Questions about the frequency and power of electromagnetic waves.thankyou - Page 1
Forum section: electronics / rf-microwave

author: tianyi
author_rank: Newbie
timestamp: August 08, 2023, 02:28:34 am
content: Hi, I'm a new registered user,I came here to learn a little about electromagnetic waves.For example, the information sent out by mobile phone base stations, wifi signals, these are electromagnetic waves.I have seen a news before, users near the mobile phone base station will have dizziness and other symptoms, because the base station transmission power is too large.Here is another example, which I saw on a forum.One user built his own high-power wifi antenna.But he said that in front of the antenna, when there is wifi data sent, he gets dizzy. Maybe it's too much powerSo I was curious.1 Antenna transmission power is large, can really make people dizzy symptoms?2 How much power would cause a person to have this symptom?3 Which frequency is more likely to make people dizzy?thank youThe first question. I don't know if this is the right place.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 08, 2023, 02:44:32 am
content: Most of the talk about physiologic effects of em waves on the internet is just nonsense.High power microwaves such as those used in large radar systems can cause heating just like a microwave oven. Microwave transmissions from portable devices and consumer devices cause very small amounts of heating and certainly much less than from the electromagnetic radiation present in sunlight which includes plenty of microwaves.Very large low frequency magnetic fields such as those produced by MRI machines can cause dizziness, for instance if you move your head while the magnet is on, although people getting MRIs may feel dizzy for multiple reasons. But that is not something most people will reproduce in their home.Without careful blind AB testing I wouldn't even bother considering someone who said they made an RF transmitter that made them feel dizzy. Psychosomatic responses are definitely much stronger than any actual effect caused by the antenna.

author: TimFox
author_rank: Super Contributor
timestamp: August 08, 2023, 03:30:32 am
content: I believe that the nausea or dizziness encountered in high-field MRI scanners is a magneto-hydrodynamic effect on the (somewhat conductive) fluid in the inner ear with motion of the head inside the DC magnetic field, not the gradient fields (roughly audio frequency pulses) or RF fields (typically 64 MHz).Since the inner ear is the sensor for bodily equilibrium, an incorrect response to head motion can cause vertigo.There are documented cases of cataracts and other bodily injury from very intense microwave energy close to radar antennas hitting a victim's eyes, and there have been RF burns caused by malfunctioning components in MRI scanners.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 08, 2023, 04:30:02 am
content: There may not be a really good forum for this information. People here will be in general well versed on the physics of EM radiation, but rarely very knowledgeable about physiology. There are forums where the reverse is true. And most forums which discuss this general topic are populated by folks with little knowledge in either field.I am personally a sceptic about most of the claims of EM field effects and think that the vast majority of the reported symptoms are psychosomatic, but can't back that assertion with a completely solid proof. The reports of dizziness in the extremely strong magnetic fields of an MRI machine would be an exception to this.There are some things that you should think about as you evaluate this.1. Whatever may be happening will be heavily dependent on the frequency of the EM radiation and on the structures it is interacting with. No generalization can be made about all EM. Some simple examples to make it obvious that this is true. You can be badly sunburned by UV radiation from the sun, but relatively or completely unharmed if the UV is blocked. This is true even though the power in the UV wavelengths is significantly lower than the power at longer wavelengths. Another example, when you microwave food in a ceramic or glass dish the food gets hot, but the dish remains cool (except where it is in contact with the hot food). The wavelength of microwaves in ovens is chosen to interact strongly with water in food, but does not interact much with glass and ceramic. These types of differences occur all through the EM spectrum.2. Power matters. Legal WiFi devices radiate only a few milliwatts of power. Cell phones are in the neighborhood of a watt. Microwave ovens are a kilowatt.3. Correlation is not causation. There are some reports of higher cancer rates among cell phone users. But other reports indicating that this is either a data collection error, or a statistical fluke in the particular data set. Even assuming the data is correct, no one has demonstrated the mechanism by which such a cancer rate increase occurred. If real it could be the EM radiation (acoustic frequencies from the speaker or the RF used for communication or some other emission from the circuitry), acoustic, physical pressure from holding the phone against the head, thermal effects from the heat of the phone and the insulating effect of holding it against the ear or some other cause that no one has though of. Jumping to a conclusion about the actual mechanism makes no sense.4. Is this really something important to worry about? If any of the various forms of EM radiation that have been generated in our society over that last dozen or so decades had a strong effect it would be easily detected and documented. It is easy to fear something you do not understand, particularly something invisible, only indirectly sensible. But the people on this forum who do in general understand EM are generally either unconcerned or only mildly concerned. You might find it appropriate to trust their attitude rather that investing the time to have more than a superficial understand of EM. Other dangers ranging from chemical contamination, to automobiles, swimming pools, lightning, hurricanes and earthquakes are more likely to cause you harm, and if you are sufficiently concerned taking some appropriate action to mitigate your risk.

