Questions tagged [radio-frequency]
303 questions
66
votes
4 answers
What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?
XKCD usually has solid (and often contemporary) science behind it. Lightning Difference, #2027 one says:
Q: What’s that trick for telling how many miles away lightning is?
A: Just count the seconds between the visible flash and the radio wave…
uhoh
- 6,089
38
votes
2 answers
How can wifi penetrate through walls when visible light can't?
I did search the question on Physics S.E considering it would be previously asked. I found this
How come Wifi signals can go through walls, and bodies, but kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
But in this…
Shashaank
- 2,837
36
votes
4 answers
How to imagine WiFi signal propagation?
When thinking about how the WiFi signal propagates through a household, can I use the following thought experiment?
Assume absolute darkness. Place a strong lightbulb where the WiFi access point is. The illumination that reaches various places in…
vektor
- 382
28
votes
2 answers
Why do I need to adjust TV antenna on the roof for better video quality?
I find it a annoying and a pain to climb up to the roof and adjust the TV antenna so that I can watch my favorite TV program without distortion. I am not using satellite service which requires a dish to capture the signal and I know the antenna is…
user6760
- 13,214
25
votes
2 answers
How do we know that these radio bursts are from billions of light-years away?
NASA just announced that they detected the first radio bursts from outside of our galaxy.
Astronomers, including a team member from NASA's Jet Propulsion Laboratory in Pasadena, Calif., have detected the first population of radio bursts known to…
user12046
23
votes
3 answers
How do you make a spherical radio wave?
A vertical rod, a usual dipole, produces radio waves in the horizontal plane, mostly in two opposite directions:
$\qquad \qquad \qquad \qquad \qquad $
If that is possible, how do you produce a spherical EM radiation? should the antenna be a (..n…
user104372
22
votes
2 answers
What is the scientific explanation for radio waves bending around the Earth?
The experiment
This experiment is documented in a documentary called Convex Earth. The exact location the following information is taken from starts at 14:25.
High frequency directional antennas are set up 14 km apart, 1.5m from water level [I…
inspirednz
- 331
18
votes
6 answers
Why put a radio telescope in space instead on the Moon?
Why are we not building a space-based radio telescope? If a lunar radio telescope is hard to build, why not a space-based radio telescope? It could be an interesting tool, I presume. Thus, what could be the advantages of a space-based radio…
riemannium
- 6,843
15
votes
4 answers
Understanding the cause of sidebands in Amplitude Modulation
I've read it many places that Amplitude Modulation produces sidebands in the frequency domain. But as best as I can imagine it, modulating the amplitude of a fixed-frequency carrier wave just makes that "louder" or "quieter", not higher-frequency…
Jason Kleban
- 455
13
votes
1 answer
Why is it that fractal antennas can filter out so many frequencies?
As known, fractal antennas are used for example in cell phones. But why is it that so many different kinds of frequencies can be filtered out of the forest of radio waves surrounding us? Is it because of the self-similarity, when you look at…
Deschele Schilder
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12
votes
2 answers
How does the human body affect radio reception?
My sister asked me why her small radio sometimes has better reception when she places her hand nearby (without touching) the antenna. I did some research and wanted to ask a few questions.
Here are some of the ideas I came across:
Electromagnetic…
12
votes
2 answers
From how far away could Earth's telescopes detect Earth-like radio signals?
The Earth has been broadcasting human generated radio signals for about 100 years now. If a nearby civilization were broadcasting similar radio signals, could we detect them with our own radio observatories like Arecibo or the VLA?
It's going to…
Paul T.
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11
votes
3 answers
If we have a cosmic microwave background should't we also have a cosmic radio wave background?
I'm a layman in physics, but here is what I understand:
What we see in the sky with naked eyes is a map of electromagnetic waves in the frequency visible to the human vision.
But that kind of observation is too limited because as the universe…
Werex Zenok
- 111
11
votes
4 answers
Why do we even need RF cavities to accelerate particles?
Wouldn't it be easier to just apply a constant $E$ field along the beam axis? This would avoid all kinds of problems like synchronicity, phase change etcetera.
Addendum. What puzzles me the most is the following assertion (taken from Exactly like…
ric.san
- 1,694
9
votes
1 answer
Photons of a radio wave
It has been said that RF consists of IR photons (here (now deleted; screenshot) however opinions seem to differ):
The smallest energy for photons is in the range of the infrared radiation. Longer wavelengths are related to modulated EM radiation…
Eugene Ryabtsev
- 190