Questions tagged [antennas]

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1answer
51 views

Basic question on Poynting flux and antennae

I am looking at a question (though my queries here are more regarding th underlying conepcts, which I don't quite understand) about a radio antenna that is polarising a conducting sphere. For ...
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1answer
101 views

Why aren't extremely-low-frequency (ELF) radio waves used for underwater radar?

Since extremely-low-frequency radio waves are used by submarines for some simple, low-transmission-rate communications, why can't those same wavelengths be used for submarine radar? It may not be ...
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0answers
32 views

What is the current in a short linear antenna in an alternating $E$-field?

Consider a short linear wire with some low but non-zero resistance, such as a copper wire. If this wire is then placed in an alternating electric field that is oriented parallel to the wire, the field ...
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1answer
51 views

What part of the energy applied to accelerate electrons radiate and why?

I understand that accelerated electrons will emit radiations, for example in a cyclotron or antenna. However I dont understand why some energy will sort of stay in the electrons and accelerate them, ...
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1answer
44 views

Is the electric field from light independent from the light magnetic field?

I was reading about antennas and I found an statement somewhat like this "loop antennas are more efficient compared to other antennas when close to the human body since the human body interacts more ...
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1answer
233 views

Spherical wavefronts from a linear electric dipole-antenna

We are given the equation \begin{equation*} \vec{E}=\hat{u}_\theta\ p_0 \sin{(\theta)}\frac{k^2}{4\pi\varepsilon_0r}\sin{(kr-\omega t)}~, \end{equation*} that describes the far field radiated ...
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1answer
34 views

Relevance of electromagnetic multipole transitions

In what kind of systems higher electromagnetic multipole transitions (like electric quadrupole transitions) become important or at least measurable? Is it for antennas in radiofrequency? Is it in the ...
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4answers
162 views

How does an antenna capture the energy of EM radiation?

I am having trouble reconciling the descriptions I have read of how antennas work to recieve and transmit radiation. My simple understanding is that in recieveing the oscilating electric field of an ...
29
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10answers
7k views

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?
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1answer
88 views

Explaining receiver antennas without using Reciprocity

I am trying to understand the physical behavior receiver antennas, but I am unsatisfied with all the explanations I have seen so far in my textbook (Stuzman & Thiele, Antenna Theory and Design) or ...
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2answers
53 views

How can be various frequencies in one place?

How can we receive different radio shows in one place, they are coded in phase or frequency or amplitude of electromagnetic waves, but shouldnt elecreomagnetic waves interact each other, and all ...
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1answer
168 views

Is a radio receiver antenna a capacitor?

I think so, but its capacitance should be very small, so how can the antenna correctly receive the signal, the low capacitance wont bother the signal?
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1answer
327 views

How do I calculate voltage in a loop antenna?

How do I calculate the effective source voltage applied to the loop by assuming that the loop is perpendicular to the wave magnetic field and that the transmitter transmits the wave evenly in each ...
2
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2answers
140 views

Is it possible to use visible light to transmit “radio” like AM/FM?

When I see a big radio antenna, I like to imagine it's a giant incandescent light bulb filament in the vertical, but of a invisible light. So that it "glows" the radio, performing oscillations which ...
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1answer
34 views

Identify the frequency antenna I can't access

I recently bought a (used) radio controlled airframe and buried deep inside the tail section is an antenna, broken out to some coax, with an SMA connector within the core of the fuselage. There is no ...
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1answer
48 views

Electrodynamics confusion - Hertzian dipole

I am studying a course in Electrodynamics and we are just covering retarded potentials and the Hertzian dipole. In my lecture notes, we have calculated the magnetic vector potential $A$ in the ...
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1answer
85 views

Why does S11 seem to be independent of transmission media in this RF transmission experiment?

Hi friends I am starting to characterize a system in which we will plan to measure the response of a sample of liquid to radio waves. In order to do this I am exciting two antennas with the ports of ...
1
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0answers
31 views

How is New Horizon able to send back data to Earth? [closed]

New Horizon is to send back images of Ultima Thule, 6.4 billion kilometres away. Barred having a very large power to send information back on a very large angle, it seems to me that the level of ...
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2answers
72 views

How doesn't signal amplification by means of antenna defy the laws of thermal dynamics?

According to this video you can amplify a signal by using the water in your head (although you could do the same thing with an antenna.) Does holding the key close to your head somehow drain the ...
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7answers
2k views

Why are longwave radio towers so tall?

They are often in the hundred meters high. See the wiki link: https://en.wikipedia.org/wiki/Longwave Why are they so tall? Because the wavelength is long, so the antenna should also be long? But to ...
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1answer
43 views

What's a word that encompasses both transmit and receive?

Is there a word that can be used to describe in a general sense the action of an antenna, whether transmitting or receiving? My best guess is "couple" as in coupling energy from a wire to EM wave, ...
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2answers
139 views

If EM waves of radio range can pass through large obstacles, why antennas do receive them? [closed]

For example a tiny antenna in the phone, why em wave do not pass through it, but pass through concrete walls?
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1answer
51 views

Best cantenna reflector type

If the reflector (closed end) of cantenna is shaped as parabolic dish, will it perform better than one with a flat reflector?
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3answers
708 views

Can a lens be used at radio frequency?

Lenses and Antennas are two means of manipulating EM waves. While antennas are predominantly used at RF / Microwave frequencies, Lenses are used for optical/IR regions of the spectrum. I understand ...
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1answer
62 views

Magnetic fields for communication or remote interaction with devices [closed]

My understanding is that the electric fields are primarily what is used in communication technology and what most antenna design based around. And I am aware of loop antennas that act as an induction ...
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1answer
36 views

Does half wave in dipole means that duration of 1/4 of signal cycle should equals capacitor charging time, if we consider that dipole is capacitor?