author: rf-fil
author_rank: Regular Contributor
timestamp: August 08, 2023, 07:36:27 am
content: Generally, claims about harm of any kind due to cell phone / wi-fi radio waves are garbage.QuoteAside from the special case of MRI, which uses spectacularly strong magnetic fields (not even much RF, I believe), the only other known possible harm from RF waves is injury due to heating. Same physics as in your microwave oven. This works from about 900MHz, onwards. Wi-Fi works on 2400MHz and 5500MHz, and if someone built an amplifier with a few watts of power and used a very directional antenna to focus this power onto themselves, then they would get burned.Every professional in the field knows this - you get trained about it. Also, every single piece of equipment that gets installed or sold to public gets assessed for this type of exposure by each national regulation agency. You can look up "SAR" regulations if you are curious. It's highly technical but it boils down to limiting RF exposure to crazy low RF levels, so there is no risk of getting burns. Every phone, cell tower, walkie-talkie must comply with these regulations. They are all safe, full stop.But yeah, if someone rigged a microwave oven to operate with the door open, and stood in front of it, they'd get cooked. One of the possible symptoms is cooking your eyes, aka cataract.I would think that the Venn diagram of people who are somehow smart enough to know how to make a very high power Wi-Fi power amplifier, but also stupid enough not to know the risk of RF exposure at these frequencies, is quite small. I suggest to google Darwin Awards for other examples of such individuals.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 08, 2023, 11:15:10 am
content: Here is a link to the ARRL RF exposure section.It has lots of links to US FCC rules and regulations concerning RF exposure:http://arrl.org/rf-exposure

author: duckduck
author_rank: Frequent Contributor
timestamp: August 09, 2023, 04:31:24 pm
content: Something else that it important to keep in mind is that received power decreases with the square of the distance from the radiating element (i.e. transmitting antenna). If you double your distance to the transmitter, you receive only 1/4 as much energy. If you triple your distance, you receive only 1/9 of the energy.What this means to you in practical terms is that unless it's an extremely powerful transmitter (75,000 Watt commercial radio station transmitter), or it's right near your body (cell phone, walkie-talkie), the amount of energy that's reaching you is very, very low.

author: TimFox
author_rank: Super Contributor
timestamp: August 09, 2023, 04:37:06 pm
content: That's true for radiated power, as in downbeam from a cell tower, radar, or broadcast transmitting antenna.With a cellphone or similar generator close to the body, you are in the near-field region which cannot be calculated directly from the radiated power.Cellphones and RF fields inside MRI machines are regulated by "SAR", the power absorbed from the energy stored in the oscillating field per mass of body tissue.A few years ago, a muckraking article in the Chicago Tribune sent several popular models of cellphone to a testing lab and found they exceeded the US regulations for SAR, which prompted software changes from the manufacturers.

