Posted: 1/27/2010 1:32:51 PM EDT
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Does the reception of wi-fi or radio waves “absorb” any of the available signal. For example, if you measured the power required for a transmitter to maintain a certain signal strength, would adding a lot of receivers require more power to the transmitter to maintain signal strength? This question came up as a result of a device that can turn wi-fi signal into enough DC volt and amps to charge a cell phone battery. I think that it is using that much power, it must be reducing signal strength. http://www.ecnmag.com/multimedia/2010/01-video-Is-the-RCA-Airnergy-WiFi-Hotspot-Energy-Harvesting-for-Real.aspx |
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No. And with the average transmitted RF of the WiFi signal in the 100 mW range, charging a battery would be a very slow process. Most antennas are omnidirectional, meaning a spherical emission pattern. Area of a sphere is 4 Pi*r2 . This means the radiated power decreases by that reciprocal, 1/4*Pi*r2 . Now with the 12 channels available, if you had 12 WiFi WAPs in a very small area and erected a cage over all of them, you could recover a good bit of radiated power but then...  |
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Quoted:
The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. I knew a ham radio operator who could recharge AA batteries using the signal from a local AM station! He lived within 45 degrees of the top of the tower, so he was very close. |
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Quoted: Quoted: The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. I knew a ham radio operator who could recharge AA batteries using the signal from a local AM station! He lived within 45 degrees of the top of the tower, so he was very close. The most powerful AM broadcasters are 50,000 Watts. Yes, it is possible but you need a HUGE receiver. The first radio-controlled aircraft were steered with a powerful RF transmitter which caused voltage-induced heating of a tuned length wire, this changed the length of the wire which changed the rudder. Those were transmitters in the 100s of Watts of radiated power, using directional antennas aimed at the aircraft. |
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No. And with the average transmitted RF of the WiFi signal in the 100 mW range, charging a battery would be a very slow process.
Most antennas are omnidirectional, meaning a spherical emission pattern. Area of a sphere is 4 Pi*r2 . This means the radiated power decreases by that reciprocal, 1/4*Pi*r2 . Now with the 12 channels available, if you had 12 WiFi WAPs in a very small area and erected a cage over all of them, you could recover a good bit of radiated power but then... But here is my argument: If you are getting any energy from the transmission, than it has to weaken the signal (I am talking possibley to a very small degree here) because energy is not free-it has to come from somewhere. You cannot take energy from something (in this case radio waves) and not pay a price (in this case reduced transmission stregnth). Just like a float imeasuring the wave ring of a rock in a pond; the float is causing a small reduction in the energy of the wave (If we filled the pond with rafts, would not the wave ring would be noticable reduced?) Another example: The earth is reducing the engergy the sun can project into space because we are absorbing some of it; we are reducing it's signal. Right? |
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Quoted: Quoted: Quoted: The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. I knew a ham radio operator who could recharge AA batteries using the signal from a local AM station! He lived within 45 degrees of the top of the tower, so he was very close. The most powerful AM broadcasters are 50,000 Watts. Yes, it is possible but you need a HUGE receiver. The first radio-controlled aircraft were steered with a powerful RF transmitter which caused voltage-induced heating of a tuned length wire, this changed the length of the wire which changed the rudder. Those were transmitters in the 100s of Watts of radiated power, using directional antennas aimed at the aircraft. Actually a tuned antenna and just a diode very simple to do |
Crystal set radios use the RF power only to provide an audio signal to the operator. They are truly RF powered radios. Therefore, at least some radios absorb the radiated RF power from the transmitter(s). So, yes, some of the photons flicked off of the transmitter antenna do get absorbed by the receiver. What is truly amazing is that these littler critters do not bump into one another and can go around the globe from a transmitter the size of a household lightbulb.
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No antenna can "suck" more power than what is radiated onto it, this is why very low power signals require a dish to collect enough signal so the signal to noise ratio is acceptable. There is no way that an antenna will "rob' another receiver of power unless that second receiver is "shadowed" by the first receiver's antenna. Radio waves behave just like light waves. |
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The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. I knew a ham radio operator who could recharge AA batteries using the signal from a local AM station! He lived within 45 degrees of the top of the tower, so he was very close. So, how large were his tumors before he passed away? I wouldn't want to be absorbing any RF signal that could do anything but be recieved by a transmitter |
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Quoted: Quoted: Quoted: The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. I knew a ham radio operator who could recharge AA batteries using the signal from a local AM station! He lived within 45 degrees of the top of the tower, so he was very close. So, how large were his tumors before he passed away? I wouldn't want to be absorbing any RF signal that could do anything but be recieved by a transmitter . Do you know what kind of radiation causes tumors? Ionizing radiation. Which begins at ultraviolet and up. Anything with a longer wavelength, like visible light, does not cause ionization.Now, what is the wavelength of radio communications? Care to take a guess? Is it longer or shorter than UV? Well, start off with AM broadcast. This is in the 550-1610 kHz. That is the kilometer wavelength. Definitely NOT ionizing. How about FM? Wavelength of meters. Still NOT ionizing. Microwave? That is "micro" only on the basis of RADIO COMMUNICATIONS. Wavelength of centimeter to millimeter. Still NOT ionizing. Next is IR...wavelength of 900-1500 nanometers. No, not ionizing....visible light? You guess, is it ionizing or not? Education, the more you know, the less you fear. And RECEIVERS receive, transmitters transmit. Transceivers do both. |
