Lightning: Newsletter Challenge (11/07/06)

A car keeps you safe from lightning due to multiple effects. Firstly, the car (except for monster trucks and SUV's) is probably lower than the top of your head, or any other shelter where you might take refuge (like a tall tree), therefore, it is less likely to be struck by lightning anyway. Secondly, the design of most modern cars being lower to the ground, with lots of glass, make it less of a target for lightning as well (unless you have a big antenna sticking up in the air!).

But the most important protection comes from the fact that you are inside the car and fairly well insulated by carpeting and seat padding from the ground or any good, low resistance path to ground. Any lightning that might strike the car will pass easily through the metallic body and chassis and thence to the axles, wheels, and TIRES on its way to the ground. It is not the rubber of the tires that provides the current path (contrary to the commonly held belief that the carbon black makes the rubber a good conductor), but the steel belts inside and impurities in the water outside (if raining) that provide a good low-resistance path to ground. The small, thin layor of rubber between is easily jumped by the high voltage.

A good example of this is how athletes who are struck by lightning often have melted rubber soles, where the current passed from their feet to ground, even though the rubber would normally NOT be a conductor. High voltage can do some amazing things!

Have you seen birds sitting on high voltage power lines! When Electricity can not conduct or pass through your body, you will be safe like birds! In addition, Faraday cage can also provide shield like protection from electromagnetism!

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Why did you reply to my posting? Are you agreeing or disagreeing with me? Did I say something wrong in your opinion?

What is the point you are trying to make that has not already been said by others?

I believe the man was pointing out two instances of the general concept, which is this: Electricity will not harm you if it cannot pass through you. In a steel vehicle, the electrical path is around the outside of the shell. Likewise, the birds on the wire are not part of a circuit and are therefore safe from the current surging past their little feet. I know of an insatnce where a fisherman crossed the coastal rail track on his way to his angling spot, and his rod touched the overhead conductor. He was fried as the current surged through his arm and body down to his feet. Yet a hiker was hit by lightning which struck his aluminium-framed backpack, and he was burned horribly from the hip down, which is where the frame touched his body. He survived.

Exactly, but he added nothing new to what had already been said, he simply restated the obvious. At least you added a few anecdotes which were somewhat interesting to read!

Chill out man. You need to drink less coffee.

The largest effect (that allows one to survive) is that the high voltage multi-frequency lightning will take the lowest impedance path to ground through the car body/skin, and over the wet/dry tires.

As electricity will take the shortest, easiest path to ground anything in that path will likely be struck also. By remaining in the vehicle and not touching any metal this can keep you from getting electrocuted. It has been documented that people who have had power lines fall on the equipment/vehicles they were in would get electrocuted by trying to leave the vehicle before the power was turned off.

At 10,000 volts per inch in air, it only takes a few thousand volts for the lightning to arc around the tires. The current flows around the occupant taking the path of least resistance through the metal.

The car is a faraday cage and the only gradient that can exist inside is the very small gradient of a low resistance path from one side to the other.

Let us say the car is struck. The voltage from where the bolt enters the car to where it exits will be a few tens of volts. Exception = corvette with fiberglass roof and side panels which may be somewhat less protective

YES! Not a perfect cage, but sufficient to keep occupants safe as long they do not touch any metal connected to the exterior of the vehicle.

If you're a sloppy person and somehow spill a liquid on the carpet of your car, the lightning may enter the antenna of your car, pass through the radio just as you were changing stations or adjusting the volume, and finding you with a path to ground via the puddle at your feet, the car body, the axle and wet tires.

Unlikely? Perhaps, but some people who were struck by lightning were not touching metal objects (such as the handset of a phone) when they were hit. The thin insulation provided by the plastic will not give you enough protection.

Lesson...don't drink while you drive .

Thank you for the correct answer, a Faraday cage. This one has always been a pet peeve of mind. When I was in college, a power line fell on a car and fried the person sitting on the outside of the car. Guess the tires did not help much there.

Your car makes an isolating cage, belief it is known as a ferraday cage.

The car.

Forget the lightning. More people are hit by falling or flying debris during a storm. Safer in the car.

Here in Colorado, somebody gets hit in the head by lightning every year - I'll stay in the car!

