# Tag Info

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Small pocket radios usually have fairly poor antennas in them, because of their small size. But by placing your hand nearby the antenna of that small radio, you are creating a capacitor in which one terminal is the radio chassis and the other is your hand. Any radio frequency signal induced in your skin by radio broadcasts will be capacitively coupled from ...

5

First of all, the photon picture isn't the best way to visualize and model the operation of a radio-frequency antenna. RF antenna engineering is best performed and understood (IMHO) by means of a model in which high frequency AC power flow through a wire creates electromagnetic waves that propagate away into space. For this to occur, the antenna impedance ...

4

I will try to summarize the classic results from 1, 2, 3. In short, a receiving antenna does not scatter as much power as it absorbs except in a few very special cases such as a very short dipole or a small loop. For these small dipoles, when the incident wave induces the same current distribution as would be seen when the antenna is used and driven as a ...

4

In this context, a resonant element would be a chunk of the system that possesses a substantial quantity of impedance (either capacitance or inductance), different from that of the system as a whole. So if the horn had a local swelling or constriction somewhere along its length, the impedance of that segment would not match that of the horn inlet or outlet, ...

4

Since the times of maximum charge separation (electric field) and maximum current (magnetic field) are out-of-phase in a oscillating dipole, the question makes perfect sense. The answer is to look at the exact dipole solution: $${\bf E}=\frac 1{4\pi\epsilon_0}\Big[ \frac{\omega^2}{c^2r}(\hat{\bf r}\times{\bf p})\times\hat{\bf r}+ \big[ \frac 1 {r^3}-\frac{i\... 3 The EM waves act on charges, and the frequency of the movement of that charges is selected by the LC reception circuit of the radio. But without enough carries present in a good antenna, there is no much signal to select. The effect of the human body is to provide movable charges for the EM waves. When touching the antenna, it increases its role. But even ... 3 They move at the drift velocity for that material and that electric field. A strong FM radio signal from a nearby station has an intensity of about 10^{-5} W/m^2, while for a weak astronomical radio source it might be more like 10^{-26} W/m^2. The equation for the drift velocity in terms of the intensity S is$$v=\mu \sqrt{\frac{4\pi k}{c} S}$$, and if ... 3 The diagrams you are looking at do not show how far the radiation propagates, it shows the power of the radiation in different directions. The portions far from the center have the highest power. If you have a receiver in one of the very weak directions, it doesn't mean that radiation cannot reach you. Just that the total power reaching you will be low, ... 3 Let me first comment a sentence from SuperCiocia’s answer. The photodetector clicks ... are caused by the photoelectric effect, that is bound electrons in the photodetector are in quantised orbits and are only capable of discrete energy jumps. (1) In addition to this statement, please recapitulate that any observation of the wave behaviour of light during ... 2 The simple answer is no. For a photon to be observed, all its energy must be collected. You cannot observe half a photon, either you observe it or you do not. The observation or detection can only happen in one place. This is often referred to as "the collapse of the wave function". As an electromagnetics engineer I sometimes monitored very faint ... 2 Light does not behave like a wave some times and like a particle some other times. Light behaves as light. Trying to categorise some behaviour as "wave-like" or "particle-like" is just an attempt to build an intuitive understanding for quantum phenomena by relating them to simpler everyday things like water waves or marbles hitting a wall.... 2 The build up of Maxwell's equations was long, and the experimental confirmation that light is the electromagnetic waves predicted by them took a while. There are "laws", relations which were defined from experimental observations, which are as an axiomatic underlayer so that the equations appear consistent and give the electromagnetic wave as ... 2 This is really just a pedantic detail, but only forces between monopoles fall off as 1/r^2. For example the force between two electric dipoles falls as 1/r^3, the force between two electric quadrupoles as 1/r^4 and so on. This detail aside, the obvious example of a force that does not obey the inverse square law is the strong force. The reason this ... 2 All elements of a circuit will radiate when there is alternating current involved. Typically though, at the frequencies involved, not very much. It is something that circuit designers must be careful of though. Capacitors themselves typically are encased in some sort of material which will damp the stray radiation, especially if it is a conductive material ... 2 An antenna, in general, is NOT a resonator (contra @oliver). The purpose of the antenna is to be a direction dependent impedance transformer that matches the wave impedance of the transmission line connecting the transmitter/receiver to space (air) having impedance \sqrt{\frac{\mu_0}{\epsilon_0}}=377\Omega in all direction. In other words, the apparent ... 2 The radiation pattern around an ideal horizontal dipole suspended in 3-D space looks like a vertical torus or doughnut centered on the middle of the dipole. This ideal picture is altered significantly by the presence of the earth's surface when the dipole is less than ~a wavelength from the surface. In this case, the radiation pattern begins to look like two ... 2 why we have never seen oscillating charges produce em waves, like for instance an antenna. You can experience this every day by looking at a mirror, which is just a flat polished conductive metallic surface. It is usually built on the back side of a sheet of glass for practical reasons: glass is transparent and it prevents the metallic surface from ... 2 at high (radio) frequencies, extension cord ("zip cord") wire has large power losses and is generally unsuitable for transmitting antennas. telecomm antennas are built so as to produce as much electromagnetic radiation as possible, and point it in the desired direction. They have a wide variety of shapes to do this. The amateur radio stack exchange ... 2 When antennas are placed very close together all bets are off. The near field coupling between antennas elements change the impedance's therefore currents and fields the far field patterns don't follow simple algorithms any more. in fact if they become close enough and fed in phase they asymptote towards a single element with two sources in parallel the far ... 2 The fields can be treated as the sum of the fields from two oscillating electric dipoles of magnitude d that are \pi/2 out of phase, with one dipole pointing along the x-axis and one pointing along the y-axis. i.e.$$\vec{d_{\rm tot}} = d\cos(\omega t) \hat{x} + d \sin(\omega t)\hat{y}, where $\omega$ is the angular velocity. I think then that you have ...

