Questions tagged [radio-frequency]
The radio-frequency tag has no summary.
307 questions
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MRI 180° RF pulse and spin synchronization
(How) Is it possible for a 180° RF pulse to inverse the net magnetization $M$ and synchronize individual spins (if spins were synchronized wouldn't that imply that there is some non zero $M_{xy}$ ...
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Off resonant driving of a MW/RF cavity
Let's say I have a cavity resonant at 10 GHz with a Q factor of 1000. Given the Lorentzian shape of the cavity, I can also drive the cavity at, say 100 MHz. Of course the response will be very very ...
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Question about 100 MHz resonator
I would like to apply an electric field of 100 V/cm, along the $z$-axis (defined by an externally applied DC field), with a frequency of 100 MHz at the location of an atomic cloud (everything is ...
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Do window screens block WiFi? [closed]
I'm thinking of putting a wifi satellite on a screened in porch outside, to cover my yard. The Internet seems to think that wire screens don't block wifi, but wifi has a wavelength of 2 inches or more,...
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Wavelength or frequency border of radio waves and microwaves
I'm unable to find a precise definition of the wavelength or frequency delimiting the radio waves and the microwaves. Some sources are saying that the frequency $f = 1\mathrm{GHz} = 10^9 \, \mathrm{...
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How is $\rm GaN$ used in Active Electronically Scanned Array (AESA) radars?
Many Active Electronically Scanned Array (AESA) radars are based on Gallium Nitride ($\rm GaN$).
I understand in principle how AESA radars work. What role does the $\rm GaN$ play? Why not some other ...
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Why is the effect of an $\alpha_\phi$ radiofrequency pulse is given by $R_{z'}(\phi)R_{x'}(\alpha) R_{z'}(-\phi)$?
In 1, page 82, it is written that "the effect of an $\alpha_\phi$ pulse can be represented by three cascaded spin rotations. In terms of rotation matrices:
$
M(0^+) = R_{z'}(\phi)R_{x'}(\alpha) ...
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Does faraday shielding suppress magnetic fields from a long AC current carrying wire?
Say we have an infinite wire carrying AC current (cycling with some frequency in the ~kHz regime). Without anything further, I assume there will be a magnetic field near the wire of the form $\frac{\...
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Why does the Spin Echo sequence refocus dephase due to magnetic field inhomogeneities and gradient echo does not?
My explanations come from Richard Buxton's book (Introduction to functional magnetic resonance imaging).
We know that Spin Echo sequence can be summerized as
90° pulse - wait TE/2 - 180° pulse - wait ...
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Is the magnetic field in an ideal coaxial cable carrying a TEM wave solely defined by the Ampere/Maxwell displacement current?
If I have a plain TEM wave in an ideal coaxial cable, a simple solution to the wave equation assumes a wave in the source free region between the center conductor and the shield. The magnetic field is ...
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RF Heating in a Dielectric
Trying to understand rf heating in dielectrics. Suppose I have a high frequency AC current around 50 - 100 MHz on a wire adjacent to a dielectric..
This creates an associated electric field $\mathbf{E}...
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Passing photoauostic audible messages via mid-infrared lasers can be done. Can we achieve this with a microwave?
In medicine, Photoauostic spectrography is used in non invansive glucose monitoring. To acheive this, we use invisible laser (Mid-Infrared) range in electromagnatic spectrum.
The photoacoustic effect ...
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3
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349
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Can refraction change a wave's frequency?
Can refraction change a wave's frequency under any circumstances ?
If yes, what conditions must apply for this to occur ?
Is this a common phenomenon or only happening in extreme circumstances (such ...
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448
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Deeper explanation of why Spin-Echo's 180 degree refocusing pulse recovers static magnetic field inhomogeneities vs Gradien-Echo which does not
I have studied the Spin-Echo pulse sequence for some time now, and read many explanations in different books and courses.
I know that the 180 degrees RF pulse (applied half way between the 90 degrees ...
