|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Thunderstorm, static and antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Theory Issues of safe operation of antennas and equipment connected to them during periods of thunderstorm activity were discussed from time to time in amateur radio literature. Nevertheless, when creating an amateur radio station, shortwave and ultrashortwave radio operators pay attention to these issues last, apparently hoping for the famous Russian "maybe it will carry over." But this is fundamentally wrong, because ... According to statistics, in Central Europe, there is an average of one to five lightning strikes per square kilometer per year. In other words, you can essentially be sure that a lightning strike will occur within 100m of your antenna once every few years (in the south and in mountainous areas, this probability is higher than in the north and on the plains). And if so, it will be much more reasonable to prepare for it in advance than to calculate losses later - in transistor transceivers, not only the input circuits of the receiver, but also the output transistors of the transmitter usually "fly out". What are the dangers for amateur equipment carries a thunderstorm? 1. Slowly accumulating static potential and its abrupt changes with discharges remote from the antenna (several hundred meters or more). If the antenna, or one half of it, is DC isolated from ground (eg, GP or symmetrical dipole), then high static potentials can accumulate on it before and during a thunderstorm. Let's consider such an example. At a height of two kilometers, a thundercloud with a potential of 2 MB (megavolts!) hangs, and the potential near the ground in this case is zero. This giant capacitor has a static electric field strength of 1 kV/m. That is, on an antenna isolated from the ground, for example, a dipole or LW, hanging at a height of 10 m, a static potential of about 10 kV will appear. As it flows down, it creates crackles and rustles in the receiver. When a cloud is discharged (to another cloud or to the ground far from the antenna under consideration), the potential of the cloud, and, consequently, of the antenna, will abruptly decrease almost to zero. A pulse with an amplitude of 10 kV formed on the antenna is more than enough to disable the transceiver. 2. If a lightning discharge to the ground occurs not far from your house (conditionally - a few tens of meters), then new dangers arise, associated not only with the antenna, but also with the power supply network and ground circuits. In addition to a sharp change in the field strength and the associated change in the potential of all nearby conductors, induced currents appear. The discharge current in the ionized lightning channel for the first 1...10 µs reaches values of 20...500 thousand amperes and then drops to zero in 200...1000 µs. These huge currents induce secondary voltages in all nearby wires. Something like a transformer is formed, where the primary winding is the lightning channel and the lightning rod, and the secondary winding is the surrounding wires. The transmission coefficient of this transformer, which depends on the distance to the wire, is, in principle, very small. But even with a transfer ratio of 0,001, current pulses in closed loops of surrounding wires (for example, a ground loop) can reach hundreds of amperes and damage devices connected to these loops. If the circuit is not closed and the gap between its ends is small, then the voltage induced in the circuit, reaching many tens of kilovolts, can break through it. An example is an all-metal gamma-matched wave channel mounted on a well-grounded mast and powered by a cable extending from the mast at an angle. In the radio station room, the cable is connected to a transceiver that does not have additional grounding. At first glance, it seems that it is not necessary - the mast is reliably grounded, the antenna is all-metal, good grounding is provided through the cable sheath. But... with a close lightning strike in an open "ground-mast-cable-transceiver" circuit, a voltage is induced, which will seek an outlet in the section of the circuit break - between the transceiver and the nearest "ground". As a result, either a ground fault will occur through the 220 V supply network, or an arc will occur to the nearest "ground" (for example, heating pipes). It is clear that neither one nor the other option promises anything good for the transceiver. 3. And, finally, the rarest, but also the most severe case is a direct lightning strike into the antenna or lightning rod-mast on which the antenna is installed. Let's start with the fact that there must be a lightning rod (that is, a path for the lightning current to the ground). In its absence, hundreds of thousands of amperes of discharge current will flow to earth along a path that seems to them the shortest. And if your drop cable and equipment meet on this path, then little will remain of them. Let's look at two examples. First example. The lightning rod is made as a separate structure and is connected with a thick wire to the common grounding of the house, the antenna is located much lower than the lightning rod. Let's see what happens when lightning strikes. Let's say the grounding resistance of the lightning rod is 2 ohms (this is a very good grounding). In the event of a lightning strike with a peak current of 200 thousand amperes (average value), a potential of about 400 kV will appear on the ground bus and on all devices connected to it (including the neutral wire of the network). Obviously, at a point far from home, the ground potential will remain zero, and all 400 kV will be applied to the neutral wire of the network, knocking out the fuses. This is the smallest loss in a direct lightning strike. Second example. On a free-standing and well-grounded mast with a ground resistance of 2 ohms, there is an all-metal wave channel. The drop cable runs along the mast and then over the ground to the radio station. The room has its own high-quality grounding. During a lightning strike with a peak current of 200 thousand amperes, the ground potential at the base of the mast will be 400 kV and will decrease away from the mast, forming the so-called "voltage funnel". The ground potential around the building will be less than at the base of the mast. Let's say it becomes 100 kV. And these 100 kV will do the same thing as described in the first example, but the matter will not be limited to this. The potential of the braid of the antenna cable will be 400 kV, and the potential of the earth in the radio station room is only 100 kV. A difference of 300 kV is applied to the cable. Its braid, due to its small cross section, will not be able to pass a large equalization current, and the cable will burn out. It will be lucky if everything is limited to this, if not, the transceiver will also be damaged. Even if the cable (as it should be during a thunderstorm) is completely disconnected, but lies not very far from grounded objects in