Questions tagged [antennas]

The interfaces between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. Antennas are essential components of all radio equipment.

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Can I create a planar inductor using salt water?

I know that in order to create an inductor for applications like NFC/antennas or electromagnets, I would need many concentric coils of a conductive material. I was wondering if it’s possible to create ...
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What is the difference between antenna input impedance and its radiation resistance?

I got to know that the input impedance of the halfwave dipole antenna is given as: $Z_{in}$=1/$I_m$* sin($\beta$(H-|Z|)) where $I_m$ is the maximum current on the antenna( when excited with a 1 volt ...
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Does a receiving antenna radiate the power received?

Let's consider the case of linear dipole antenna or Hertz dipole. I know that the antenna acts as a transducer and it radiates the E-M waves due to the acceleration and decelaration of the electrons ...
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(Why) is the real life observation space spherical?

It is a quite philosophical question. To explain it, let's consider a dish antenna radiating a EM field like in the following picture: Let's image you are an observer who can look the antenna ...
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How important are polarization and magnetization to far-field antenna properties?

Magnetization sources a magnetic field and polarization sources an electric field. Presumably, as an RF current is run through an antenna, it develops locally some magnetization and polarization. In ...
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EM scattering cross section, antenna directivity and its interpretation

The formula (35.25), see Slater, $\bar A_r = \frac{\lambda^2}{4\pi}$ is probably the most important formula in radar and/or antenna engineering. It says that the average absorbing cross section of ...
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QFH antennas and their construction

I just wanted to ask the reason for having two loops in a QFH antenna. and what is the advantage of having them oriented the way they are?
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Wireless Power Transfer in Layered Material

I’m looking to develop a mathematical model for near-field electromagnetic power transfer through layers of human tissue, from an external transmitting antenna to an embedded antenna. Each tissue ...
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Why do field lines kink when a charge is accelerated and not at constant speed?

The emission of electromagnetic waves is sometimes explained by drawing field lines that "kink" when the charge is accelerated. The kink is then said to be an EM wave. The kink formation ...
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Are the electric and magnetic fields of an EM wave also perpendicular in the near-field or the intermediate zone?

Are the electric and magnetic fields of an electromagnetic wave perpendicular to each other even in the near-field zone or in the intermediate zone (when the radiation zone approximation is not valid)?...
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Does changing magnetic field create different Electric field in EM waves?

In em waves does changing magnetic field create different Electric field or it just help already created electric field by ac generator to propagate?
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Scattering off upside-down parabolic reflector (radio frequency)

Suppose I have a steel frame parabolic reflector with a handful of steel wire supports. The size is approximately 10m high and 20m in radius. Assume the gaps in the triangular trusses are roughly 3m. ...
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Finding a rigorous EM book with applications

I'm a mathematician who would like your help getting the right text for EM to suit my needs. I want to learn more a bit about applied EM. Mainly about antennas and transmission lines, reciprocity, ...
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How do electrons flow in a "random single wire antenna" of a Raspberry Pi, which is DC and just have one path? (no gnd)

I have been amazed by the fact that just by inserting a cable in a GPIO of the raspberry pic acts like an antenna, as far as I know, electromagnetic waves are generated when there's current, and also, ...
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Do two antennas need to have the same range in order to communicate?

If one antenna has a range of say 200 feet and a second antenna has a range of only 20 feet, can communication still occur if the first antenna is outside the 20 feet range of antenna 2?
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Em radiation incident on an atom

What happens when the EM wave is incident on a atom? Do the de-localized electrons in the atoms or the valence band electons move in the direction of the electric field? Also when the electrons move, ...
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Potential difference in dipole antenna :

Can some one explain this concept in dipole antenna that how potential difference is working? And it is correct to say when connected to the ac source positive charges move to one pole and negative ...
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Using ham radio to communicate with an underground crew [closed]

In "Stranger things 3" Dustin uses a ham radio named Cerebro (an image https://www.reddit.com/r/StrangerThings/comments/fhrdml/will_cerebro_make_an_appearance_in_st4/) stationed on a hill ...
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Why are smartphone antennas not $\lambda/2$ of the wavelength? Why are patch-antennas used instead of simple dipole antennas? [closed]

Why are smartphone antennas not $\lambda/2$ of the wavelength? Why are patch-antennas used instead of simple dipole antennas? I'm very new to this but really interested in using basic physics to ...
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Can lenses and mirrors be described in terms of beamwidth and directivity?

As a first background, I am an Electrical Engineer with experience in antennae design for microwave bands. Lately, I have been interested by optical devices, and I notice one strange phenomenon: when ...
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How do horn antenna work?