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Why does this RLC behavior differ from what equations predict?
In order to better understand RLC behavior under current sources, I constructed the simple circuit below and attempted to analyze it. However, my equations do not agree with simulation results.
The ...
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Phase-Based Ranging
I've been reading a lot about Phase-Based Ranging (PBR), and it's all starting to become very intuitive.
BUT; There is one technical, but extremely important, detail that seems to escape me. When the ...
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Are these levels of RF radiation harmful?
I live on the top floor of a building that has four 5G antennas on the roof. The house is curved, and from my viewpoint, I can partially see one of the antennas pointing towards me. Curious about the ...
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Prime operator for sinusoidal testing function in self-term approximation for Thin Wire using Method of Moments (MoM) $s_m^{'}(x_p)$?
I am implementing thin wire method of moments using W.Gibson's "Method of Moments in Electromagnetics". The approximation for self-term using piecewise sinusoidal basis function is given as
...
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Synchronous phase value for CERN LHC [closed]
How to find out that the value of the synchronous phase for CERN LHC in the case where beams undergo collision, and the RF cavities provide only longitudinal focusing with no net acceleration.
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Slice selection in MRI
Studying MRI, I found that excitation is done by applying B1 (RF) and Gz simultaneously.
But, I think that if those are applied at same time, slice selection cannot be done correctly because Gz will ...
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Book Recommendation about radio frequency antennas and satellites
I have recently obtained a job in the field of radio frequency antennas and satellites, and I am interested in delving deeper into the subject. Could you recommend any books that cover these topics ...
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Why is 180 degree pulse applied after half echo time (TE) in Spin Echo sequence?
The definition of Echo Time from Radiopaedia:
The echo time (TE) refers to the time between the application of the radiofrequency excitation pulse and the peak of the signal induced in the coil. It ...
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Gravitational waves vs ULF Radio waves
During the recent merger of two Neutron stars the lead up to the merger was detected as gravitation waves. This was the merger of two spinning bodies that had very strong magnetic fields and they were ...
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Is the Fourier Transform a reliable way to infer the physical phenomena producing an RF signal?
Consider the following thought experiments:
Scenario 1:
A person standing far away shines 3 light beams at you, each beam having a narrow spectral distribution centered around different frequencies F1,...
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Can I recover the physical wavelengths present in a light source from a time-series measurement of its amplitude?
Consider a mixture of different wavelengths being emitted from the same point (ex: a star). This light consists of a mixture of wavelengths and intensities at each wavelength. When measuring the ...
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202
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How to measure time varying electric field strength accurately in space and time?
I would like to know of any techniques that can be used to measure the electric field strength precisely and accurately in both time and space. I know that there will be physical/ practical ...
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Question about CPU manipulation with RF waves [closed]
Is it possible that by sending a specifically modulated RF wave, you can manipulate the flow of electricity to the CPU, and maybe cause binary code Execution? Like typing something like Hello?
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105
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How to increase the angle of light (without going from a higher to lower index of refraction)?
I would like a device to spread out/increase the angle of rays in a light source. In other words, I would like to reproduce the behaviour of light traveling from an area of higher to lower index of ...
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When does Lorentz reciprocity not apply to an EM system?
I'm trying to understand the limits of when Lorentz reciprocity does and doesn't apply to a given system. I know that it only applies to linear systems, but based on a couple of examples I believe ...
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Is there an intuitional way to tell which physical quantities are angular frequency and which are frequency?
Such as the natural linewidth is defined to be angular frequency, while the absolute frequency of laser is frequency.
By far I haven't found a good way not to learn it by roting.
Would anyone be ...
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How a does a RF magnetic field interact with trapped atoms in evaporative cooling?
In the paper “Evaporative Cooling of Trapped Atoms,” [Advances in Atomic Molecular And Optical Physics 37, 181–236 (1986)] by Wolfgang Ketterle and N. J. Van Druten, they claimed the matrix element ...
