Questions tagged [antennas]

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18 views

Do EM waves produced by a dipole antenna propagate along 1 plane?

The plane would be perpendicular to the dipole and passing through it. Do em waves propagate in all directions of that plane or in any direction of any plane?
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Effect of bends on the radiation pattern in monopole and dipole seawater/liquid antennas (Reference Request)

I'm aware of the radiation pattern and the electromagnetic theoretical analysis behind monopole and dipole seawater/liquid antennas (cf.Investigations of Water-Based Liquid Antennas for Wireless ...
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Voltage distribution around a dipole for different wavelengths [closed]

It is well known that the alternating current distribution in a center-fed dipole antenna (black line) is as follows: Despite the different wavelengths, the current distributions depicted above all ...
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1answer
19 views

What does an external test charge feel along the dipole axis of radiating antenna?

I have been reading about how radio dipole antennas radiate "bubbles" of electromagnetic waves, due to the kinking of the field lines from the acceleration of the charges. The bubbles that ...
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3answers
78 views

Why is it that we never see visible spectrum waves being generated by oscillating currents in an Antenna?

Studying Maxwell's equations brings surprising clarity and odd questions to my mind. In this case, I was going through the Point form and Time Harmonic form of the four equations and a line in my ...
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1answer
25 views

Why do we need special geometry for telecommunication antennas?

In many youtube videos about building low power radio transmeters , I have seen a piece of regular wire or extension cord used as a antenna. But , it seems like telecommunication antennas have ...
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3answers
180 views

What is the difference between a radio antenna and a capacitor?

As I understand it, a mobile phone and a base station communicate by exchanging photons. The antenna is like one half of a capacitor. Pulsating voltage is applied to the conductive antenna and that ...
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2answers
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What is the diff in operational range underwater between a dipole trimmed to resonate @2.45GHz versus a dipole tuned to resonate using a pi network?

(Sorry about the use of "diff", but I needed the characters.) In air, a half-wave dipole can be physically trimmed to resonance and will radiate well. It can also be tuned using a pi network ...
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3answers
44 views

Why is the half-wave dipole the most used antenna design?

When producing em waves using a dipole antenna (of length L), you could theoretically use any L and adjust the frequency of the oscillating voltage to get the desired wavelength. Then why are most ...
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4answers
140 views

Why are the electric and magnetic fields in phase in EM waves?

Consider a radio wave emanating from a dipole antenna. Close to the antenna, I would think the electric and magnetic fields would be 90 degrees out of phase (i.e. if the strength of the magnetic ...
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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 ...
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S Parameters in HFSS

I'm new to HFSS Software and I'm trying to plot Scattering parameters in HFSS for a dielectric resonator excited by a probe (shown in image). I know that the top cylinder has a higher dielectric than ...
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2answers
49 views

In an antenna how fast do electrons move when receiving a signal?

In an antenna how fast do electrons move when "receiving a wave"?
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What is the distribution of photon frequencies emitted by an antenna powered by AC?

This is a follow up on answer by Holger Fiedler here: https://physics.stackexchange.com/a/622802/230132 He writes that an antenna powered by alternative current (AC) will emit other frequencies than ...
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1answer
35 views

What is a resonance element?

I'm looking at a horn antenna and why its a good addition to a waveguide. I've read that the horn can operate at a wide range of frequencies since it has no resonant element. But I can't find a good ...
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1answer
33 views

How do the arc and the straight line have equal length in this figure?

I have the following derivation in one of my physics problem related to range of a transmitting antenna: How is the distance QT equal to the length of arc PT? Can someone provide a quick explanation ...
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Is the curl of this electric field zero and is ampere law true?

let's consider this couple of endless conductive cones connected to an AC voltage source: As shown in the picture, it generates an electric field on circumferences along $\theta$, and a magnetic ...
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3answers
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Can molecules absorb radio waves efficiently through quantum state transitions?

In the classical world, a radio antenna designed for operation at a certain wavelength must be close to the same size as the wavelength – typically within about one order of magnitude. Otherwise, the ...
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23 views

What is the gain of an isotropic antenna?