author: radiolistener
author_rank: Super Contributor
timestamp: August 09, 2023, 08:24:12 pm
content: Quote from: tianyi on August 08, 2023, 02:28:34 amit depends on many factors, such as power flux density, exposure time, individual human properties, such as physiological and psychological state, individual sensitivity, etc.Usual cell-phone and Wi-Fi router emission power is about 1-3 W. Usually such a power with usual antenna produces power flux density which is not enough for human feelings. But again it depends on exact antenna and room environment. For example 1 W laser can easily damage human eye or skin, this is because it focus all energy within a small point, so the power flux density is very high within that point of focus and can make damage.Very long continuous exposure can lead to some physiological changes in human body which also can lead to such effect. But such area is not well known so exact limits and duration for such effect is unknown.Quote from: tianyi on August 08, 2023, 02:28:34 amas said above, it depends on many factors such as power level, antenna radiation pattern, environment geometry, surface materials and personal properties of human. So there is no exact numbers. But 1-3 W used in cell phone or Wi-Fi router looks pretty safe, if you don't use it for a long continuous exposure.Quote from: tianyi on August 08, 2023, 02:28:34 amit depends on many factors. But in general, more higher frequency is more dangerous, because all emission power can be focused withing smaller area which leads to higher power flux density so it can make more damage. At some extreme high frequency it can even make direct physical damage for example X-ray or gamma radiation.In short, cell-phone and wi-fi transmitter power is pretty safe, so you can test it, if you don't feel dizzy, then it's ok for you. But it's still better to not keep your body for a long period of exposure. Just try to use it for a short period of time and other time keep on safe distance from such devices...

author: radiolistener
author_rank: Super Contributor
timestamp: August 09, 2023, 08:45:35 pm
content: Quote from: CatalinaWOW on August 08, 2023, 04:30:02 amAs I know, max power for WiFi is limited to about 36 dBm = 4 Watt.For example, WiFi module on my Raspberry Pi 4 has output power 1 Watt.Cell-phone has about 2-3 W. But note that cell-phone base station has total power about 1000 W (this power is shared between many channels and many antennas within antenna module, as I remember it has about 50 W max per channel and about 20 channels working in parallel). But this is the max emission power, it's power is software controlled and usually is limited to about 50% or even smaller if there is too short distance between nearby cell base stations.I cannot agree that this is small power, even 5 watts is enough to cause a burned wound under certain conditions, but if you don't take it too close to your body, you can assume that power below 5W is almost safe. At least if you don't use it for a very long period of exposure...Regarding to a cell-phone base station, it's unsafe, because it has emission power more than microwave oven at almost the same frequency, so it's very dangerous to keep your body close to such antenna. It can simply fry your body, but if you stay at about 200-300 meters away it's pretty safe.Note that distance is a natural shield from such emission sources... The most dangerous area is a near field zone of antenna, because it has extreme high field strength even for a small transmission power.There is a rule for antennas - the smaller antenna is more dangerous than larger antenna. This is because smaller antenna needs to swing much stronger fields in it's near field zone in order to transmit the same power as larger antenna.

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2023, 04:45:33 pm
content: The "skin effect" also limits penetration of RF radiation and fields into the body.Early calculations for MRI, based on a continuous container of "normal" saline solution, suggested that RF field penetration of a human body would limit the maximum frequency to ballpark 10 MHz.Since the saline in your body is contained in cellular structures, the skin depth is in fact deeper than that, but there are still problems above, say, 100 MHz for high-field MRI into the body interior.MRI has been used with very high magnetic fields, about 7 T = 70 kGauss, at roughly 300 MHz for smaller volumes, such as head imaging.GHz radiation from radar, for example, can cause serious surface burns or cataracts, even though it won't penetrate into the center of the body.

author: radiolistener
author_rank: Super Contributor
timestamp: August 13, 2023, 07:32:38 pm
content: Quote from: TimFox on August 13, 2023, 04:45:33 pmPiece of meat placed into microwave oven has very good heating at several centimeters under skin, so I don't think that skin effect can help to protect human bodyRegarding to radar, yes, they have very high power pulses (up to megawatts or even gigawatts), so it can make damage even if human body is placed outside it's main radiation lobe. When a bird enters the main lobe of the radar at short distance, it just flares, produce smoke and their burned body falls on the ground...So, it's better to stay away at safe distance from such devices. By the way, eyes and bladder are most sensitive organs for high power microwave emission.