the room, these 300 kV are able to pierce several tens of centimeters of air with an arc discharge. That is why all cables coming from the antenna must be completely disconnected during a thunderstorm and removed far enough away. It should be borne in mind that the protective zone of the lightning rod (in which you can not be afraid of a direct lightning strike) is a cone with a vertex at the end of the lightning rod and a radius near the ground of about 3/4 of the lightning rod height. How to prevent destruction? It should be clear that the three reasons outlined in the previous section are equally likely. Static potential is something that everyone will encounter many times. And not just during thunderstorms. The induced currents from a nearby lightning strike will also have to be experienced by almost everyone on average once every few years. Perhaps fate will save you from a direct lightning strike, but it’s better not to rely on chance, but to think in advance about such a possibility. It'll be cheaper! So, it is better to start the fight against static potential at the antenna design stage. It is almost always possible to choose a design that is completely closed to ground by direct current - loop dipoles on a grounded traverse, loop GP, antennas with gamma and omega matching, J-antenna, etc. If the antenna is not closed to ground, then noticeably improve the situation one (for an unbalanced antenna) and two (for a symmetrical) two-watt resistor of 100 kOhm, connected between the antenna sheet and the grounded mast (or coaxial cable braid). These resistors create a circuit for the removal of slowly accumulating static and significantly, up to several tens of volts (depending on the height and potential of the thundercloud), reduce voltage surges at the receiver input during discharges. But only for discharges, the path of which is significantly removed from the antenna. With strong static discharges, it makes sense to attach home-made arresters to the antenna sheets - M5-M8 bolts sharply sharpened at the ends. The tip of the bolts must fit 1...1,5 mm (adjustable by turning the bolts) to the ground plate. To prevent the occurrence of induced currents, earth busbars made in the form of a ring should be avoided, all devices should be connected in star form to one common ground. Carefully analyze your wire economy for the presence of closed circuits with a large area in it and eliminate them. The danger here is not so much for the closed circuit itself, but for the devices connected to it. Very significant voltages are induced in loop antennas, for the removal of which spark gaps should be installed at the power point, with the smallest possible gap (1 ... 2 mm) - the resistor is not enough here. If possible, it is better to lay the antenna reduction cable in a metal pipe or bury it in the ground. To protect against a direct lightning strike, two different tasks must be solved. The first is to make a high-quality lightning rod with good grounding. The lightning rod itself and its ground wire must be made of a material with a cross section of at least 50 mm2 and not have sharp bends. This increases the inductance, and for a pulse as short and high as lightning, even a small amount of inductance will present increased resistance. An extremely large voltage will be released on an inductive reactance of the order of a few ohms at currents measured in thousands of amperes. The second problem arises because, in practice, a rare radio amateur will not be tempted to use a lightning rod mast to place his antennas (in fact, when will there be lightning, and here the high mast is idle!). And this task is to ensure that the lightning discharge current mostly goes through the grounded mast and minimally through the cable supplying the antenna to the equipment, i.e. it is necessary to pave a path for the lightning current to the ground with much less resistance than through the cable. For this, it is highly desirable that the top of the mast be 1 ... 1,5 meters higher than the antenna. The mast can be extended with a piece of metal pipe or a thick rod (wire), which will divert most of the atmospheric electricity directly to the mast with its obligatory lightning protection grounding. The antenna itself must be properly grounded to the mast. If this cannot be done due to its design features, spark gaps should be installed. From the antenna power cable, make a few turns just below the antenna feed point. That part of the current that is still going to "fly" into the cable will meet the inductive resistance of the coaxial choke, which is considerable for a short pulse, and create a voltage drop across it. This voltage will break through the gap of the arresters, the resulting arc will create a leakage path for the current to earth through the mast with less obstruction than through the cable. The grounding of the mast must be connected by a separate wire of large cross section (at least 50 mm2) to the grounding of the house in order to equalize the ground potentials in case of a lightning strike. All of the above measures do not completely eliminate voltage surges on the equipment, but allow them to be reduced to acceptable, non-destructive values. Nevertheless, it is desirable to take additional protective measures in the equipment itself - it is desirable to install a resistor with a nominal value of 100 ... 200 kOhm at the receiver input. On the antenna connection connector there is a spark gap with a minimum ignition voltage (if only it does not work from the signal of its own transmitter). In the presence of a SU or LPF, made according to the P-loop scheme, this role is successfully performed by the output KPI with an air (minimum possible!) gap. T-shaped SU, standing at the output of most industrial transceivers in this situation, are unsuitable - the discharge spark "flies" through them, right to the transmitter output. In the circuits of wires (cables) for controlling gearboxes and switches coming from the antenna, it is necessary to install varistors, or better, arresters. And, finally, it should be remembered that when a thunderstorm approaches, it is necessary to completely disconnect all antenna cables from the equipment, and the latter from the network! Author: I.Goncharenko
Artificial leather for touch emulation
15.04.2024 Petgugu Global cat litter
15.04.2024 The attractiveness of caring men
14.04.2024
▪ GaAs-solar panels for roofs of electric vehicles ▪ External drive DashDrive Durable HD650 for extreme sports ▪ Cypress CYFB0072 4,8Gb/s video buffer chip ▪ Miracle laptop with 6,8 GHz processor 1 TB RAM
▪ site section Tone and volume controls. Article selection ▪ article Man-made disasters. Basics of safe life ▪ article How candles were used as clocks and alarm clocks? Detailed answer ▪ article Loading and unloading of viscous oil products. Standard instruction on labor protection ▪ Article Life with new energy. Encyclopedia of radio electronics and electrical engineering
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page