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Impedance at Feed Point and End of Antenna
Watching this pretty great video from 1947 about antenna fundamentals. I have a question about one part of it though.
The video states that the impedance at the feed point of the antenna is 72 ohms, ...
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Antennas and Wavelength
I'm clearly missing something here, but I'm trying to grasp basics of how antennas work.
Relating it to standing waves on a string of length L, the lowest frequency possible is a wavelength of 2L, due ...
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Wireless Power Transfer Modelling
I am trying to develop an analytical model for RF wireless power transfer from an external antenna to an implant antenna, embedded inside layers of lossy tissue.
The external antenna will be ...
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How would we create a device to detect communication using the x-ray and gamma bandwidth? Like a radio. But x-ray gamma spectrum [closed]
I realize the dipole would have to be small enough (in the nucleus of an atom range) and we don't have any mechanism that is small enough to demodulate the frequencies at this rate. But is there some ...
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How does one pick Echo time (TE) in MRI machine?
The definition of echo time (TE) in MRI is the time difference between a 90-degree RF signal and the echo peak, which feels like an intrinsic property of the proton.
How can we even control how long ...
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Mathematical Modelling of Wireless Power Transfer
I am trying to understand the interaction between a transmitting antenna with a certain transmitting power (denoted PTX in the diagram) and how the radiation pattern of the TX antenna is affected by a ...
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How does radio wave get reflected?
Actually recently I posted a question in h-bar ($\hbar$) relates to radio wave reflection from mirror, and one of the user replied that it can't, only ionosphere can reflect it, so my question what is ...
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Feynman physics on least time of light
What does light checks all paths mean by Feynman? Especially the statement is labeled by yellow. Why there is only one path that leads radiowaves to D’? And how wave check all paths, that is, why it ...
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What is physically happening in a medium in which an evanescent wave is propagating?
Let's consider for instance a metal, for which the dielectric function reads:
$\epsilon = 1 - \frac{\omega_{p}^{2}}{\omega^{2}}$
where $\omega_{p}$ is the plasma frequency. The dispersion relation for ...
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Can radio waves with multiple frequencies have the same power?
I was watching data from a spectrum analyzer that shows the frequency of the wave and its power in dBm. I noticed that all frequencies were operating at the same power; however, I don't understand how ...
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Interferometry maximum wavelength for a set of baseline lengths
I am trying to understand phase ambiguity resolution for interferometry. Unfortunately, all I have are old power points and not really any notes or textbooks on the subject.
I have found in a power ...
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Is there an optical/electromagnetic equivalent to the mechanical lever?
From Wikipedia: "Levers can be used to exert a large force over a small distance at one end by exerting only a small force (effort) over a greater distance at the other."
Is there any ...
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Cryogenic and vacuum safe cable labeling or tagging
Not the most in depth of physics questions, yet this feels like the right community for trying to crowd-source this type of expertise.
Essentially it is a question for those of you experienced with ...
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Mechanism of NMR
I am doing research into NMR right now in several different sources, and it seems as if there are two competing models which are used to describe how radio waves can impact nuclear spins. The first ...
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Implementation Question about Thin Wire Method of Moments
I am currently trying to implement a simple $1 \textrm{D}$ thin-wire method of moments code in Python to practice E&M and numerical methods. I am following Gibson's "The Method of Moments in ...
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Time difference of arrival between radar pulses
Consider the following geometry:
Where $\bar{s} = (s_x,s_y)$ is an object of interest, and $\bar{r_c}$ is the location of the radar.
Let the echo delay time of a radar pulse to the scene center be $...
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What type of equipment could generate interference on the 1.6ghz spectrum [closed]
From the perspective of physics - other than wireless computer network devices - what domestic or domestic or commercial equipment could generate seemingly random interference on the 1.6ghz range?
...
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How do lasers work if the phase shift from reflection is 180 degrees?
I am having a hard time wrapping my head around the phase shift that is imposed by reflection. I'm specifically thinking about things like visible light of off mirrors or RF reflecting off of silver, ...
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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 ...
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