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2023, 08:43:40 pm
content: Yes, but MRI often needs to penetrate more than one inch into the body.It’s generally known that if a human directly contacts a wire with high RF voltage, it results in a nasty skin burn.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 14, 2023, 02:33:14 am
content: Quote from: radiolistener on August 09, 2023, 08:45:35 pmI did bolix the WiFi power levels.But something doesn't add up on the RF levels of cell phone base stations. Near where I used to live there was an informal walking path that passed within a meter or so of a short cell phone tower - something on the order of 5-10 meters. Neither I nor my wife noticed any thermal or other effects from this. The informal path was widely used by folks in the neighborhood, I would estimate that dozens of people a day passed near this thing. I can't prove that there were no effects, but really believe that if there was anything serious going on that something would have turned up from epidemiology. I would also be surprised if all of the antennas on a tower were disabled when new antennas are installed or older ones services. The coverage outages would have customers screaming loudly. Some of these towers have a dozen or more antennas. If each is radiating a kW, the total power is pretty impressive.I would use caution in equating power with danger. There is some logic in saying power below a certain level is safe, but details really matter. The power required to stop your heart under ideal (ideal might be the wrong word here) is measured in milliWatts or less, but that doesn't translate into an assertion that milliwatts are always hazardous.

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2023, 03:34:30 am
content: mA of current through the heart is dangerous.

author: radiolistener
author_rank: Super Contributor
timestamp: August 14, 2023, 09:26:20 am
content: Quote from: CatalinaWOW on August 14, 2023, 02:33:14 amIf you power on microwave oven (0.8 kW) with opened door, and stay at about 5-10 meters away you also cannot notice any thermal effect. This is because distance reduce the power and microwave oven doesn't have sharp beam. The same antenna on cell base station is not directed it's beam into people on the road. You can check if there is high power emission with neon lamp, it will glow in your handsFor example you can put your hand on powered 10W LED and feel heat, but if you put your hand on 10W laser, it will make serious damage for your hand. The same power, but laser focus all energy within smaller area.But if you stay in the main radiation lobe of cell base station antenna at 5-10 meters, I think you can feel some heating. The reason why people don't feel it in your example is because antenna beam is not directed directly into peoples or that was some low power base station for very short distance coverage. Such low power base stations can be found inside building and underground.Quote from: CatalinaWOW on August 14, 2023, 02:33:14 amusually their emission power is less, 1 kW is the max power which is used when cell phones are located on long distance. When there are a lot of cell base stations, the distance between them is short and they using max distance limit to prevent interference with nearby base station, so the power will be limited. Usually one antenna module of cell base station has about 200-500 W total power. But if cell base covers long distance due to lack of coverage its total emission power for all channels can be up to 1 kW. This is used outside of the city and on the sea coast, where distance between cell phone and base station can be up to 70 km.As I said, their power is software controlled, so the base station can tune power level depends on it's settings for max distance coverage and actual distance to the cell phone (it is measured in real time as signal propagation delay).

author: radiolistener
author_rank: Super Contributor
timestamp: August 14, 2023, 10:15:19 am
content: Quote from: CatalinaWOW on August 14, 2023, 02:33:14 amyes, it depends on exact case. For example you can stay near megawatt power MW transmitter with glowing neon lamp in your hand and don't feel any discomfort. But at the same time you can get painful RF burn from 5W transmitter. I had these burns from 5W, they are very unpleasant and much more painful than from usual fire, probably because the skin is damaged at a relatively large depth

