Home |
Search |
Today's Posts |
#22
|
|||
|
|||
David Robbins wrote:
"Andy Cowley" wrote in message ... CW wrote: Effective lightning protection can be done in the amatuer station for a reasonable cost. Most though, don't do so. "Andy Cowley" wrote in message ... As I understand it, there is nothing that can work if a direct lightening strike occurs. How? How do you deal with thousands of amps? It's for certain sure that a simple spark gap will be blown to kingdom come in the first millisecond, so what happens in the next millisecond? and the one after........... I think your method must be untried, untested and 'whistling in the dark'. Andy, M1EBV lightning doesn't go on for milliseconds, 50 micro-seconds is a relatively long stroke. 30kA can go through a 12ga copper wire with no damage for 10-20 microseconds. in most cases there will actually be very little voltage between wires of a coax or twin lead just because their insulation will break down or the feedpoint of the antenna will arc over... both are naturally occuring spark gaps that actually work very well to protect equipment from direct strikes. assuming of course the tower and feedline have good grounds. where people have problems is they don't ground the shield of the coax to a single point ground along with the power lines, so they get differential voltages between grounds that has no place to go but through the equipment. properly grounded installations with relatively small arresters to limit voltage on the center conductor of the coax relative to the shield are very effective. for tube type receivers a simple spark gap is adequate, for transistorized stuff you may need lower voltage protection and should probably get something commercially made for the job. I had not realised that the current was so 'spikey' I was aware that the duration of a stroke, including restrikes, was of the order of a second for large strikes. I found this info: 0.2 MA for 200 uS, then ~10 kA for another 200 uS, then 300-500 A for ~0.75 seconds. This followed by an average of 3 to 4 (max 26) restrikes at 0.1 MA (possibly decaying to 25 kA) for 200 uS for a big stroke. There is a MIL standard for this - MIL-STD-464. If your Coax breaks down then you have a PD in the tens of kV range for common coax and arc over in solid dielectric is permanently damaging. Sorry if I mislead anyone but we don't really suffer badly from lightning in G-land. Here are some interesting links. http://www.weighing-systems.com/Tech...Lightning1.pdf http://www.lightningsafety.com/nlsi_lhm/NFP_780.html http://www.mil-std-464.com/ http://www.kolacki.com/MIL-STD-464.htm I'm a little wiser now. Thanks vy 73 Andy, M1EBV |
#23
|
|||
|
|||
Mark Keith wrote:
"There is plenty the average ham can do to reduce damage." True, and the ham needs a good ground anyway. Most commercial radio installations operate 24-7 and are nearly unaffected by lightning. Protection comes from common-sense lay out and usually does not include many expensive arresters. One arrester salesman said his business was exemplified by the story of a bar patron who had a pipe on a lanyard about his neck. Bartender asked about the thing pending from his neck. Client said it was an elephant whistle. Bar tender asked why? as no elephants were to be found in the environs. Bar patron says: See, it works doesn`t it? Best regards, Richard Harrison, KB5WZI |
#24
|
|||
|
|||
David Robbins wrote:
SNIP lightning doesn't go on for milliseconds, SNIP. More specific information. The continuing portion of a lightning stroke, values up to 600 amperes, can go on for almost 300 milliseconds. Reference, WS-118-41129, Paragraph 3.9.XX [XX= I forgot the sub paragraph], Lightning [USAF system specification for the WS-118 Missile]. There is a USAF model, released in 1982, that encompasses 90% of all USA lightning strokes. It is as follows: Peak stroke; 0 to 100,000 amperes in 1 microsecond. Peak decay; 100,000 amperes to 25,000 amperes in 25 microseconds First tail; 25,000 amperes decaying to 600 amperes in 1 millisecond Continuing current; 600 amperes constant for 300 milliseconds. There may be up to 6 continuing strikes with amplitudes at 1/4 to 1/2 of the above. 50 micro-seconds is a relatively long stroke. 