The fact that the car is a Faraday cage, providing a low impedence past for the electric discharge around you is correct. Interesting, the car itself can be several 10's of thousand volts from ground when struck, so if someone touches the car and ground at the same time they can be shocked. Being inside the car/cage where the voltage difference over the car/cage structure is low reduces chance of electric shock.

This assumes a normal car with metal roof. My ragtop convertible provides little protection because the I'm not inside a Faraday cage even through I have four tires insulating the car from the ground.

Lastly, I am astonished that ordinary people believe the tire provides insulation from the lightning bolt! Think about it, the bolt jumps several thousand feet, sometimes up to 5 miles between clouds and ground. But it can't jump the last 4-6 inch gap of rubber and air between the rim and ground provided by the tire?

lightning is electricity and it will allways take the path of least resistance hence the metal car body. it is the same reason that you would wear a wire mesh coveralls when working on powerlines (the best path is not you, it is around you). the tires being rubber are probably contamanated and would not offer a lot of insolation anyway.

The car body is most probably a "unit" meaning all the sheet metal and structural members are solidly welded together. Aside from the windows, any occupant in the car body, is encased in a sheet metal ball or cage. Such metal structures, even those like a barred wall in jail cells, form a "Faraday Cage".

What happens is when lightening strikes it in one place, the flow of electrons tries to flow through, but the corresponding magnetic field generated by the electron flow, generates a force keeping the flow on the outside of the sheet metal skin. This is also known as the "skin" effect.

The occupants then are never exposed to the flow of electrons, thus do not feel a shock or worse.

The actual better place to be is inside a jail cell, or better yet, a metal safe. Even if they are not grounded, the same effect takes place.

A metal shed, with loosely applied panels, may not work as the same force that acts on the electrons will also force the panels apart and can actually blow the metal panels away, exposing the occupants. The metal structure requires good structural integrity.

Lightning arrestor conductors are braided instead of simply twisted for this reason. If twisted stranded conductors are used, the magnetic force causes them to unwind and break apart, losing the conductive path and causing damage due to the plasma that ensues from the arc. The cable should not have sharp bends, but gradual ones. The step function of the current and resulting changing magnetic fields acts to make a sharp bend look like an open circuit to the flow of electrons.

Best Regards, Nick Abbatiello

You overhear a discussion at work where two friends are talking about the safest place to be in a lightning storm. One says to the other: "You're always safe in your car because the rubber tires insulate you from the ground." At this point you feel compelled to interrupt. You point out that yes, you are indeed safe in your car, but the reason has absolutely nothing to do with the tires. What keeps you safe from lightning when in car? answer: The car body acts as a Faraday cage.

metal shielding diverts any bolts to ground.

The safety inside a car or a bus or any other closed vechile is a result of Faraday cage effect. When lightning strikes the car the charge warps around the car body and will not penetrate it, thus provid9ing safety to the occupants of the vehicle. One can see a typical demnostration of this in science museums.

One has to be careful that it is not an open top car.

Raja Padmanabhan

You overhear a discussion at work where two friends are talking about the safest place to be in a lightning storm. One says to the other: "You're always safe in your car because the rubber tires insulate you from the ground." At this point you feel compelled to interrupt. You point out that yes, you are indeed safe in your car, but the reason has absolutely nothing to do with the tires. What keeps you safe from lightning when in your car?

The reason you are safe is due to the Faraday cage effect, the electrical charge stays on the outside of the conductor and therefore no electrical field is present within the conductor.

The main protection would arise from sitting inside something closely approximating a Faraday shield.

I guess you wouldn't be as safe in a Corvette(fiberglass) or a Saturn (plastic) as in a car with a sheet metal body. What about carbon fiber airplanes? Are they in danger?

A plane in mid air that is an insulator like carbon fiber or other non conductive stuff would enclose you in an insulated box. The lightning would not find an easier path from easily if that was true. If there were a number of antenna wires etc that compromised the integrity of the insulated box you may find a risk, but it would need a bassing strike to 'see' a slightly easier path and divert the main body to go through you and the plane. Leaders that feel the way for lightning would probably not select you as an optimal path. Few people glide or even fly small planes in thunderstorms.

