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1answer
77 views

Light is/produces a electromagnetic wave, but is the opposite also true?

A similar question has been asked, and to some extent the answer is: you need an antenna with $L \approx 200nm$. But it's not entirely clear to me if this is completely hypothetical, as "Marek" also ...
3
votes
2answers
73 views

How do radio antennas follow electrical rules

Power = current times voltage. However current requires a complete circuit to 'flow'. How can a 1,000 Watt antenna work if the output of the transmitter is connected to a single emitting wire or ...
1
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2answers
47 views

Will radio waves bend to reach receiver?

I was wondering if receivers just catch the radio waves that pass through, or if they actually attract the waves like a magnet. In other words, will a radio wave moving in a straight line bend in ...
1
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0answers
21 views

Is it possible to make an antenna that's more compact than a split-ring resonator antenna? [closed]

I'm aware that a split-ring resonator antenna works efficiently by being half the size of a classic antenna. But is there any design that takes even less space? I have in mind millimeter or micrometer ...
0
votes
2answers
79 views

How exactly do radio waves work?

So I understand that a radio wave is created by electrons oscillating in the antenna. But how does this create a radio wave? How are the electric and magnetic field "emitted" by the electrons? If you ...
1
vote
3answers
93 views

How does the tuner really work in a crystal set?

I would like to know how and why the tuner works in a crystal set radio. I already know all the theory and the "formal definitions" (this creates a resonant circuit which responds to an alternating ...
0
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0answers
16 views

Radio wave propagation path loss coefficient below 2?

We did a series of measurements of radio wave signal strength. The setup was between stations on top of flat roofs of neighbouring houses (with line of sight). We are interested in the path loss ...
1
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1answer
53 views

Dipole antenna $E$-field direction

I was looking up dipole antennas on Wikipedia, among other sites. I came across this animation: and was wondering why there wasn't any field lines in perfectly vertical direction. In the case of a ...
1
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2answers
37 views

Do a receiving antenna interfers with an emitting antenna?

As I understand: Accelerating electrons generate electromagnetic waves. An emitting antenna have an alternating current (electrons are moving) which generates an electromagnetic waves. The ...
1
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0answers
65 views

How is my guitar cable picking up clear radio-signals?

I've recently noticed radio-signals coming through my guitar amplifier. I'm on the top floor of a building in a densely populated city, with a clear visual path to many other buildings. My guitar ...
0
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0answers
30 views

If we had a radio antenna produce visible light? [duplicate]

If we had a radio antenna produce visible light would it degrade the quality of the antenna due to the light production. How fast would be the degradation?
0
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0answers
9 views

Getting H-Plane Cut from Near to Far Field Transformation

I am using the Near to Far Field Transformation method described in http://www.keysight.com/owc_discussions/servlet/JiveServlet/download/72-15457-47956-2405/36-tkadlec_roman.pdf. The method describes ...
0
votes
1answer
32 views

Connecting two antennas to provide signal

Problem: I am in a room without FM signal. I would like to provide FM signal to the room. If you have a hole for coaxial cable, which can connect a place with signal and the room, is it possible to ...
0
votes
2answers
45 views

There is no reverse flow of energy in a mismatched transmission line, is this correct?

I have already posted this question on the Amateur Radio site, but I would value opinions from a more theoretically based group. Is this text correct? I use it when helping people to study for ...
2
votes
0answers
26 views

How to mathematically model a realistic aperture illumination?

I want to know a mathematical expression that I can use to model a realistic aperture illumination to produce the primary beam of an antenna so that the radial distribution of this aperture ...
0
votes
2answers
58 views

Is a longer antanna better for reception ? Or just more directional?

Let's say I have two antennas, A1 and A2, and they have identical radiation profiles. So the relative radiated energy in a given direction is the same for both antennas. Let's assume also that we ...
0
votes
1answer
73 views

If electromagnetic waves can be generated by oscillation of a coil why can't light? [duplicate]

I was recently reading how electromagnetic waves are generated using oscillation of a current in an antenna. Why is it that this principle cannot be used to generate light?
2
votes
2answers
106 views

Rigourous Treatment of Antenna Theory/Physics

As a rising senior who has finished his QM sequence but not yet done an E&M/optics sequence, but is about to start a research project that will be a large part antenna design and optimization, ...
8
votes
1answer
165 views

Why do I hear voices when I touch my turntable needle?