30kA can go through a 12ga copper wire with no damage for 10-20 microseconds. in most cases there will actually be very little voltage between wires of a coax or twin lead just because their insulation will break down or the feedpoint of the antenna will arc over SNIP: Arcs can sustain 100s of volts in the dynamics of the arc [plasma or carbonized material]. .... both are naturally occuring spark gaps that actually work very well to protect equipment from direct strikes. assuming of course the tower and feedline have good grounds. where people have problems is they don't ground the shield of the coax to a single point ground along with the power lines, so they get differential voltages between grounds that has no place to go but through the equipment. SNIP: Great advice! properly grounded installations with relatively small arresters to limit voltage on the center conductor of the coax relative to the shield are very effective. for tube type receivers a simple spark gap is adequate, for transistorized stuff you may need lower voltage protection and should probably get something commercially made for the job. SNIP: for a high power solid state station, 1500 watts, the matched RMS voltage is 274 volts, the maximum peak to peak is 274*2.828 = 774 volts p-p. Any surge device must accommodate the high RMS voltage and yet the receiver/transceiver front end must tolerate 774 volts p-p without damage. My solution, disconnect the antennas, radio power lines, etc. Deacon Dave, W1MCE |
#25
|
|||
|
|||
Richard, the local radio station has a line to ground with a large gap
which regularly arcs because of static build up. Most radio stations go off the air momentarily when lightning strikes. Is it the static arc that drives a disconnect relay for a few milli seconds and is this used as a primary protector for lightning? Regards Art (Richard Harrison) wrote in message ... Mark Keith wrote: "There is plenty the average ham can do to reduce damage." True, and the ham needs a good ground anyway. Most commercial radio installations operate 24-7 and are nearly unaffected by lightning. Protection comes from common-sense lay out and usually does not include many expensive arresters. One arrester salesman said his business was exemplified by the story of a bar patron who had a pipe on a lanyard about his neck. Bartender asked about the thing pending from his neck. Client said it was an elephant whistle. Bar tender asked why? as no elephants were to be found in the environs. Bar patron says: See, it works doesn`t it? Best regards, Richard Harrison, KB5WZI |
#26
|
|||
|
|||
Art, Kb9MZ wrote:
"---the local radio station has a line to ground with a large gap which regularly arcs because of static build up. Most stations go off the air momentarily when lightning strikes.' AM broadcasters use unbalanced vertical radiators driven against a ground radial system. The vertical radiator is nowdays the insulated tower irself. It sits on a base insulator, held erect by insulated guy wires. An arc-gap is fitted across the base insulator. This is either a pair of spheres or a pair of boomerang forms which are adjusted for a close spacing. Though galvanized, these gap fixtures get tower paint applications. Towers often get direct lightning hits. The paint remains pristene in all the gaps I`ve seen. The arc to ground is always to the Faraday shield between the tower coupling coils. That picket fence between the coils is pock marked like the face of the moon from tower strikes. Splattered copper abounds. You hear momentary disconnects during lightning strikes when listening to an AM station during this kind of storm. This is a defense mechanism. When lightning creates an arc, the conductive plasma path allows RF to continue feeding the ionization. This allows an arc to keep alive that the r-f is too feeble to strike for itself. Transmitter output into the plasma short circuit is an overload capable of transmitter damage. To counter the arc problem, the coax is d-c isolated with capacitors at the ends of the center conductor. The close-spaced coax usually gets an arc when the antenna system is overloaded. The coax has a high common-mode impedance. A relay d-c power supply and a d-c relay coil are connected in series and this series combination is connected between the center conductor and coax shield. An arc completes the d-c path for the relay coil. Relay activation is used to momentarily kill the transmitter, extinguishing the arc. Best regards, Richard Harrison, KB5WZI |
#27
|
|||
|
|||
"Andy Cowley" wrote Here are some interesting
links. http://www.weighing-systems.com/Tech...Lightning1.pdf http://www.lightningsafety.com/nlsi_lhm/NFP_780.html http://www.kolacki.com/MIL-STD-464.htm Excellent reading, thanks. And thanks to all who contributed. In all these informative discussions, the precautions seem to be centered only around towers. My HF antennas consist of 3 long wires and 1 dipole suspended from and between pine trees, all some 80' in the air. Of course disconnecting constantly in thunderstorm season works, but should the feedlines all be connected to a ground system outside at time of disconnect? Is grounding a dipole for instance just guaranteeing a fry job when there might have been only dielectric-puncture? The latter is certainly an easer repair. I would think grounding might help to disintegrate the Balun also - but you guys are clearly the experts so I look forward to your advice. As far as rooftop antennas go, I now plan a much better down conductor system than the rado shack aluminum ground wires that probably melt just a wee bit slower than solder ;-) Jack Virginia Beach |