Large planes get strikes from time to time

http://en.wikipedia.org/wiki/Air_safety

Yes in fact Nasa and many other manufactures have done many years research into the very problem of composite materails and lightning.. I believe they may have solved the problem by using speicalized paints and embeding conductive material into the composite material ... .and the answer to the question is the that the lightning would pass on the outside of the vechicle (faraday/skin effect ie electrons travel on the surface of a conductor) also modern tyres are conductive (very high resistance) but this allows the body to maintain a ground potenial without the use of the old earth strap you use to see on many cars. But there is no conclusive proof that you would survive a lightning strike in your car as each bolt from the heavens is completly different to other .... so was it 2 million volts or 20 million or 200 million that hit the car !

I agree. When volts of this magnitude are concerned the equation becomes a little more kinematic. Despite the miniscule mass of an electron when 10 to the 50 (give or take) approach your car at an alarming rate the impulse may be so great that it simply blows holes in anything in its way. This explains so some extent (I realise there is usually an amount of moisture either on or in) why trees are hit by lightning and 'explode'. Also consider the problem or arc welding at too higher amps; the metal is a very low resistance path to ground (or to complete the circuit) but you can still 'burn' holes in the metal very easily.

This has much more to do with Faraday shielding, no?

No, a flying plastic cage is not a faraday cage. A plastic/wood/fiberglass/carbon fiber plane/glider is not a faraday cage. It must be a conductor that is fairly continuous to the frequency involved. Cell phone work through car windoes, but AM radios have trouble unless in a strong signal area. FM are better, the short wavelengths gets through the 'mesh' of the windows.

The military do build a faraday cage into their plastic planes, but it is under the attennuation/deflecting layer and ground radars do not see this flying metal box. They do it to stop EMP and also to limit jamming from other sources as well as to keep their own radiation inside

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The military do build a faraday cage into their plastic planes, but it is under the attennuation/deflecting layer and ground radars do not see this flying metal box. They do it to stop EMP and also to limit jamming from other sources as well as to keep their own radiation inside

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Some how an EMP pulse has me thinking it would propable fry the planes electronics and you forget the yanks and russians signed a treaty banning high attiude nuclear test basically banning the use of EMP style attacks !

Remember, the signal fire that tells all that the war has started is lit with the tinder of the treaty...

And do you think that North Korea, China, India, Israel, Pakistan, Iran, or even France would be under the same restrictions? Even Russia could break their treaty if they felt it was in their national interest.

No, it still make sense to "harden" these strategic aircraft from EMP attack as well as other electronic "jamming".

Apparently the main protection is because with a Faraday cage the current passes round the occupants with a relatively low Voltage drop. For an imperfect cage such as a traditional car I would estimate this at about 20-V for 20-kAmps, and this should be of such small duration that even 200-Volts would not be dangerous. But perhaps there could be risks from insulated cables entering the car (antenna, accelerator cable?)

However, I have heard that adhesives are finding increasing use in the assembly of cars. How might this affect the safety? And how would adhesive bonds hold up following a lightning strike?

The metal body shell acts as a Faraday Cage. At high voltage like charge electrons repel each other pushing the charge to the outer surface of a conductive body and little or no charge is felt inside the cage. The higher the voltage the more the electrons push outwards.

It is a myth that tires are non-conductive. Carbon lamp black is added to the rubber during manufacture, this makes the tire conductive, albeit with a fairly high resistance. in a local incident. a row of hydro poles were knocked over in a storm and the high voltage cables landed on top of a car. Due to their conductivity the tires tires heated up so much that they caught fire.

The car actuates as a faraday's case, so the current always go through the shortest and less resistive way, in this case car plates.Because you are a more resistive and longer way for current, it wont circulate through you.

Alejandro P.

You can look up lightning on Wikipedia, and it'll tell you, you're safe due to the car acting as a faraday cage. No big suprise there. Of course, a car doesnt guarentee safety. If you have a pacemaker for example, the emp generated by the lightning can cause cardiac arrest. Also, a bolt of lightning can be as hot at 28,000 degrees C, and it has been well documented (I've even seen it firsthand) that lightning striking a tree can cause the moisture inside to turn into steam which blows the tree apart. The lightning itself might not kill you in the car, but I wouldnt be suprised that with the right type of lightning, you could get cooked pretty easy from the heat, or at least suffer some burns.