So I was trying to figure out the reason why my old (and probably sufficiently damaged) needle on my phonograph (turntable) was not working like it was a little while ago. With my headphones on, I ...
0
votes
1answer
32 views

Wideband metamaterials in mobile-telecommunications bands

Were there any metamaterials (materials, settings, geometry) studied that are non-resonant (not frequency selective) in the frequencies between 700 MHz and 2700 MHz, therefore useful for mobile ...
1
vote
1answer
34 views

Layer of graphene on reflector of an antenna

First of all I have no idea how well graphene "sticks" to other metals, but let's suppose it does (well, if I may please ask you to reply to the "stick" question too...). Given the material great ...
1
vote
0answers
44 views

Why are most antennas in cellular networks +/- 45° polarized?

I've just been asked a strange question that I cannot find an answer to (even on the internet it seems I can't find any explanation for this) and I ended up wondering why most of the antennas which ...
1
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0answers
32 views

How to describe frequency spectrum with samples?

I posted this same question in math, but nobody is answering, so I thought I'd give a try here too... To me the question is pretty straightforward, but nevertheless I have to explain its background ...
0
votes
1answer
46 views

$H$ field from corresponding $E$ field (antenna)

The $E$ field at $kr >> 1$ due to a short dipole oriented in the $z$-direction and located at the origin is: $$E_0 = \frac{10^3e^{-jkr}}{r}\sin\theta\space\space(V/m)$$ Can I just solve for the ...
0
votes
0answers
51 views

Finding skip distance/one-hop distance for a radio wave?

I've been racking my brains all afternoon trying to figure this out. Here's the question we're given: At an altitude of 400km is the reflective layer in the atmosphere. What is the maximum distance ...
1
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0answers
38 views

The relation between electric and magnetic potential equation [closed]

Use the principle of duality to derive: $$\textbf{A} = \frac{\mu e^{-j\beta r}}{4\pi r}\int\int_S\textbf{J}_s\, e^{j\beta\, \hat{\textbf{r}}.\textbf{r}'}dS'$$ from this $$\textbf{F} = ...
0
votes
0answers
83 views

What have we built? A resonant loop antenna? Rhombic?

Some background, my partner and I have built an antenna we are supposed to characterize for an assignment, and compare it to physical predictions. However, we don't know what type of antenna we have ...
1
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2answers
84 views

Explanation for: A monopole antenna must contain a resistor (or equivalent) and therefore must have 2 terminals?

Could someone explain why this sentence makes sense: A monopole antenna transfers energy from electrical domain to the electromagnetic domain, hence must contain (equivalently) a resistor, hence ...
0
votes
1answer
75 views

Proof the directivity antenna formula in degrees

For a single-lobed pattern the beam solid angle is approximately given by $$\Omega_A \approx HP_E.HP_H$$ where $HP_E$ and $HP_H$ are the half-power beamwidths in radians of the main beam in the E ...
0
votes
1answer
53 views

Effective Area of Isotropic Antenna: Explanation?

I'm reading some introduction to antenna theory and I've often puzzled on the equation: $$ A_{eff} = \frac{\lambda^2}{4\pi} $$ which relates the effective area by which an antenna captures radiation ...
0
votes
0answers
26 views

Effective aperture of an isotropic antenna; question on polarisation

In this answer to Effective aperture of isotropic antenna, a factor of 1/2 was included to account for polarisation, but the antenna was isotropic. How can this discriminate polarisation?
1
vote
1answer
287 views

Current distribution on a dipole antenna

It is known that the current distribution on a thin dipole antenna is always sinusoidal: http://www.antenna-theory.com/antennas/dipole.php How can I prove this?
6
votes
2answers
540 views

Why doesn't my mobile phone pick up the electro-magnetic wave emitted by my water heater?

I admit my knowledge of physics is very little, but I did study topics on EM and stuff in school. I was reading about computer networks and I read about antennas and how they are just wires or strips ...
2
votes
1answer
47 views

Does array gain violate the laws of physics or not?

