The interfaces between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. Antennas are essential components of all radio equipment.
Questions tagged [antennas]
395 questions
141
votes
2 answers
How many photons are received per bit transmitted from Voyager 1?
As of 2024, according to https://voyager.jpl.nasa.gov/ , Voyager 1 is around one light·day away from Earth and still in radio contact. When Voyager 1 sends messages to Earth, roughly how many photons are (1) transmitted and (2) received per bit?
Craig Gidney
- 7,172
54
votes
3 answers
Can I use an antenna as a light source?
Can I use a normal metal antenna to emit visible light?
oneat
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33
votes
7 answers
Radio antennas that are much shorter than the wavelength
From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall having to mess around to compensate for the fact…
user4552
30
votes
10 answers
Is every open circuit a capacitor?
I think that even open-ended wires can let AC current flow through them, just with a low capacitance. I also think an antenna could be a capacitor and open ended. Am I thinking correctly?
Bálint Tatai
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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
3 answers
Detectability of interstellar messages
Recently a debate started whether it is a good idea to send more messages into space in the hope of having alien civilizations receive them. There are some predecessors, most notably the 1974 Arecibo Message to globular cluster M13, which is an…
Jens
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18
votes
4 answers
Does a radio receiver "collapse" a radio wave function?
Does a radio receiver "collapse" a wave function when listening to a radio broadcast generated via a transmitting antenna?
Background:
There has been much discussion on this forum (here) about the nature of radio waves, photons and quantum…
kdtop
- 327
17
votes
2 answers
Why is Near Field Communication (NFC) range limited to about 20cm?
Near Field Communication (NFC) operates at 13.56 MHz.
Near Field is the region situated at a distance r << λ
λ = c/f
c = 299 792 458 m/s in the vacuum.
f = 13.56 MHz = 13.56*10^6 Hz
λ = 299.8/13.56 = 22.11 m
Of this distance, the Reactive Near…
Mister Smith
- 313
15
votes
7 answers
Why does the length of an antenna matter when electromagnetic waves propagate perpendicular to the antenna?
The optimum length for a dipole antenna is a multiple of half the wavelength that it is designed to receive or emit. Why is this? If an electromagnetic wave has E in the x-axis, B in the y-axis, and propagates along the z-axis, it will be generate…
user411769
- 161
15
votes
2 answers
Why do we need an antenna of size comparable to the wavelength for surface wave transmission?
Why do we need an antenna of size comparable to the wavelength for surface wave transmission?
I am unable to understand why such condition is imposed.
Hanzala
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13
votes
1 answer
Average force between two parallel finite wires with AC
Imagine 2 parallel antennas (wires) of equal length (a) with a distance r between them.
Both have AC currents with identical sine waveforms (equal frequencies and amplitudes). They are also in phase with each other.
Edit: The distance between them…
Xynon
- 331
13
votes
6 answers
How can current flow through an open wire (like a dipole antenna)?
I am trying to understand how current can flow through a dipole antenna (with length $\frac{\lambda}{2}$ or anything else) which actually is nothing more than an open wire.
Could someone clarify this please?
Thank you
privetDruzia
- 393
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…
11
votes
3 answers
How do radio telescopes work?
If I search online for how radio telescopes work, the found articles talk about how RF is on the spectrum, etc, how the parabolic collector is the aperture which contributes to the sensitivity and reflects the signals into the detector at the focal…
Jason Kleban
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