My question is simple: What is the gain of an isotropic antenna in dBi? I cannot find a consistent answer on the internet. Some sources define it to be 0dBi and others define it to be 1dBi. Thanks.
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1answer
21 views

Why we express the radiation pattern of an antenna array in terms of electric field and not magnetic field?

Is this because in the farzone we are talking about TEM waves and the relation of the magnitude of the field is E=ηH, therefore the patterns of the fields should be identical in terms of shape and ...
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2answers
33 views

Radar lobes vs forever propagating waves

I'm trying to learn some more about radar and I'm confused about "lobes". My understanding of EM radiation is that it radiates forever or until something physical blocks it, but directional ...
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1answer
66 views

Are Field Beams Possible?

I was recently wondering why the field force between two objects is proportional to the square root of the distance (sometime you just take things for granted). This comes from the inverse-square law, ...
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21 views

How is the synthetic aperture of a radar done?

In the case of SAR (synthetic aperture radar), the Doppler effect is used to simulate an antenna (syntetic) whose resolution is better. What I would like to know is how this is done in detail, what ...
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11 views

How to find the dimensions of a patch antenna of a horn shape?

So i was asked to make a horn-shaped patch antenna, but i'm having hard time to find its dimensions. Is there a relation between a real horn antenna dimensions and a patched one?
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Are there two groups of charges associated in an alternating current half wave?

Edit: to better express the problem: As alternating current passes along a wire: Individual current charges produce a magnetic field after zig zagging rapidly locally with a small resultant vector. ...
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1answer
42 views

Need 50Hz detection antenna for reception of AC mains signal

Anyone know how can i design antenna for electric field detection. i m trying to make contactless Ac mains Detector which range is near about 1-mtr long. my question is about possibility of antenna ...
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How do minimum scattering antennas extract energy

In our antennas and fields course, we had one task where we simulated a line antenna in an incident plane wave. There, we specifically looked at the fields produced by scattering from the antenna. ...
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1answer
132 views

Circular antenna array

At page 20 of http://read.pudn.com/downloads773/doc/comm/3065154/5237234.pdf the two formulas that characterize the behavior of a linear and a circular antenna array are compared. The reason why I ...
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Is the phase of the retarded electric field between the antenna tips the same as that of the magnetic field in contrast with a standing wave?

In case of λ/2 dipole antenna, is the phase of the retarded electric field between the antenna tips the same as that of the retarded magnetic field in contrast with a standing wave? I know dipole ...
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2answers
131 views

Exactly what is the cause for the zero reactance seen in the impedance at the center feed point of a resonant half wave dipole?

Lots of explanations i've read on the internet written by ham radio operators and in some text books state that a half wave dipole is resonant when the inductive and capacitive reactances cancel out. ...
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42 views

How to apply Maxwell's equations to electromagnetic waves? [closed]

You know how antennas work and create EM waves...They oscillate charges which create EM waves Well I'm not satisfied with that explanation so it would be great if you could use maxwells equation to ...
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2answers
147 views

Why do electromagnetic waves detach from the antenna?

In most explanations of how antennas work they say that because of kinks in the electric field of an antenna meets at one point and detaches... I looked up reasons for why this happens but all I got ...
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2answers
43 views

Does an antenna emit EM radiation in all directions?

You know how an antenna works? It creates EM fields and oscilates them so that it creates electromagnetic fields...Well does that electromagnetic wave travel in a straight line or spread out ...
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24 views

Capacitance of a dipole antenna

let's see this evaluation of a dipole antenna capacitance. Here my reference. The text states that: The key assumption is that the electric field lines from one arm of the dipole antenna to the other ...
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32 views

Finding the integral equation for the current on a thin solid PEC by using the equivalence principle

There's a thin solid PEC (perfect electric conductor) cylinder of length $L$ and radius $r$ aligned with the z-axis and a plane wave $\vec E$ is illuminated. When neglecting the current on the endcaps,...
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25 views

How to measure low-frequency electric fields?

