Questions tagged [antennas]

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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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23 views

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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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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2answers
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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
32 views

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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33 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
20 views

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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1answer
12 views

How to calculate the reflection efficiency for an antenna? [closed]

I want to calculate the reflection efficiency of a transmitting monopole antenna which is connected to a 50 ohm lossless transmission line. And I know that the input impedance of a monopole antenna is ...
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24 views

In an AC Current, if the voltage is high, does this “amplify” the radiowave emissions without changing the frequency?

Hello My question comes from my interest in antennas i suppose: I understand that antennas utilize alternating current to generate EMF Waves along a particular frequency depending on how quickly the ...
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1answer
18 views

Converting $\frac{dB\mu A}{m}$ to $\frac{dB\mu V}{m}$? [closed]

I am trying to convert $\frac{dB\mu A}{m}$ to $\frac{dB\mu V}{m}$ So, I know that; $\frac{dB\mu V}{m}$ = $\frac{dB\mu A}{m} + 51.5$ However, I cannot find a source explains where 51.5 comes from. ...
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Dipole oscillators question from Feynman lectures 1.28-29

I am confused by the Feynman lectures Vol1Ch28-29. In all the pictures, there is an electric dipole oscillating vertically. We're assuming the intensity is $E^2$ and that in the far r limit $E$ ...
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1answer
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Derivation of the Gaussian Beam Pattern

I am currently reading this book by Mireille Levy. It says (on page 40) that the Gaussian beam pattern with half-power beamwidth $\beta$ is gien by: $$B(\theta)=A\text{exp}\left(-2\text{log}2\frac{\...
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1answer
33 views

Why is the effective area of a dipole antenna receiver greater than it's actual size?

The receiving ability of a dipole antenna is much greater than what is attained when doing calculations involving a plane wave interacting with its actual size. Why is this? How can an antenna pull ...
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1answer
100 views

How do antennas work?

I know the basics about em radiation. However, I’m not in college, and I’m not a genius. I’m really looking for someone that’s willing to help explain this to me in a relatively simple way, without ...
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Simplified model of antenna polarization loss

I need to write a very simple model to estimate the $dB$ value of the polarization loss between two antennas (in realtime). Such antennas are common (yet identical) omnidirectional dipole antennas, ...
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15 views

Electromagnetic radiation and angular power distribution in time vs. frequency domain

So I am taking an electrodynamics course right now where we are currently learning about radiation from antennas. In our lecture, my professor had derived a formula for angular power distribution ...
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72 views

Source of cylindrical electromagnetic waves

I have a question about waves with cylindrical wavefront. Precisely, I have read that they may be generated by a linear source, while for instance plane and spherical waves are respectively generated ...
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29 views

Why are the electric and magnetic waves produced by an antenna in phase?

From my understanding, an antenna produces an electric field when charges are separated, and produces a magnetic field when they are flowing. However, when the magnetic field is at a maximum, the ...
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31 views

Inverted antennas

Im trying to draw the E and H plane for radiation pattern for two-parallel-dipoles-in-out-of-phase-by-180, in other words I have two inverted anntena dipoles with same distance from each other in ...
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1answer
15 views

Derivation of the total received field in ground reflection model

I am reading this book by J.D. Parsons. The problem concerned, is how to find the propagation model for propagation over a curved reflecting surfac, i.e.: I would like to know where is the ...
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2answers
147 views

When does an EM wave become a photon?

Consider a small LC circuit which is oscillating at some rate. The wavelength is VERY long compared to the diameter of the inductor. (100's of meters) Beyond the radiansphere, where the EM waves are ...
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How does a mobile patch antenna receive the correct signal in any orientation and rotation?

For instance if we take a dipole and then rotate the receiving dipole 90 degrees such that it forms a cross against the transmitting dipole, would the signal be retrieved at all? If it were rotated ...
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1answer
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Is there any factors depends for electromagnetic waves?

I am staying in hostel. One person said me that he is getting better connection with the wifi when the door is opened. I am wondering that what an electromagnetic wave can do with the door? Is there ...
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2answers
44 views

Near-field energy transfer

If you absorb energy in the near field of the antenna, it will produce a loading effect on the source. Whereas in the far field it will not. Is there an intuitive explanation why this is true for one ...
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1answer
58 views

How is cosmic microwave background radiation experimentally detected?