Does half wave in dipole means that duration of 1/4 of a one signal cycle should equals capacitor charging duration, if we consider that dipole is a capacitor?
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2answers
33 views

Antennas behavior in different dielectrics

I tried to set the antenna in the dielectric, thinking that the minimum frequency of the S-parameter would move, but it began to behave very strangely. What shoudl happens to the operating frequency ...
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1answer
67 views

How does energy transmission work with antennas?

If we surround an energy emitting radio antenna by large number of energy absorbing radio antennas, do the surrounding antennas have a large effect on the central antenna? The emitting antenna emits ...
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0answers
42 views

How can we excite transverse magnetic or electric ( TM or TE) modes on metallic mediums?

I would be very thankful if someone can point me at references on explaining theories of excitations for transverse magnetic or electric modes. In engineering, the literature has plenty of papers on ...
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0answers
40 views

Dipole Receive Antenna: Derivation of circuit representation

Many textbooks cover generation of electromagnetic radiation via transmit antenna and then invoke a reciprocity theorem for the antenna acting as a receiver. I'm wondering, can you derive the ...
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3answers
478 views

How do near-field EM fields change to far-field EM waves at an antenna

What is the mechanism by which the near-field non-traveling electromagnetic fields in the vicinity of an antenna are changed into the far-field electromagnetic waves traveling at the speed of light? ...
4
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4answers
430 views

Antenna Radiation

I am confused by something I have seen discussed but not with full clarity: it has to do with understanding antenna behavior at the quantum level for its own sake (yes, classical approach is easier ...
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1answer
201 views

Has the double-slit experiment been performed with an antennae detector?

My layman's understanding of one aspect of the wave/particle duality is that when we attempt to detect wave behavior we observe wave behavior, and when we measure to detect particle behavior we ...
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0answers
26 views

About the center of phase and the geometric center of an antenna

Suppose we have two antennas $A$ and $B$ in a near-field configuration. Let $P_A,P_B$ centers of phase and $G_A,G_B$ the geometric centers of $A,B$ respectively. $A$ emits a signal which is captured ...
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1answer
350 views

Why is the beamwidth of this antenna shown like this ?

I am not yet super familiar with the way beamwidth are calculated, but from what I've read, it's based on the point where the strength of the signal reaches -3dB. Here is an excerpt from CCNA ...
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2answers
290 views

How to tune the frequency of a coil?

I'm doing a scientific fair project. I want to do it about wireless energy transmission. In practice consists of two coils NOT physically connected. One is the transmitter of energy, which is ...
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1answer
64 views

Radiation Pattern for a Dipole Antenna with non-sinusodial forcing voltage

I was wondering, how does one derive the radiation pattern for a dipole antenna if you don't assume a sinusoidal forcing function? It seems this assumption kind of ignores the most important question ...
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1answer
93 views

Why do satellite dish come in many different sizes?

Is it to do with how many photons can be absorbed in a given area? more surfaces mean more photons are absorbed but does it translate to better quality? btw can a single atom be as good as the largest ...
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1answer
532 views

Beamwidth of reflector antenna problem

The beamwidth of a reflector antenna of diameter $=70 $ cm at $30$ GHz is: a) $3.7$ degree b) $0.5$ degree c) $1.1$ degree d) $1$ degree My Approach: We Know Directivity : $D= \frac{4 \pi}{\...
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1answer
399 views

The radiation pattern of two parallel dipoles in out-of phase by $90°$ [closed]

How can we find the radiation pattern of two identical and parallel dipole antennas which are kept apart by a distance of $\lambda /4$ in the $H$-plane? Given: They are fed with equal currents but ...
2
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2answers
415 views

How does a receiving antenna work given that the electric field is 0 in conductors?

The question of how a receiving antenna works has been asked on this site before, such as here How does a receiving antenna get an induced electric current? and here How does a receiving antenna work?....
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3answers
179 views

L-band for search radars?

I'm making my way through the Linesman-related section's of Gough's Watching the Skies. Linesman was developed to counter the carcinotron jammer. The main solution was the Type 85, which had 12 ...
2
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2answers
139 views

Electromagnetic wave [duplicate]

Light is electromagnetic wave. There is continuous vibration of electric and magnetic field in the light. It means that I I place a conducting coil in the light. There will be change in magnetic flux ...
4
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2answers
213 views

Receiving AM radio through household wiring

A friend's post on FB made me recall a similar situation. In his case, his IR-linked headphones now play a local AM station at very low volume whenever he's near an electrical outlet or switch in his ...
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1answer
161 views

Far field horizontal polarization component of a dipole antenna

An infinitely thin dipole radiates an electromagnetic wave which in the far field is vertically polarized, with zero horizontally polarized component. I am trying to understand what happens when the ...
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1answer
235 views

Patch antenna excitation

How can we excite the patch antenna in different modes like TM10 and TM30? If a microstrip patch antenna resonates at TM10 mode can it resonate in other modes also like TM30?
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1answer
128 views

Does a Dipole Antenna have modes?

I often see solutions to radiating, center fed dipoles with "approximations" of the current distribution being sinusoidal. When describing this, people often refer to the "Boundary Conditions" as no ...
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1answer
81 views

Energy conservation in antenna

When electrons oscillate in antenna the do work against resistance which becomes heat so from where it gets energy to create radiation
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1answer
2k views

How do antennas generate photons?

I understand that in order to most quickly understand all aspects of electromagnetic radiation, the photon representation should be used in some cases and the macroscopic electromagnetic wave ...
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1answer
1k views

Standing waves in antenna: why is the current wave symmetric with respect to the centre?

In trasmitting antennas the current is described with a standing wave (in resonant configuration). Since the current must be zero at both ends of the antenna (it could not be otherwise) the mechanical ...