I see a lot of discussion here about the conductivity of tires. The conductivity of the tires is irrelevent. If they were perfect insulators (I know that a perfect insulator doesn't exist) consider for a moment that the lightening "bolt" has travelled thousands of feet from the clouds to the car in an effort to reach the ground (or the other way around) do you really think that a couple of extra inches is going to stop it? It will simply jump across the air gap outside of the tires.

As for the use of adhesives in automobile pannels instead of welding reducing the "faraday cage effect"; the non conductive joints would mearly look like a small capacitance to the lightning. Considering that lightning is essentially an impulse, it's frequency approaches infinity and the small capacitance would look like a dead short. A faraday cage does not need to be continuous to direct current, only continuous at the frequency of the voltage (or current if you prefer).

Why a faraday cage works at all, is best described in reply #16 by guest. The magnetic field created by the current forces the electrons to the outside of the conductor (this is why high tension power transmission lines are hollow). The entire car appears as a single conductor in the circuit so the current travels on the outside.

You are right about the tyres being irrelevant, but the comment about the capacitance of glued joints looking like a "dead" short" needs serious justification. Lightning simulation injectors for circuit testing typically inject ~10-mC in each of a closely spaced series of pulses - and I believe this only represents the current that would be induced in a single pin - not the effect of a direct lightning strike on the tip of a conductor. Even at these levels, however, it would take quite large areas of very thin adhesive to keep the Voltage to "trivial" levels.

Many replies have stated electricity (and lightning) takes the path of least resistance. This all-to-often heard statement is actually false. Please remember Kirchoff's Laws and Thevenin's Theorems. Where there exists more than one conducting path, the current will divide proportionally among the multiple paths where the most current, but not all, flowing on the path of least resistance. Current flows in all paths but the higher resistance of the other paths may result in currents too low to have any appreciable effect on whatever is conducting along those other paths.

This over-simplification often lead to people misunderstanding the use of grounding rods on electrical systems. The ground rod does not protect people from electrocution. The resistance of the earth is much too high to conduct much current. However, the green ground wire run with the circuit conductors (probably better called bonding wire) ensures (is supposed tto ensure) there is a low resistance path back to the service to ensure enough current flows to cause the overcurrent protective device to trip, assuming the wiring has been properly installed.

In example, people and animals have been shocked or electrocuted by wiring faults in manholes (through the metal cover) and street light poles. People have wondered why the manholes or poles do not have ground rods when rod serves to only disipate large surges such as from a lighting strike. What is really needed is a continuous bond wire back to the service point, where the bond wire is usually also tied to a ground rod.

What does this have to do with lightning striking a car? Not much. However, as mentioned the metal car body will serve to conduct the lightning strike to ground around the occupants much like a lightning protection system on a building, as long as the occupants do not also form a secondary conducting path.

Just in case your curious, Ground Fault Circuit Interrupters (GFCIs) do not need or use the "ground" wire for proper function.

A couple of things. First, lightning is probably lazier than the rest of us geeks. It will take the easiest path(s) to dissipate its static energy. When lightning has been plotted traveling over 30 miles, you have to wonder about this some but that just happened to be the easiest path at the moment. Lightning is more impulsive than my ex in Wal-Mart!!! The current rise time of lightning is quite fast; placing it in the VHF region of the radio spectrum. This high frequency also has its 'skin effect' when the electrons prefer the outside of the conductor. However, the best reason the car is a safer place to be is what kills is the difference in potential. The person inside a Faraday Cage or suit is at some potential in reference to some other potential. We usually reference ourselves to ground but sitting inside a car that might actually be hit by lightning would put us several tens of thousands of volts above ground. Even the dirt under the car would be many thousands of volts above the dirt a mile away as the spark dissipates. Being inside a structure with substantial low impedance conductive material surrounding you will keep the difference in potential voltage (and hence current flow) to a survivable level.

Unfortunately, the Faraday Cage of a jail cell is actually a more hazardous place to be. Since, on average, there are only 66 people killed by lightning and Texas alone has executed 24 this year, I'll take my chances standing on top of a barren hill flying a kite!!!