Can it be detected from the earth with a single dipole antenna or a series of such antenna forming an interferometer (with an electric circuit to filter out the noise due to radio stations and ...
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Can Coupled Mode Theory be used to calculate Antenna Aperture?

A typical metric when discussing electromagnetic wave scattering problems is the scattering cross section, typically defined as the ratio of scattered power to field power density: $\sigma_{scat} = P_{...
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Finding Flux-linkage of incident field on square-loop antenna

I'm having a hard time grasping this problem. I have previously found and sketched a time-harmonic E and H field in a source-free area of a lossless medium, coming from a WLAN hub. I have an antenna, ...
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32 views

How can the electric field lines be separated from an antenna?

I found many YouTube lectures about the antenna. For example, check the following one: E&M Radiation (8 of 22) Dipole Antenna Radiation Pattern. Question. Why don't the electric flux lines ...
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Broadband Wireless Access Technology

I am looking for a textbook or online resource that covers the physics of 4G and 5G, perhaps even 3G. having read about the new energy levels of each generation, I am curious and interested in ...
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2answers
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What are some examples of physical phenomena where time varying magnetic fields are used for generating electromagnetic radiation?

The most common method of generating electromagnetic radiation is by using oscillating charges, for instance in antennas. However, I couldn't think of an example where varying magnetic fields are ...
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131 views

How did German radio beams reach distant English cities during WWII? [closed]

During WWII, the Germans were using radio beacons (the "Knickebein" system) to guide their bombers into English territory. They set up two beacons, one in Kleve, a city in West Germany, and one at ...
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Are there sources for potential wave solutions for which the electromagnetic far-field is zero?

Are there sources for electromagnetic potential propagating wave solutions for $\phi$ and $\overrightarrow{A}$ for which the electromagnetic far-field $\,>\!\lambda\,$ in vacuum is zero $\...
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51 views

Where does the energy go to for a charged sphere with pulsing radius?

In the question A charged sphere with pulsing radius the answer says that the charged sphere does not radiate. However, compressing a sphere of charge to a smaller radius requires work, so where does ...
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1answer
26 views

Does the microwave generating component in a microwave operate under the same physics as an antenna radio transmitter?

Essentially does the component of a microwave which generates those microwave photons operate using an AC power supply connected to a broken circuit which causes an oscillating dipole to radiate ...
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1answer
103 views

What is the maximum and minimum of antenna gap of dipole antenna?

Consider the picture below is a design of half wave length ($\lambda$) dipole antenna. I read some explanations saying that it is the best design to get the optimum transmit power. Or it is cut to $0....
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1answer
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Radiation EIRP impact on organisms

I read some documentation about how harmless the non ionizing radiation can be and it looks like the arguments are around the frequency(hence the energy) of the wave. In the sense that a particle with ...
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49 views

Which is the physical reason for why a dipole is an isotropic radiator?

Good morning. I was wondering which is the exact physical reason for why a dipole (e.g. a $\lambda/2$ dipole) has the equal radiation pattern in all directions. I have founded many formulas but I can'...
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81 views

Poynting vector flux on a closed surface

What does the flux of the Poynting vector through a closed surface represent? I know that the flux of the Poynting vector on a certain surface represents the power flowing through it. But if the ...
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EM waves from AC current in a loop?

The video How does an Antenna work? shows a loop of wire and says it can't be used as an antenna. I am confused because I thought electrons under AC current could be considered as accelerating ...
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1answer
50 views

Electric Field around an avalanche beacon?

I'm currently working on a scientific work regarding electric fields created by avalanche beacons. My question is what kind of field is created around the antennas of such device? Is it a dipole (2 ...
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43 views

Proof of differential and integral identities used in electromagnetics radiation

I am starting my studies in antenna theory and need help to prove the following identities: I) $\nabla^{'}\left[\mathbf{J}(\mathbf{r^{'}})e^{j\mathbf{k}\cdot\mathbf{r^{'}}}\right] = j\mathbf{k}\cdot\...
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1answer
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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
98 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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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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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
227 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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