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Anyone who said NO has failed physics. You can't get something for nothing. I work with radio equipment every day. I have radios which have receivers that only need .02 millionths of a volt's worth of signal in order to quiet the receiver enoiugh that the received signal is intelligible. It takes that .00000002 volt's worth of signal and uses it to drive the first stage of the receiver. It doesn't get re-radiated. Though the effect is negligible, yes, receivers do ABSORB the RF energy that makes it into them via the antenna. A theoretically perfect spherical radiating antenna, called an isotropic radiator, radiating one watt of RF power, will radiate the power into a spherical volume, with the signal strength attenuating by the inverse square law. But if you were to place a large spherical shell around the antenna, perfectly centered on the antenna, and of ANY diameter, and it was a complete shell that absorbed all RF energy, it would recover the full watt that was initiall transmitted, minus losses in the antenna material itself as it will not be a perfect conductor. What's interesting is that AM and related receivers DO regenerate and re-radiate the signal that they're tuned to, but the re-radiated signal is very weak. However, it's not so weak that it can't be detected. When I'm listening on my HF amateur radio, I often hear the chirps and squeaks caused by other people with HF radios tuning across the frequency I'm listening to. Sometimes, if you're really lucky, you can actually track that other tuning chirp across the band. CJ |
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(a) Does the reception of wi-fi or radio waves “absorb” any of the available signal. (b) For example, if you measured the power required for a transmitter to maintain a certain signal strength, would adding a lot of receivers require more power to the transmitter to maintain signal strength? there are two very different questions above. (a) YES (b) NO Quoted:
This question came up as a result of a device that can turn wi-fi signal into enough DC volt and amps to charge a cell phone battery. it is marketing BS. it doesn't work. it is solely designed and marketed to part you from your money. ar-jedi |
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This concept (true for electricity, heat, and other forms of energy) is soooooooooo overlooked or ignored in TV shows and movies.
No antenna can "suck" more power than what is radiated onto it, this is why very low power signals require a dish to collect enough signal so the signal to noise ratio is acceptable. There is no way that an antenna will "rob' another receiver of power unless that second receiver is "shadowed" by the first receiver's antenna. Radio waves behave just like light waves. |
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The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. Just plug the attennas into a huge receiver that will amplify the signal to many volts. That will recharge the batteries in a hurry, ya dummy. Now that I think about it, you could "recover/mine" radio waves by putting up receivers right next to the radio stations, and charge many batteries through the receiver FOR FREE. Maybe with enough receivers you could convert to 120V AC and power the whole block. Holy cow, this is the answer to the energy crises..... ...WOOT |
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No. And with the average transmitted RF of the WiFi signal in the 100 mW range, charging a battery would be a very slow process.
Most antennas are omnidirectional, meaning a spherical emission pattern. Area of a sphere is 4 Pi*r2 . This means the radiated power decreases by that reciprocal, 1/4*Pi*r2 . Now with the 12 channels available, if you had 12 WiFi WAPs in a very small area and erected a cage over all of them, you could recover a good bit of radiated power but then... But here is my argument: If you are getting any energy from the transmission, than it has to weaken the signal (I am talking possibley to a very small degree here) because energy is not free-it has to come from somewhere. You cannot take energy from something (in this case radio waves) and not pay a price (in this case reduced transmission stregnth). Just like a float imeasuring the wave ring of a rock in a pond; the float is causing a small reduction in the energy of the wave (If we filled the pond with rafts, would not the wave ring would be noticable reduced?) Another example: The earth is reducing the engergy the sun can project into space because we are absorbing some of it; we are reducing it's signal. Right? That reciever is not going to draw power from the other recievers, unless it blocks another reciever. If the signal is not received, it is wasted. Think of it this way, a sprinkler in your yard spraying 360 degrees. If you have 5 people with buckets, the same distance away, they are all going to catch the same amount of water ( assuming the sprinkler is spraying equally in all directions). Anywhere there is not a bucket, the water is going to hit the ground. Now it would be possible to surround the fountain with a gutter system ( Faraday cage) and collect all of the water, but it would be useless. |
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The premise of charging a battery via received radio signal is rather crazy, especially with WiFi which is an extremely low power service. Receiver inputs are typically on the order of microvolts even from large antenna arrays for much higher power services. Just plug the attennas into a huge receiver that will amplify the signal to many volts. That will recharge the batteries in a hurry, ya dummy. Now that I think about it, you could "recover/mine" radio waves by putting up receivers right next to the radio stations, and charge many batteries through the receiver FOR FREE. Maybe with enough receivers you could convert to 120V AC and power the whole block. Holy cow, this is the answer to the energy crises..... ...WOOT Don't tell the liberals. They'll try to mine Rush Limbaugh! |
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... When I'm listening on my HF amateur radio, I often hear the chirps and squeaks caused by other people with HF radios tuning across the frequency I'm listening to. Sometimes, if you're really lucky, you can actually track that other tuning chirp across the band. ... Wouldn't it be because of most radios using heterodyne? At least that's how it was when I was interested in this stuff. Also, to people talking about tumors because of radio waves: in college I lived next to a big radio antenna for about 6 years. We could hear their radio transmissions through speakers even when the audio system was not powered. We had digital circuits fail because of interference. As far as I know nobody got sick, 20 year later everyone is alive and well. |