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 14, 2023, 10:56:16 am
content: I have had a small RF burn also. Entirely accidental, touched the wrong thing. Quite Painful, for quite a while, as I recall.Here is an interesting article that summarizes the use of RF in surgery. It does explain RF burns. In our world of radio this is today relative low freq.What we do not really know it the long term effect of lower power (distance from antenna), higher RF freq on the human body. I know there are studies that do not show any bad results. Time will tell.https://www.asit.org/assets/documents/Prinicpals_in_electrosurgery.pdf

author: jjoonathan
author_rank: Frequent Contributor
timestamp: August 14, 2023, 07:21:36 pm
content: I nearly got an RF burn from the poorly maintained microwave in the grad school lounge. I don't normally rely on interlocks to turn off the microwave, but one day I was chatting and didn't want the obnoxious 5 second beep to interrupt the conversation so I just opened the door with a couple seconds left. I stuck my hand in and pulled back when I felt very strong 120Hz on my hand and forearm. At first I was confused, but then a second later the obnoxious beep went off, making the cause clear: the interlock had failed and I had just stuck my hand in a running microwave. Great. Fortunately it didn't result in any noticeable burns. It was a 1000W microwave, but it's unclear how much of that actually turns into RF and how much of the RF coupled into my hand. There were no noticeable lasting effects.In any case, this is why the literature focuses on RF burns: they happen, we have data on them, they seem to behave more or less like thermal burns, and it is easy to characterize the dose-dependent relationship because we can see what happens at dose * duty cycle 1000000x a typical cell phone. Phones are waaay down on the "no symptom" side of the curve. Is it possible that there is another mechanism at play that still causes chronic effects at the comparably tiny dose? Sure, but it has to stop scaling with dose at some point or we would have already seen it. That narrows down the search space considerably. It also has to be low enough in effect size to not show up in population statistics, or we would have already seen it. Is anyone looking? Clearly yes, because a "mouse in a microwave" study circles every few years when a research team exposes mice to very intense RF, measures slightly elevated cancer, and speculates a link to cell phones. These papers tend to die out when they get in front of an expert body that can ask questions about how representative the power levels actually are of typical cell phone use, if the elevated cancer follows the rates set out by the existing thermal models or if the results are actually new, etc, but the papers always continue to circulate around the more gullible parts of the internet for a while after they die of natural causes. I take this as a sign the system is working: researchers are paying attention, the public is paying attention, censorship isn't killing the stories before they circulate and if one of these papers found something that was actually concerning there would be a rush among labs to get in on the RF story of the decade, it would break out into the media, FCC limits would be lowered, high bandwidth wireless service would get shitty for a while and then we'd find a way to compensate.Oh, and that's another reason why I don't put much credence in the cranks: you can often ask "how would you establish the new safe dose limits" and the answers you receive ("no safe dose!") will usually prove that conspiracy-brained people aren't trying to be reasonable. Not only does "no safe dose!" indicate ignorance of background radiation, but our telecom systems have orders of magnitude available on several dimensions of compromise (peak power, duty cycle, average power, bandwidth) and this means that any effect which actually exists could almost certainly be alleviated without giving up *all* telecom, but none of the conspiracy-brained people are ever interested in that discussion because the actual problem is that they have no evidence at all, let alone enough evidence to reason about. At some point you have to put down your foot and say "proof or GTFO." Anything else leads to madness.

author: LM21
author_rank: Regular Contributor
timestamp: August 14, 2023, 07:25:06 pm
content: Quote from: tianyi on August 08, 2023, 02:28:34 amThat person should see a doctor about his dizzynes. Maybe high or low blood pressure.

author: radiolistener
author_rank: Super Contributor
timestamp: August 14, 2023, 11:09:59 pm
content: Quote from: LM21 on August 14, 2023, 07:25:06 pmAre you sure? Usual WiFi has max 4 Watt power, which is already enough to get noticeable effect in some specific conditions. And what he meaning when talking about "high power WiFi"? And with high directivity antenna...I hear one story when the woman called the police and began to complain that her neighbor was irradiating her with electromagnetic radiation... The police thought that she is crazy and needs to go to the doctor, however, in order to calm her down, they agreed to talk to a neighbor and ask him not to irradiate her. And what was their surprise when they called a neighbor and saw behind him dozens of microwave ovens with an open door aimed at the neighbor’s apartment...