The antennas tag has no wiki summary.
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1answer
64 views
Astronomical-wavelength radio (AWR) transmissions between cosmic plasmas?
My son asked me if electromagnetic waves longer than radio exist. I told him that even though physics permits such waves, there are no antennas long enough to radiate or detect them.
However, on ...
2
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3answers
116 views
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 ...
1
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1answer
93 views
The length of an antenna is twice the amplitude of the wave
I have seen it remarked in some problem sets that if you have an electromagnetic wave traveling in the $x$-direction with it's $y$-coordinate given as
$y(x,t)=y_0\sin (\omega t +kx)$
and you want a ...
1
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0answers
29 views
Does shadow fading change if obstacles are fixed?
Given a RF sender and a receiver, suppose the obstacles between them are fixed, both their material and dielectric properties, does the path loss caused by their shadowing vary? If yes, what causes ...
3
votes
1answer
60 views
Effective aperture of isotropic antenna
I have always taken for granted that 'the aperture of a loss-less isotropic antenna is $\dfrac {\lambda^2} {4\pi}$'.
On a whim, I tried to look up how this expression was derived, but so far I have ...
0
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0answers
90 views
Can a salt solution in water function just fine as a dipole antenna? [closed]
I found that it is known that ultrahigh frequencies of 900 and 1800 megahertz electromagnetic radiation
can generate excellently the following phenomenon: electrostriction
- the under the influence of ...
0
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0answers
62 views
I need to learn about Antenna physics (Books are preferred) [closed]
Soon I will be a graduate physics student at MIT... I need some references to study antenna physics, because it is one of the topics of my Doctoral General Exam... Until now every book I have ...
1
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0answers
21 views
Is the inducing signal weakened to the extent it induces a signal in the receiving antenna?
Silly question ...
An antenna is necessary to receive a transmitted signal from atmosphere, or otherwise. The transmitted signal weakens as it travels through the atmosphere. When the signal ...
0
votes
1answer
58 views
Numb3rs TV series - Antenna
Most of you might have watched the TV series 'Numb3rs'.
http://en.wikipedia.org/wiki/Numb3rs
In one of those episodes they were discussing about how to capture the contents on the screen of a ...
0
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1answer
376 views
How do EM waves get detached from an antenna?
How does an electro-magnetic waves get detached from an antenna and spread to the space?
While an antenna receives an EM wave, which quantity of the EM wave (electric or magnetic) is used for ...
1
vote
1answer
69 views
Getting a given wavelength radio signal given an antenna with real-world constraints
Supposing you are given a transmitting antenna of whatever type of metal is most commonly used these days, and supposing that you are applying an AC current with the intent of transmitting a 1 m ...
8
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2answers
2k views
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
...
1
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0answers
71 views
Can mu-metal reduce the negative effect of metal close to low frequency RFID antennas?
I have an 134.2 khz RFID antenna for sport events which is basically a 1x1 meter square sheet of rubber with 3 loops of antenna cable embedded. The problem is that the transponder recognition range is ...
3
votes
2answers
201 views
Radio communication and antennas
I'm trying to understand how an antenna is able to understand, but have a few quesions:
I understand that for an antenna to distinguish between 2 (or more) signals, they have to be in different ...
0
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0answers
89 views
Wireless signal strength
My question is possibly somewhat misplaced, but I'll try to explain as best as I can. Suppose I have a transmitter with a frequency of 2500MHz and a power of 1W. It radiates uniformly in all ...
0
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0answers
114 views
Antenna Power and gain calculation [closed]
I have a wireless security related question, the second part confused me:
Your wireless network usually has a range of 100 feet. However you
are having a (confidential) meeting in a 10’ x 10’ x ...
1
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2answers
211 views
Quantum Mechanics- Antenna emitting electromagnetic radiation
Radio signals are being transmitted in a frequency of $ 8.4 \times 10^9 s^{-1} $ and being received by an antenne that is capable of receiveing power of $ 4 \times 10^{-21} watt $ ($ 1 watt = 1 J ...
1
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2answers
75 views
What is difference between the miltary radar in 1940's from commercial antenna that is for the use of TV
What is difference between the miltary radar in 1940's from commercial antenna that is for the use of TV?
I have read article from some of the WW2 history website that call the German radar the ...
0
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1answer
84 views
Is this claim from historician true for physicist point of view?
"The original Naxos I had a vertically polarized antenna, with poor results as the British radars initially used horizontal polarisation. (This seems to have been a case of the German designers being ...
1
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2answers
351 views
How to capture electomagnetic radiation/waves?
If I wanted to find out what kind of electomagnetic waves "travel" through my room at which frequency, what kind of equipment would I need? Suppose I want to view frequencies from 0 Hz to 6 GHz.
1
vote
1answer
186 views
What are the optimal shapes, if any, for fractal antennae?
A DIY fractal antenna project is described at http://www.htpc-diy.com/2012/04/diy-flexible-fractal-window-hdtv.html
Mainly idle curiosity, but I was wondering what the principles behind these might ...
2
votes
2answers
347 views
How is antenna gain correlated to beam width?
Let's say you have two dipole type antennas. Antenna A has a gain of 2.15 dBi, a horizontal beam width of 360 deg and a vertical beam width of 45 deg. Antenna B is similar to antenna A, but has a ...
1
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3answers
740 views
Geometry of wireless signal strength
How does wireless signal strength correspond to distance? RSSI lies between -100 and 0 (at least, on my computer). Let's say I walk a distance x towards the router, and my RSSI goes from -60 to -50. ...
2
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3answers
997 views
Some questions about car radio and cellphone antennas
1-Why the antenna of the radio of cars is located outside the car and not inside?
2-If the answer to 1 is because that cars are like Faraday cages then how come my cell phone can receive signal ...
2
votes
1answer
340 views
How does holographic radar work?
I'm trying to figure out how the mechanics of holographic radar work.
AFAIK there is a continuous 3d transmission signal (a dome-shaped antenna?)
But because there isn't a direction or time-domain, ...
3
votes
2answers
322 views
Near-field around parabolic antenna?
Having a discussion at work about the $H$-field around a big parabolic antenna.
All of the safety tests done around the antenna only mention $E$-Field. They state in the radiating near-field the $E$ ...
6
votes
1answer
286 views
Why are fractal geometries useful for compact antenna design?
While most of what I've read about fractals has been dubious in nature, over the years, I keep hearing that these sorts of self-similar (or approximately self-similar) geometries are useful in the ...
5
votes
1answer
2k views
Adverse Health Effects of Strong Radio Waves
A while ago, I was hiking near the Hollywood Sign in Los Angeles. When I got to the sign, I found out it was fenced off; with several signs and a security camera promising prompt enforcement. As I was ...
1
vote
3answers
656 views
Radio waves and frequency of photon
How radio waves create the current in antenna in terms of photons? If it is Compton scattering then why is not changed the freuency of photons?
5
votes
1answer
262 views
How is an inflatable parabolic antenna created?
I'm intrigued by this, and how it would work:- 3 sub-questions if I may:
Construction:
As I understand it's a flexible sphere constrained by a rigid edge.
a. Do we simply glue 2 flat circular ...
7
votes
3answers
974 views
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 ...
17
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3answers
2k views
2
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1answer
247 views
What is the effect of ice on an antenna?
A local FM radio station transmitting at 89.3 MHz recently announced that it would be running at 50% power due to freezing weather and a forecast of ice accumulation, as "when ice is forecast ... it ...