Several years ago, ~2000, a small car at the Tucson Electric Park in Tucson Arizona was hit by lightning. The car exploded, probably due the energy disipation as heat in the poor steel conductor... Luckily the passengers were watching the side winders game...

I don't think cars are safe...

Sonoranbill

well, bolt energy varies. A very large bolt can indeed deposit a lot of energy onto anything, like a car. If the arc went to a ground under the plastic gas tank(outside the faraday cage) you may well get an arc that could burnthough the tank and cause an explosion. In addition, unibody = thin sheet steel welded together = bad his resistance path and IxIxR can get to be a lot if I = 100,000 amps

Just saw the answer. Doesn' lightning start from the ground, making the car 13inches closer instead of 13 miles?

There is a great illustration of this (faraday cage) in the Deutsche Museum in München, Germany. The museum curator climbs into a wire mesh cage suspended from a cable from an overhead crane. The cage is raised to be between and in line with two 2 or 3 meter diameter globes about 5 meters apart connected to a Van De Graff generator. As the globes are charged they eventually reach the arcing voltage and discharge. The most dangerous part to the curator is the noisy discharge.

Jürgen Amtmann

You overhear a discussion at work where two friends are talking about the safest place to be in a lightning storm. One says to the other: "You're always safe in your car because the rubber tires insulate you from the ground." At this point you feel compelled to interrupt. You point out that yes, you are indeed safe in your car, but the reason has absolutely nothing to do with the tires. What keeps you safe from lightning when in your car?

The car's metal body acts as a Faraday Cage to prevent from lightning.

I'd like to know if lightning is dc or has ac components. If the current rise times are so small, does that mean a lightning strike can be considered to be a delta function, which has a flat frequency response? So would that mean all the radios and tvs in the vicinity of the strike, no matter what the frequency they are tuned in to, would get a brief noise signal?

Phil

The peak of each lightning pulse lasts in the order of (I think) microseconds. But the pulse creates a plasma, which radiates over a very broad spectrum (VLF to UV). This interference lasts until the plasma is discharged. As the plasma represents an "easy" discharge path once it is formed, there will be a sequence of strikes down the same path, so the interference can last for a significant fraction of a second; with most types of broadcast, this gives a characteristic crackle.

So there is indeed interference irrespective of radio frequency, but the reason is not primarily the shortness of the pulse.

You will hear lightning crackles with a radio receiving Amplitude Modulation signals, as on the AM broadcast band and Shortwave radio bands, however, NOT on the FM broadcast band which uses Frequency Modulation, unless the lightning is so close its energy overloads the radio and some of the circuitry forms a primitive detector circuit or even the speakers react to the varying electrical pulse, like when you hook a speaker to one battery terminal and tap the other speaker wire on and off of the other terminal.

You will also hear lightning crackle on the Citizen's Band (CB) radio frequencies which use AM also, but not on commercial or amateur ("Ham Radio") two-way radio frequencies which use FM. (OK, some Hams still use AM on certain bands and a popular mode, Single Sideband (SSB) AM is susceptible to lightning interference as well).

Broadcast Television using external antennas (not Cable or the newer digital TV broadcasting) is also affected, since the video portion is sent via AM and the audio via FM (in most countries except France). That is why the picture can get "messed up" during thunderstorms, but the audio is usually OK.

This is not what I've observed, and (to the best of my knowledge) there is no theoretical reason that it should be correct - unless you know different.

As the noise from a plasma source is essentially uncorrelated, half of the signal on either side of the FM carrier appears in relative antiphase. So it looks just like part of the FM signal. For a given noise level, however, the effect will be smaller - by the ratio of the peak frequency deviation to the acoustic frequency. The end result should be that moderate lightning interference produces a much quieter crackle on FM (for a signal with a 75-kHz peak excursion the difference in level will be about 25-dB at 4000-Hz), but the level increases with acoustic frequency. This is what you should hear if signal is strong and the lightning is distant. Naturally, where I live Murphy is dominant, so the FM signal levels are rather low and local lightning completely overwhelms the signal. The result varies from room to room, as it depends on the type of demodulator used in each wireless.