What is the technique to measure the spectrum of an electric field (in empty space) between 1Hz and 10kHz? Is there some kind of antenna which can be connected to an oscilloscope/spectrum-analyser? ...
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4answers
124 views

Wave and particle nature of light during detection in a single-photons double slit experiment

I am just a curious physics student. This question is about the nature of light. In a single-photons double slits (or multiple slits) experiment, the interference pattern or the distribution of the ...
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Lossy electromagnetic wave and reactive power

Consider an electromagnetic wave in a lossy medium. This medium has a complex permittivity $\varepsilon_r=\varepsilon_r' +j \varepsilon_r''$ and a wave impedance given by $$Z=\frac{E}{H}=\sqrt{\frac{\...
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2answers
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In LC circuit, the EM waves specifically originate from where?

In LC circuit, we know that there will be charge oscillations, so emw will be produced. I just want to know emw will originate from where? Will it originate from within the capacitor? Or will it ...
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How to estimate power received by an antenna considering multipath effects?

I have the gain plots of a transmitting and receiving monopole antenna as shown in the image below. The axis of both antennas are coincident thus using the Friis Transmission Equation gives zero ...
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81 views

Calculating magnetic field from vector potential for half wave dipole antenna

Given the far-field vector potential... $$\vec A_{\text{ff}}(\vec r) = \left(\frac{\mu_0I}{2\pi kr}e^{-jkr}\frac{\cos\left(\frac{\pi}{2}\cos(\theta)\right)}{\sin^2(\theta)}\right)\hat z$$ I need to ...
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1answer
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How do you polar-plot the complex-valued array factor (AF) of a phased antenna array? [closed]

If a normalized array factor for an $N$-element linear antenna array (without per-element phase shifts) is calculated as $$ AF = \frac{1}{N}\sum_{m=0}^{N-1} e^{jmkd\cos\theta} $$ where: $\theta$ is ...
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1answer
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Waveguide operation frequency range

Google told me the operation frequency is 125% - 190% of the cutoff frequency. But I did not find any other material for reasoning. My question: What happens if frequency was too high?
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Strange approach to the line-fed slot antenna analysis

There is a beautiful demonstration, available in the text Robert S. Elliot, Antenna theory and Design, Wiley-IEEE Press, page 17 (Stratton-Chu solution), which shows how the electromagnetic field at ...
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1answer
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Why does an ideal antenna not absorb all incident power?

http://farside.ph.utexas.edu/teaching/em/lectures/node94.html states: So, in the optimum case half of the power absorbed by the antenna is immediately re-radiated. Clearly, an antenna which is ...
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Help understanding the “Nyquist Criterion”

I am researching the split-step parabolic equation and its split step solution as in: Ozgun, Ozlem & Apaydin, Gokhan & Kuzuoglu, Mustafa & Sevgi, Levent. (2011). PETOOL: MATLAB-based one-...
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I'm unable to wrap around the concept of antennas and more specifically microstrip antennas. How does a microstrip antenna which is bended work?

So, I had come to the agreement that to understand how antennas work, it is best to assume light/electromagnetic waves act like photons/particles in the presence of recieving antennas. And therefore a ...
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1answer
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Loop antenna $E$ and $H$ modes without approximations as to the distance or wavelength

In Jackson's 9.14 we calculate the expressions for $E$ and $H$ of a loop antenna with current $I=I_0\cos(\omega t)$ in the $x-y$ plane in the radiation zone, where we approximate the distances to be ...
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1answer
101 views

Magnetic field closed loop lines in an electromagnetic wave

let's consider the simplest example of electromagnetic waves: If we consider only this picture, it seems that the magnetic field lines are not closed loops, which is clearly not possible. I know it ...
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1answer
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Polarization of Patch antenna - Intuitive explanation

I was looking into the polarization of patch antennas (for GNSS use). I found this website (http://kempbros.github.io/antennas/Patch_Antenna_Generator/) which gives the geometry of the antenna ...

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