Questions tagged [antennas]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
0
votes
0answers
14 views

Dipole antenna in water and air at the same time [closed]

A dipole antenna is partly immersed in air and partly immersed in water. More precisely, the middle part of the antenna is in air and the two dipole ends are in water: The question is: what is the ...
3
votes
2answers
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 ...
0
votes
0answers
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 ...
0
votes
1answer
12 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 ...
1
vote
1answer
105 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 ...
0
votes
0answers
16 views

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 ...
0
votes
1answer
23 views

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 ...
1
vote
2answers
40 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 ...
1
vote
1answer
57 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 ...
0
votes
0answers
10 views

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_{...
0
votes
0answers
19 views

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, ...
0
votes
0answers
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 ...
0
votes
0answers
22 views

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 ...
0
votes
2answers
48 views

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 ...
0
votes
1answer
121 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 ...
0
votes
0answers
32 views

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 $\...
0
votes
2answers
42 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 ...
0
votes
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 ...
0
votes
1answer
94 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....
0
votes
1answer
16 views

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 ...
0
votes
1answer
60 views

How a real dipole physically works

I'm trying to figure out how to 'see' this object, specifically a $\lambda /2$ dipole. The ideal $\lambda/2$ dipole model is represented by a current distribution along a line (of this length, in fact)...
0
votes
1answer
45 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'...
0
votes
1answer
51 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 ...
1
vote
0answers
63 views

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 ...
0
votes
1answer
45 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 ...
1
vote
0answers
36 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\...
0
votes
1answer
45 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 ...
0
votes
1answer
85 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 ...
0
votes
0answers
31 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 ...
0
votes
1answer
48 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, ...
0
votes
0answers
22 views

Electrons cannot sense electrostatic force?

Why there is not voltage or current just for 1ms if I connect a multimeter ground to the negative terminal of a DC power supply or charged capacitor? Why electrons in measure lead and DMM device ...
0
votes
1answer
43 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 ...
1
vote
1answer
190 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 ...
0
votes
1answer
32 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 ...
3
votes
4answers
141 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 ...
28
votes
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?
1
vote
1answer
80 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 ...
1
vote
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 ...
1
vote
1answer
119 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?
-1
votes
1answer
229 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
votes
2answers
124 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 ...
0
votes
1answer
33 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 ...
0
votes
1answer
42 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 ...
1
vote
1answer
77 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
vote
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 ...
0
votes
2answers
68 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 ...
11
votes
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 ...
1
vote
1answer
40 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, ...
-1
votes
2answers
135 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?
1
vote
1answer
50 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?

1
2 3 4 5