#28
|
|||
|
|||
Sorry if this doubles, it didn't show up after almost an hour the 1st time:
"Andy Cowley" wrote Here are some interesting links. http://www.weighing-systems.com/Tech...Lightning1.pdf http://www.lightningsafety.com/nlsi_lhm/NFP_780.html http://www.kolacki.com/MIL-STD-464.htm Excellent reading, thanks. And thanks to all who contributed. In all these informative discussions, the precautions seem to be centered only around towers. My HF antennas consist of 3 long wires and 1 dipole suspended from and between pine trees, all some 80' in the air. Of course disconnecting constantly in thunderstorm season works, but should the feedlines all be connected to a ground system outside at time of disconnect? Is grounding a dipole for instance just guaranteeing a fry job when there might have been only dielectric-puncture? The latter is certainly an easer repair. I would think grounding might help to disintegrate the Balun also - but you guys are clearly the experts so I look forward to your advice. As far as rooftop antennas go, I now plan a much better down conductor system than the rado shack aluminum ground wires that probably melt just a wee bit slower than solder ;-) Jack Virginia Beach |
#29
|
|||
|
|||
On Wed, 17 Dec 2003 00:05:58 -0500, "Jack Painter"
wrote: "Andy Cowley" wrote Here are some interesting links. http://www.weighing-systems.com/Tech...Lightning1.pdf http://www.lightningsafety.com/nlsi_lhm/NFP_780.html http://www.kolacki.com/MIL-STD-464.htm Excellent reading, thanks. And thanks to all who contributed. In all these informative discussions, the precautions seem to be centered only around towers. My HF antennas consist of 3 long wires and 1 dipole suspended from and between pine trees, all some 80' in the air. Of course disconnecting constantly in thunderstorm season works, but should the feedlines all be connected to a ground system outside at time of disconnect? Is grounding a dipole for instance just guaranteeing a fry job when there might have been only dielectric-puncture? The latter is certainly an easer repair. I would think grounding might help to disintegrate the Balun also - but you guys are clearly the experts so I look forward to your advice. As far as rooftop antennas go, I now plan a much better down conductor system than the rado shack aluminum ground wires that probably melt just a wee bit slower than solder ;-) Although some have already addressed part of the issue with wire antennas, I'll try to elaborate a bit without repeating...and will probably fail...but... Two things about ungrounded wire antennas and ungrounded verticals. Static electricity (precipitation static) and lightening strikes (nearby and direct hits) BE it rain of snow and snow is particularly bad, just the precipitation and build many thousands of volts on an antenna. Some years back...welllll...actually quite a few (back in 1966) I was in the process of living in a mobile home while building a new home. I had a 40 meter quarter was vertical set up about 100 feet from the trailer. The station was used on the kitchen counter and stored in a broom closet. One evening as we st there watching television I heard a popping noise. It was pretty loud. A bit of searching showed the noise to be coming from the closet. When I opened the door I was greeted by a blue white flash accompanied by a loud "pop". The static was arcing across the PL259 with enough current that the arc was extending a good half to one inch out from the connector and it put any ignition I've ever used to shame. That includes some pretty strong magnetos. To top it off the thing was flashing every few seconds. Now this was one of those things where the choke across the coax connector would have bled off the charge big as it was. A lightening arrestor (spark gap) would have kept the voltage down, but most likely would not have protected a receiver input without a choke across the connection. Nearby lightening strikes do something similar for ungrounded antennas although they also induce a current in grounded ones as well. Now we are getting into the realm where the choke across the terminals may not be enough to protect the rig and I'm assuming the rig is properly grounded. That a protective device across the input will protect the rig is some what problematic. It just depends on the strength of the induced voltage and current. OTOH, some protection is better than no protection. Now