1

Isn't induced EMF in an antenna dependent on the power of the electromagnetic wave? Indeed it is not - for given antenna geometry, the induced EMF due to external wave depends only on electric field of the wave where the antenna is. The total induced EMF includes also self-induced EMF and the latter depends on behaviour of current in the antenna and its ...

1

I think your question might be valid in some non-technical sense. The truth is, the signal does lose energy. As someone previously stated, it is the power of the signal and the impedance of the atmosphere that matter. As the broadcaster of the signal, you have to push enough power through the air to send the signal. If you want to think of these bodies as ...

1

Consider an oscillating electric charge (part of an oscillating current). The moving charge introduces a transverse distortion into its (preexisting) radial electric field (strongest when moving most rapidly). As part of a current it also produces a magnetic field (wrapped around the direction of motion), also strongest when the charge is moving most ...

1

Yes, atoms and molecules can absorb or scatter radio waves if they are excited into very high quantum states (so called 'Rydberg States'; see https://en.wikipedia.org/wiki/Rydberg_atom ). These states are for instance created through recombination of ions and electrons in a plasma. The density of atoms in these states is very small but the absorption and ...

1

You have answered your own question in a sense, by part of your statement that: "This image shows that the applied voltage is in phase with the current" this is the definition of the "real" or purely resistive part of the complex impedance. there is no standing wave at the feed point but a travelling wave from the source to the feed point,...

1

The "electric field detachs" from the antenna because it is not simple electrostatic field, but instead it is transverse wave generated by accelerated electrons in the antenna. Accelerated charged particles create such propagating waves of electric field. Poynting vector is a measure of EM energy flow, by itself it does not explain how electric ...

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If your circuit was oscillating since the beginning of time: Many photons are generated simultaneously, and the collection of these packets of multiple photons being produced at a constant rate produce the E-field. (As a technical point, it's actually the coherence between these possibilities that produce a nonzero E-field, but you can think of it like their ...

1

EM waves are generated in a circuit having a physical inductor and capacitor but in an ideal "$LC$" circuit charges move and oscillate without EM waves being generated. The reason for no waves is the assumption that the circuit consists of infinitesimally small elements and connections, in practice, having linear extent negligible compared to \$\...

1

It will originate from everywhere: the wires, the capacitor and all the elements carrying the current or the displacement current. Indeed, thee emission of the EM waves is described by the Maxwell equations where the sources are the time-dependent currents and fields. What is confusing in LC circuit is that it is not suitable for the description of the ...

1

The maximum operating frequency of a waveguide is typically limited by the higher order modes. If you try to use the waveguide at a frequency where multiple modes can propagate, you will get some coupling between these modes, which will cause some signal power to travel faster than the main signal. This generally causes problems depending on how the signal ...

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