FM modulation is detected by a discriminator (classicly) and by a phase locked loop in a modern set. So unless the lightning creates changes in frequency it will not be detected. Lightning is essentially broad band shot noise whose contribution at a particular frequency varies inversely as the square of the frequency. The main bolt and precusrors are quite low in frequency, and we use AFMAG from global lightning storms to detect the return current, again a low frequency situation. That we can hear it at all at 1 Mhz AM tells us the fundamental bolt has a high intensity. This noise is a lot less on the 125 Mhz AM airport tower to air radio, but it is there.

You cannot hear it on a FM radio, save as a desensing of a weak frequency station or as high field rectification within the radio...which may also suffer a breakdown due to these induced fields if attached to an antenna out the window. Few people use antennas on FM, so this is rare.

A bolt can also destroy fridges, radios, you, etc if the house is hit, as there is a really high field.

Hi Aurizon

Acknowledging that the lightning signal will be weaker at 100-MHz, the fact remains that it is audible. Theoretically, the attenuation (relative to AM) would be due partly to the difference in levels (at 1-MHz the interference will include the bolt current, whereas at 100-MHz it will be largely the plasma noise unless the strike is quite close), and partly due to the modulation ratio (75-kHz peak modulation and 3.6-kHz re-weighted peak audio sensitivity will give a further 25-dB suppression). However, for stereo FM radios the added suppression due to FM is only about 3-dB, and what you hear will be affected by the muting arrangements. Although there can also be issues with imperfect filtering on discriminator sets, I now suspect that this is the reason I hear such different results on different sets.

Fyz

Plasma noise does not have an FM component. You can hear AM noise up to over 1000Mhz close to the source. It may go higher as you get closer. I doubt you can hear any true FM noise from plasma at all. With a high field you can get transients induced into the wires that are amplified and heard as sound, however they were not produced by the FM demodulation circuit and were not heard as FM.

Hi Aurizon

Plasma noise has a "white" spectrum, and emissions are effectively uncorrelated. When added to a carrier, 50% on the energy on either side of the carrier will be in phases (relative to the carrier) that correspond to amplitude modulation, 50% of the energy will be in phases that correspond to phase modulation. So half of any energy that lies within the spectral bandwidth of the receiving system will appear as phase noise. Obviously, the only difference between phase modulation and frequency modulation is a scaling factor, which gives the effective attenuations I referred to previously.

If you want a reference for this, conversion of a sinusoidal signal to frequency modulation is reasonably straightforward, and equations can be found in "Reference Data for Radio Engineers" (often referred to as the ITT handbook), for example. I don't have a copy to hand, but my recollection is that, in editions after the fourth, you will see that the sideband amplitudes are given by Bessel functions. Obviously, the general case and the reciprocal effects are not so readily susceptible to analysis - but it should be clear that, if signal in the phase domain transforms to sideband spectra, sideband spectra must conversely transform to signals.

If you want more information on how white noise transforms to noise in the demodulated signal, you will need to consult a specialised text on FM radio analysis. The first place to look would be the sections that deal with the effects of thermal noise, as the statistics are indistinguishable from plasma noise (for the milliseconds or so that the plasma persists).

Regards

Fyz

Well, I am not sure this would produce any output from a discriminator or a PLL, since the duration at any instant would not be sufficient for any system I know of to extract the (non existant) modulation from

The plasma lasts longer than the inverse of the information bandwidth that has to be conveyed; if the system doesn't respond in that time, it won't respond to legitimate signals either. This is as true for the AM response as for the FM.

Regarding the "non-existence" of the modulation, I think it would be appropriate if you would take the trouble to check your data. Modulation and decoding theory are well trodden areas - if you can't find a reference, maybe you can find a colleague or ex-colleague whom you respect to discuss the issues with - or maybe a HAM with the appropriate educational background? Or you might just try listening to stereo radio in a thunderstorm with the muting turned off. The crackle will not be quite identical to AM in the same frequency band - but it will be very similar, albeit at a slightly lower level.

Sorry if this comes over as a bit abrupt. I'm doing my best.

Fyz

P.S. I know the old saying that "when theory and practice agree they could both be wrong" - and it can be true if the theory is half-baked stuff that is forced to match sloppy measurements. But in this case, the theory has been checked over decades by using it as a predictor of performance.

