away from conductive structures replaces all the exotic insulation you could imagine. Besides, if we are talking about your T2FD and coax, I cannot imagine this is a problem in the first place. You are going to want to drive the coax shield to ground as soon as possible, not isolate it with insulation. You are going to want to isolate the T2FD ends, but any induced potentials will have to reckon with the built in resistor snubbing them. Such potential does not merit heroic insulation efforts. OK, I'm abandoning the heroic insulation efforts. I'll ground the coax up on top, and introduce an insulating transformer to decouple grounds only if double grounding introduces noise - that can be done later at the shack end of the coax. What is the spectrum of an EMP? It is mostly Pulsed DC with very little RF over 1MHz. The pulse shape is characteristically described as several µS rise and about 20 µS fall. If you consider the strike contains 100,000A and integrate this pulse over one second, then the duty cycle reduces this to 10 - 100 A. Now I know what to expect when I wile away a thunderstorm sucking an antenna plug. I NEVER saw baluns with windings atop each other To maintain a wide bandwidth and to preserve balance. Got it! I have repaired many such sets, professional and consumer alike. Yes, some included the NE-2. That practice was discarded just as many years ago too. There are probably a billion TVs out there, and not one of them with this kind of protection. Things change. Just as many more people now enjoy the other side, most TVs get their storm thrills through the power socket nowadays - especially if served by cable :0. Open a PSU, and you'll likely find MOV's. Not good for RF work because they tend to have too much capacitance.... And TV antennas tent to be 1) tiny 2) narrow beam, 3) tuned to UHF. No protection againt a direct hit, but non that sensitive to near hits. The market for gas discharge devices for many kinds of RF applications is still quite lively though, and it's not for protection against nearby transmitters. mine the archives of rec.radio.amateur.antenna for Richard Harrison, KB5WZI I'm grabbing my steel hat as I type! 73, thanks again. Filippo N1JPR/I2 |
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Richard Clark wrote in message news:
Returning to the issue of lightning, I would suggest you mine the archives of rec.radio.amateur.antenna for Richard Harrison, KB5WZI More silly musings: I read, among other things, that the advice to introduce a coax shield RF choke as lightning protection has been dropped, mostly on the grounds that a choke is an EMP pickup. Hmmmm... That applies to a helical wound inductor, not an isotropically near-deaf winding like a toroid. In turn, a single-layer toroid winding would offer a flashover point between the hot and cold ends of the winding. OTOH if the inductor is made - with a couple of windings on straight drainpipe - electrically in series - axes parallel - oppositely oriented - spaced enough to reduce mutual inductance (say, one yard or more), what you get is high insulation, no flashover point, limited pulse pickup (as long as the distance between the windings is that the distance between them and the putative pulse source). Another trick would be to make it a single shielded winding. (Insert warning about inductively shorted winding here -- ) Yet another would be the SPL, the satanist pentagonal layout. It's like a toroid, cept the winding is split in 5 parts, each in the middle of the 3-foot side of a pentagon. The two ends are kept well apart, of course. A weatherproof chandelier is to be lit in the middle as storms approach. Sounds like fun. If I find the inspiration I'll put in a phase canceling split-winding choke. |
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Climb to the top of the building (you HAVE done this, haven't you?)
The top of my condo became very popular after I taught local kids how to pick the locks many years ago, a long time befor I got my own keys. Problem was, they would not limit themselves to our own condo roof. Now the place looks like a WW1 front line, as other condos erected barbed wire everywhere to keep them out. Luckily they can't be seen from the street level... and LOOK at those antennas and feed lines already there. Do they have this protection you fantasize about? Do they treat anything as ground? Do their lines tie to it? The other ham has a mess of some 20 or so lines, including meteosat x 2, sat-tv x 3, HF wire, HF beam, HF vertical, 2m GP, 2m collinear, 2m yagi, 70cm 2 elements x 16 array, 23 cm dish, VHF discone, weather station, 70 cm moonbounce, plus controls for rotor and azimut mount for the dish. All bonded in several places, and to the lattice tower as well. Plus the guy has a ground from shack on down. My obvious wish is that any rogue moving charge will prefer his invitation to mine. molten [...] unobtainium AFAIK, properly grounded gear rarely melts, although you tend to get some burn marks where the plasma kissed the metal. I have climbed as far as about 5 feet above the rotor, i.e. some 30+ feet from the roof and about 140 ft above street level. Any higher would be unwise without a harness (I do not intend to emulate Col. Armstrong, esp. with no chick looking on, although I did drag a few up to the roof). No burn marks up to that height. The unobtainum ingots that lie neatly stacked on the roof look pristine. is there any evidence of a fine metal particulate mist deposited along side the building (it will look glittery like Disco Makeup for a drag queen)? We do get a lot of that, but it must come from other local sources. Probably drag queens. Ask yourself, what makes your antenna so much more attractive when the Eiffel Tower looms over it. Intrinsic beauty? hearses in the parking lot Right, thinking of my numerous pacemaker-equipped neighbors you must be right. They do not fall like flies in an electrical storm. People killed by lighting in this area tend to be caught by exploding trees in parks. In the last serious case (2 dead, I think + several hurt, about a years ago) one lady's arm was was temporarily paralyzed by the EMP she caught from an umbrella she was holding about 50 ft from the tree that got hit. Those closer to the hit were hurt mechanically, not electrically. It was really bad luck. It had just started raining, and it was the very first bolt, while they were all leaving the park for a nearby cafe' - they were not braving the storm right under the trees. building those pentangle chokes may if fact incur a vengeful bolt delivered directly to you. Plus, any "pentagon" could also get the local jihadis all worked up. Hmmm, should I get some local priest / imam to come over and bless the hell out of it? A friend of mine did that to all his motorbikes, and they did work more reliably than mine. Now that I think of it, some local catholic priest indeed comes over for a round of apartment-by-apartment blessing once a year, and I always opted out. There must some neighborhood aura of blessedness that got layered on year after year. Who knows, in this modern age that religious metaphor may be embellished to a nuclear cruise missile to fulfill the prophecy of EMP. (Ah! Images of Monty Python's Holy Hand Grenade come to mind.) Pass the Lord and praise the Ammunition! Let us know how it turns out. ;-) It may take a while, esp. as I have to reuse (steal) someone else's abandoned & forgotten flue line to bring the coax to my place. A former underground garage & repair shop was turned into an unheated undergroud parking facility. They just ripped out the furnace (which for many years had provided me with an excellent local ground, btw!, although they never noticed). Now I have to drill a hole in a little known structure to access the chimney up on top. A straight 110ft drop! I hope I won't hit asbestos, which is a definite possibility. When the wire is up, I'll post some pix, I promise! 73 & thanks N1JPR/I2 |
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