The tag has no wiki summary.

learn more… | top users | synonyms

1
vote
0answers
30 views

The relation between electric and magnetic potential equation [on hold]

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
20 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
vote
1answer
33 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
31 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
0answers
25 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
9 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
64 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
420 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
36 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
68 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
38 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
172 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
51 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
34 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
53 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
65 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
89 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 ...
2
votes
5answers
146 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
39 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
312 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
4answers
203 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
110 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
vote
2answers
112 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
vote
2answers
626 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 ...
0
votes
1answer
210 views

Electromagnetic waves in an antenna

There is a few questions that need to be answered in detail but in an easy way... What does it mean to describe the 'plane of polarisation' of electromagnetic waves? Why will some antenna have rods ...
0
votes
2answers
222 views

Does a simple copper wire with an AC current create EM waves?

I want to know if a household AC current flowing through a simple copper wire will radiate EM waves? If yes, up to what range and can they be called radio waves?
0
votes
1answer
98 views

Magnetic field of a Herzian dipole antenna

If I am given the dipole moment of very short dipole antenna as $P = P_0 sin (\omega t)$, what will be the magnetic field and polarization of far field radiation? Do I need to consider the time ...
0
votes
1answer
41 views

Wave frequencies and barrier width?

I know a fact that says a wave can go through barriers thinner than its length. This is why for example FM radio can be picked anywhere while antenna TV needs direct sight to the transmitter. Is this ...
4
votes
2answers
196 views

What is happening to the electrons, and E & H fields, in an antenna with a standing wave inside?

Diagrams like the one shown below are often shown to explain antenna theory, but I have always had problem with the concept of voltage being a wave, and because of this the diagrams never make any ...
7
votes
2answers
203 views

How is extremely low frequency (ELF) radiation collected by a submarine antenna?

The U.S. Navy Project ELF managed to generate extremely low frequency (ELF) radiation at down to $\approx 76$ Hz (implying a wavelength of $\approx 3,945$ km!). I was curious, what kind of receiving ...
2
votes
0answers
112 views

Signal strength drop-off from a geosynchronous satellite as a function of angular pointing error

If we have a compact antenna used to communicate with a satellite in geosynchronous orbit somewhere between the $K_u$ and $K_a$ bands of the spectrum (for high broadband applications where we can't ...
3
votes
0answers
97 views

Did Penzias and Wilson need such a large horn antenna to discover the CMB?

Penzias and Wilson discovered the 2.7K Cosmic Microwave Background (CMB) Radiation using a 6 meter horn antenna, along with a cryogenic low-noise detector measuring at 4 GHz: ...
0
votes
0answers
343 views

explain how does Rectenna work to someone with some college level physics

I understand that rectifying antenna (rectenna) is supposed to convert electromagnetic energy to electric current however I do not understand how it's really working. I do get that it's kind of like ...
1
vote
1answer
223 views

Fresnel Zones-How are they Formed?

How are Fresnel Zones formed? What phenomena of light allow ellipsoid areas to be in phase? I've tried reading articles, but they more or less introduce me to characteristics of light, and then tell ...
1
vote
0answers
54 views

Why do (most) metals cause problems in a microwave oven? Which metals (if any) don't? [duplicate]

Most of us have seen microwave ovens with metal racks or shelves, which challenges the common notion that you can't (safely) put metal in a microwave oven. What's going on here? Is it a matter of ...
0
votes
1answer
341 views

How/Why does voltage reflect?

I'm trying to understand antenna theory again, and I'm again stumped by the concept of VSW (voltage standing waves). I understand standing waves, I remember these from a physics clases, however I do ...
3
votes
2answers
141 views

Is there a differential equation that can represent a circuit with an arbitrary voltage source connected acrorss an antenna?

An RLC circuit with a voltage source can be characterized by the differential equation: $$ LC\;\ddot{I}\left(t\right) + RC\; \dot{I}\left(t\right) + I\left(t\right)-C \;\dot{V}\left(t\right) = 0 $$ ...
5
votes
0answers
79 views

Is there any antenna with a single null?

If we designate the origin (the reference point from which all displacement vectors are measured) $\vec{0}$, and If we consider a sphere $\mathbb{B}\left(\vec{0},\mathcal{R}\right)$ of radius ...
2
votes
3answers
325 views

Equivalent circuit for an arbitrary receiving antenna

This Wikipedia entry tells me that the Thevenin equivalent circuit for an arbitrary receiving antenna on which an electric field $E_b$ is incident is a voltage source $V_a$ in series with an impedance ...
2
votes
0answers
49 views

Is the electrical signal delivered to a load by a receiving antenna always $\propto \frac {\partial}{\partial t}$ of the $\vec{E}$ or $\vec{B}$ field?

A time-varying $B(t)$ field through a loop antenna induces a voltage proportional to $\dot{B}(t)$. A Hertzian dipole along a time-varying $E(t)$ field also induces a voltage across a load--while I ...
1
vote
0answers
97 views

Hertzian dipole in time varying electric field

Suppose we place a Hertzian dipole (short, ends loaded with capacitance) in a time varying electric field $\vec{E}\left(t\right)$, with magniture $E\left(t\right)$ and direction as shown in this ...
0
votes
1answer
292 views

Very short ($\ell \ll \lambda$) dipole as a receiving antenna

I'm trying to understand how a very short dipole of length $\ell \ll \lambda$ works for receiving radiation. (It is center-fed, and has two thin perfectly conducting arms each of length $\ell/2$ ...
2
votes
5answers
956 views

Radio antenna producing waves in the visible spectrum

If a radio could produce waves in the visible light spectrum, what would the result be? This is a thought experiment that I've pondered for a few years now. I realize there are a few/many real-world ...
2
votes
1answer
123 views

Is there a way to tell what frequency an unmarked antenna is designed for?

I have accumulated a large amount of R/C gear over the years. I have several antennas which are not labelled as to their original use. This antenna is either for 5.8ghz, 2.4ghz, or 910mhz. The ...
6
votes
1answer
113 views

Astronomical-wavelength radio (AWR) transmissions between cosmic plasmas?

My son asked me if electromagnetic waves longer than radio exist. I told him that even though physics permits such waves, there are no antennas long enough to radiate or detect them. However, on ...
5
votes
4answers
2k views

Radio antennas that are much shorter than the wavelength

From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
1
vote
1answer
264 views

The length of an antenna is twice the amplitude of the wave

I have seen it remarked in some problem sets that if you have an electromagnetic wave traveling in the $x$-direction with it's $y$-coordinate given as $y(x,t)=y_0\sin (\omega t +kx)$ and you want a ...
1
vote
0answers
51 views

Does shadow fading change if obstacles are fixed?

Given a RF sender and a receiver, suppose the obstacles between them are fixed, both their material and dielectric properties, does the path loss caused by their shadowing vary? If yes, what causes ...
5
votes
2answers
1k views

Effective aperture of isotropic antenna

I have always taken for granted that 'the aperture of a loss-less isotropic antenna is $\dfrac {\lambda^2} {4\pi}$'. On a whim, I tried to look up how this expression was derived, but so far I have ...