I am a bit disturbed lately since I don't know the answer this basic problem. Say we have a standard isotropic antenna with some fixed parameters (load impedance, etc), and we feed this antenna with ...
18
votes
3answers
1k views

Detectability of interstellar messages

Recently a debate started whether it is a good idea to send more messages into space in the hope of having alien civilizations receive them. There are some predecessors, most notably the 1974 Arecibo ...
2
votes
0answers
103 views

Phased non linear array antenna - First Sidelobe

I have a problem I cannot seem to solve and I REALLY need some help. It's about phased-array antennas whose dipoles are not equally spaced, not equally phased, not equally fed (amplitude). Let's ...
1
vote
0answers
125 views

How is it that 62" is the best length for 72Mhz receiver?

According the this article (which was published somewhere in 2005-2008 I think, but it's still comes up first in Google) the best length for a 72Mhz receiver antenna is 62" (~157.5cm). This puzzles me ...
2
votes
4answers
717 views

How do radio waves reach receivers without being canceled out by interference?

When I think of waves traveling through a medium, I tend to think of the double slit experiment or waves in a pond. In those cases, waves are canceled out by destructive or constructive ...
0
votes
2answers
65 views

Does the power weakening of an electromagnetic transmission over distance depends on the beam's width?

Does the power weakening of an electromagnetic transmission over distance depends on the beam's width? If I emit an omnidirectional electromagnetic transmission, its power weakens in the ratio of ...
0
votes
1answer
48 views

Power spectral density of noise: what is $\mathcal{N_0}$ in $S_X(f)=\mathcal{N_0}/2$?

My question is regarding power spectral density of white gaussian noise. It is known that the power spectral density of white gaussian noise is $$S_X(f)= \frac{\mathcal{N_0}}{2}$$ My question is does ...
2
votes
1answer
76 views

The influence of the antenna height

I am working on a model of a transmitter. The transmitter is attached to the wheel of the vehicle and thus constantly changes it's height. In other words capacitance between antenna and ground is ...
3
votes
1answer
110 views

Power loss due to dipole antenna position mismatch

If we have two dipole antennas, it is well known that in order to transfer maximal power, two dipole antennas should be parallel and on the same height, which means that line that connects their ...
0
votes
1answer
288 views

Difference between directional and omnidirectional transmission or reception?

It is suggested that the next generation wireless technology will be highly directional since this increases the signal gain (MIMO) and decreases interference. How will the receiver still be able to ...
3
votes
5answers
452 views

If an antenna must be $\frac{1}{4}$ of the wavelength, how can car antennas be so small?

If the transmission antenna has to be $\frac{1}{4}$ of the wavelength, how can the car antennas' size be much less than that and properly receive the radio signal?
0
votes
0answers
53 views

Could a 1kHz EM field be detectably disturbed by an action potential?

This might be completely off base, but I'm wondering if this would be possible. Neurons fire action potentials that can be modeled as low-intensity (~100mV) EM pulses of 1kHz frequency (up and back ...
2
votes
2answers
418 views

What happens to a photon after it is absorbed by an antenna?

I recently have read about interception of wireless information, however this mentions that people can intercept the information, and then somehow the recipient also gets the information. Regardless ...
0
votes
5answers
246 views

Sequence of E and B field in radio waves and in single photons

In antenna technology we distinguish between nearfield and widefield. In the nearfield the electric and the magnetic fields are shifted by 90°. If you look closer you can see that there are two ...
2
votes
2answers
162 views

Can the magnetic fields of EM radiation be harnessed or measured?

We use the electric component of EM radiation to create the EM radiation and to detect it (antennas and Etc.), but does anyone know of a situation where the magnetic component of EM radiation is used? ...
1
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2answers
146 views

Physics of antenna requirements

I'm currently a junior at San Diego State University and ever since I learned on my intro physics courses that antennas are best 1/2 wavelength. I've read wimpy explanations that it is so it can ...
1
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2answers
2k views

What happens when length of antenna >> lambda

Length of a dipole antenna according to antenna theory should be lambda/2 for best reception. I am just curious about the outcome when length of dipole antenna >> lambda. Impedance will be zero in ...