I have indeed asked an experienced person. I will let you know what he says.

Some of the crackle will be wiring induced and not indicative of the detection of an FM signal

Unless your filters are all acoustic resonators (crystal filters or ceramics), wiring induced crackle via RF is likely to be far more of an issue with lower frequency signals - because of the size of the resonator coils. That also gives FM radios an additional advantage in practice - the small physical size of the boards, the practice of using ground planes, and the small value of the coupling capacitor between the antenna and the tuner all help to reduce the crackle that will be coupled via the RF demodulation. In a dual-band radio, the audio section will naturally see the same pulses regardless of input signal type. That would make automotive radios a good test-bed for the purpose, as they are relatively well screened from the lightning pulse; they also require good power supply isolation for other reasons. Living and travelling where I do, I hear much the same crackle on the car radio as I do with most of the radios in the house.

That said, I take your point regarding noise coupling via the wiring - now you point it out, it does seem likely that coupling via wiring and the power supply is the source of the very different performance of one of the radios (rather than the primitive mute control which is also a feature of this radio). You would probably recommend that I dispose of this beast - but it's an early 1960's FM-only valve radio, and the sound is superb when there's a decent signal.

Fyz

My friend says that it can indeed be detected as a high frequency swishing sound.

details

yes, amplitude variations are transformed to phase noise and phase mod is the derivitive of freq mod, so it will be heard but in actual sound generally as more of a higher frequency swishing sound.

Thanks. But I don't see how you can describe it as AM in the first instance. If the noise is sensed with respect to an uncorrelated carrier, it is always half and half. There is one difference in type when the noise exceeds the inband signal level: properly filtered FM and PM will not give outputs that significantly exceed the normal peak signal, but AM can in theory be essentially unlimited (though the better designs recognise the practical limits of the signal modulation, and suppress anything that would significantly exceed it).

I've never heard the continuous noise source described as a "swishing" sound before, but I think I know what you mean. However, you can sometimes also hear what I would describe as swishing, which is due to a time-variant white noise source; this would be heard at the same times as long-distance communications using short-wave radio.

True FM mono with continuous white noise wouldn't sound like either of these, however - it would be a much sharper sound, a bit like some of the early tape recorders. What you actually hear on a mono FM radio with continuous white noise is a noise spectrum that rises up to 2-kHz or 3-kHz, and then flattens out, so the audio noise is more like white noise with the bass reduced. The flattening out of the noise at high frequency is due to enhancement of the higher frequency audio at the transmitter, which starts at one of the frequencies mentioned. So, in reality, the modulation approximates FM at low frequency and PM at higher frequencies. With stereo, the spectral variation of the audio noise is much less pronounced, because the difference between the channels only occupies an octave, and the part to which the ear is most sensitive occupies about a third of an octave.

But you I doubt you would be aware of such subtle distinctions when the noise is induced by lightning, as the noise pulses only last a few milliseconds.

Regards

Fyz

I think he is referring to the fact that the lightning is AM, however since some of the terms in the expansion of an AM signal share some of the terms in an FM expansion this causes some response in the reception, even though modulation by periodic change in frequency is absent. These terms in the expansion will be higher in frqeency and be tine variant = swishing.

He is a consultant in receiver design, and speaks with an authority I lack. Possibly a direct dialog would help. replace at at with @

jramseyatrochester.rr.com,

Lightning consists of a sequence of current impulses, each of which is associated with an envelope of white noise. The filtered current pulses can be described as AM, but SFIK, the spectrum of that part of the signal falls dramatically above about 100-kHz unless you are very close to the source. Again SFIK, constant level white noise should not cause variable level swishing - that would need some other effect to come into play.

Unfortunately, direct communication with Dr Ramsay would mean more public exposure than is realistic for me at the moment.

Regards

Fyz

OK. He is not a PhD, but well versed in RF matters

Lightning AC or DC?...

Lightning is not ac or dc....It is Static

J.P.M.

the cars chassis work as a cage leading the current from the lihgtning around you...

the lightning jumps several km thruogh air.. i do not think 30cm of rubber would save your neck..

The reason is the car is a Faraday cage!

The same goes for an airplane!

Cheers,

Nuno