When looking at horn antenna simulations the strength of the propagated wave is directionally dependent.(this is the entire reason they exist) I am wondering how this is able to be done? Basic ...
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Far-field distance of antenna and a wave phase

In a following reference [David M. Pozar 2012, 4ed, Microwave Engineering, p 661], it is told that the far field distance of a relatively large antenna is given from the formula \begin{equation} R_{ff}...
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Electromagnetic energy from a dipole antenna

Looking for references on previously done analytical development of the radiated field very close to a dipole antenna. I am not interested in a spherical coordinate system-based analysis, because that ...
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Far-field in air versus vacuum

I think the far-field is significantly different in vacuum versus air, is this true? More specifically, when we talk about far-field, we mean the electromagnetic wave as a whole etc. but in the near ...
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Why half-wave dipole is the most efficient one in comparison to other electrical lengths?

I have a problem on the understanding of a half-wavelength antenna. Namely, why is the half-wave antenna has a highest power emission. Considering Far Field factor F($\theta$), which is known from the ...
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Quantum description of radio antenna

I am trying to form a clearer picture of how a radio transmitter emits waves from a quantum point of view. The classical description is quite easy: the electrons oscillate in the antenna and, as ...
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Infer 3D location (Height, Latitude, Longitude) based on Signal Strength

Sorry if my question is irrelevant. Let's consider two cases: $1)$ Suppose that we have an Antenna and a tagged individual that can be pinged from the Antenna. Suppose that the individual is pinged ...
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Walkie talkie antenna

how is it possible that the antennas of the walkie talkies are so small, if the frequencies they receive have wavelengths of more than 30 cm? Maybe the why's are quarter wave antennas? and for pulsed ...
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Is it really true that radios work better in mountaneous region than in cities?

I was recently travelling somwhere with my family. During the journey we had to pass through a mountaneous terrain. My mother suddenly switched on the radio of car and it was so clear. Is this true ...
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Josephson Junction - characteristic Impedance

Currently, I am working on antennas connected to a Josephson Junction to detect terahertz radiation. The junction shows a normal state resistance $R_n$ of approx. $0.9~\Omega$. To simulate the ...
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Physical temperature and antenna temperature

I have read some documents on the subject, but until now, unfortunately, I still do not have a good understanding of them, most likely due to personal shortcomings that start from physics. In this ...
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Does antenna Q cause distortion?

Everyone knows that a 1/2 λ dipole has on the elements a standing wave the voltage and current of which are a bit less than 90° out of phase with each other, and the departure of this phase difference ...
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Which terms in the equations for dipole impedance include inductance/capacitance and velocity factor?

In the below equations for the impedance of a center fed half wave dipole, exactly which terms include self inductance/capacitance, and which terms cater for an antenna embedded in a homogeneous and ...
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Is self-inductance and velocity factor included in equations for half wave dipole imdedance?

Can anyone tell me if the equations below for the impedance at the center feed points of a half wave dipole include terms for self inductance and velocity factor of the elements, and if so provide an ...
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Does the self-inductance and velocity factor of the elements of a dipole antenna change its resonant frequency?

Below are well known equations from Wikipedia for the resistance and reactance in the impedance of a dipole antenna. where a is the radius of the conductors, k is the wave number 2πf/c, η0 denotes ...
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Hertzian dipole fields transformation after rotation and translation

For a z-directed Hertizan dipole at the origin, the field equations in spherical coordinates are $$E_r\left(r\right)=\frac{Il}{2\pi}e^{-jkr}\left(\frac{\eta}{r^2}+\frac{1}{j\omega \epsilon r^3}\right)\...
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Would increase the size of an antenna theoretically improve the ability to capture signals, if we allow changing the shape?

I have an argument with my friend about drone radio systems. My assumption is this: the bigger the receiver the better reception is possible to achieve. The logic behind this is geometry: transmitter ...
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Microwave relay antennas

Looking over the history of microwave relays I can see there are four distinct antenna design "periods". The very early systems used simple parabolic reflectors with dipole antennas, often ...
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What is the explanation behind radio shadowing by people?

I called it shadowing but masking may be more appropriate, feel free to suggest a better question title after you read the following. I have an old clock alarm radio receiver that plays at the center ...
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What is the radiation pattern for an electric dipole rotating in the horizontal plane?

In trying to answer $\sigma$ transition and angular momentum conservation the following question arose. Suppose we have an electric dipole $\boldsymbol{d}$ which is oriented and rotating within the ...
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Can EM (radio) wave oscillation interact with an antenna without photon absorption?

EM wave interaction with an antenna has been covered, e.g.: How do we visualise antenna reception of individual radiowave photons building up to a resonant AC current on the antenna? Most answers ...
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Isn't induced EMF in an antenna dependent on the power of the electromagnetic wave?

The induced EMF in an antenna is often given as $E\cdot h$, where $h$ is the height of the antenna and $E$ is the electric field strength in V/m, but this does not seem to capture the power of the ...
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Does less than half a wavelength antenna spacing help?

In Metamaterial based Holographic Beamforming (Holographic Beam Forming and MIMO), the spacing of the elements is much lesser than half wavelength. The inventors claim that it works better than the ...
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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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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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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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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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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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3 answers
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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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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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