as to direct hits to wire antennas and ungrounded verticals. There are precautions to take such as cable routing and grounding of the shield at the base of the antenna and prior to entering the house/ham shack, but again these come with no guarantee. *Generally* installations using a few wire antennas and ungrounded verticals are configured in such a way that the antennas can be disconnected. With these stations I would always disconnect and ground the coax. Then unplug the AC mains from the station. I would resort to one other step which is to ground any other cables and even rotor cables if any exist. The easiest way to ground a group of coax cables is to take three aluminum plates (or copper). clamp the plates together and drill them to take bulkhead connectors (the clamping only assures the connectors will properly align after the thing is assembled.) One plate is for the connectors going into the house, one is for the coax cable connectors from the antennas and the third is for the grounding. I should really make one of these up and shoot some photos to show how well they can work. At any rate, The plates can be configured several ways as long as it allows the user to unplug the cables from the antennas from the house and plug it into the grounding plate. The slip on PL-259 equivalents work very well for this. The same thing can be done one cable at a time, but I find that with a rapidly approaching storm I'd not want to be spending time disconnecting one cable at a time and then reconnecting said cables to a grounded set of connectors. I'd go so far as to attach a pair of rack panel handles to the one plate to allow for easy unplugging and reinserting it into the grounding receptacle. I's also ground the metal plate that holds the feed throughs into the house and install PolyPhasers for each line. it and the grounding receptacle both need to be thoroughly grounded and make no sudden or sharp turns in the cables. Another tract would be to have all cables enter the house through a well grounded panel using bulkhead connectors and PolyPhaser. By well grounded I don't mean tieing the plate to a single ground rod with a #8 wire, but rather bonding the plate to at least two 8' ground rods and connecting those to a grounding network. Even then in this kind of installation I'd consider disconnecting the cables and grounding them. BTW, ground rods can be easy to install if you don't have rocky soils, by using a hydraulic drill. (Another thing I need to write up and photograph). Actually I have the photos. I just need to write up how to make and use one. Maybe soon. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair?) www.rogerhalstead.com Return address modified due to dumb virus checkers Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair?) www.rogerhalstead.com Return address modified due to dumb virus checkers Jack Virginia Beach |
#30
|
|||
|
|||
Jack wrote:
"My HF antennas consist of 3 long wires and 1 dipole suspended from and between pine trees, all some 80 feet in the air." I worked for years in a shortwave broadcasting plant. One building contained 12 transmitters, 8ea. 50KW, and 4ea 100 KW, plus several lower powered transmitters. We had dozens of antennas which included several curtain antennas for each of 3 frequency ranges, low, medium, and high. The curtain array requires 4 towers for support. It has 4 dipoles in a radiating plane, all driven in-phase. It has 4 reflecting dipoles in a parallel plane directly behind the radiators. Height of the drive point of the array, its midpoint, was about 1-WL, or about 165 feet as I recall. That makes the tower height well over 300 ffeet at 6 MHz. For economy, the curtains for a particular frequency range are hung from a double row of towers which support curtains on both sides. The transmission lines from nearly all antennas, curtains, rhombics, or whatever, are brought into a cross-bar switching area so that any transmitter can be connected to any antenna. Transmission lines are all open-wire spaced at about 15 inches, or more, for a 600-ohm impedance if memory serves. Where the antenna line enters the switching area, boomerang arc gaps provide a flashover-point opportunity, line-to-line, and line to ground.This works. Open wires, like coax, have a high common-mode impedance. This tends to make a gap more conducive than the transmission line. Use large ground wires to keep inductance and resistance low. This keeps voltage drop low and handles the kiloamps for the short period of the surge. Best regards, Richard Harrison, KB5WZI |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Lightning Strokes, Masts & Volts | Antenna | |||
Antenna mast grounding question | Antenna | |||
Lightning protection question revisited | Antenna |