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## Hot answers tagged antennas

33

A result known as Birkhoff's theorem forbids spherical electromagnetic radiation. The statement of the theorem is that any spherically symmetric vacuum solution to Maxwell's equations must be static. It is rather simple to prove. In a spherically symmetric solution $\mathbf E$ and $\mathbf B$ must be radial. Make an Ansatz, \mathbf E = E_0 \exp(i(\mathbf k\...

33

You are right, every circuit possesses some unintended capacitance, which is called "stray" capacitance. Whether or not it affects the operation of the circuit depends on the frequencies that the circuit is intended to operate at. The amount of stray capacitance that a circuit has is typically tiny, but at high enough frequencies even a very tiny amount of ...

15

Some numbers come from a review paper by Cullers (2000), who discusses the SETI Phoenix project. There, it is claimed that the Arecibo dish is capable of detecting a narrow band, coherent signal of $f=10^{-27}$ W/m$^2$ given a 1000 second observation. Assuming that this is an isotropic signal, then the implied power at distance $d$ is $p=4\pi d^2 f$, which ...

14

This is a problem of impedance matching for even an infinitesimally small antenna, i.e., a Hertz dipole (or monopole above a ground plane) can perfectly transmit and be perfectly matched to a pure sine wave. What it cannot do is to be matched to a signal of finite bandwidth. The impedance of a short dipole of length $h$ and radius $a$ is approximately $Z_{... 14 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 ... 13 You might want to have a look at Does light induce an electric current in a conductor?. It's probably impossible for a radio aerial to emit visible light as the frequency of light is around the plasma frequency of the metal that the aerial is made of. We're not really supposed to address hypothetical questions, but if you could find some material with a ... 13 Summary: The fact that the length of an antenna is of similar size to the wavelength of light is a coincidence due to the similarity of the speed of light in air and the speed of light in the antenna (which are usually copper wires). For other waves, this may not be the case. Different guitar strings, for example, resonate at different frequencies despite ... 13 My answer is completely different. Longwave antennas usually are quarter-wave antennas, also known as Marconi antennas from its inventor. As the name says, they have length about 1/4 the wavelength to be transmitted. Compare them to dipole antennas generally used for shorter wavelengths, both in transmission and in reception, which are one half-wave long. A ... 11 The idea behind the quarter wavelength antenna is that it is self-resonant: it is "tuned". You can however use an antenna of any size to pick off some electromagnetic energy - and you can tune the antenna by adding some inductance in series (or inductance and capacitance). The reason that you tune an antenna is simply this: you want it to have real impedance,... 11 The radiation pattern of any dipole antenna looks similar to what you are showing in the 2D plots - but in your interpretation of the 3D pattern you have the axes wrong. A dipole antenna with the main axis vertical will transmit power in the horizontal plane, with less and less power as you go further away (inverse square law). If you measure the power as a ... 10 http://www.antenna-theory.com/antennas/shortdipole.php is a website with useful info., including formulas. To oversimplify, it seems to say that once the antenna is a tenth or less of the wavelength, the exact ratios don't matter so much. The antenna is inefficient, but it works for both sending and receiving. If you can detect the signal, of course you can ... 9 Yes, this is correct. However, you should also keep in mind$-$particularly when you're describing any type of antenna$-$that any such residual capacitance may very well be competing with the inductance of the circuit, coming from nonzero interactions between the different currents in different parts of the circuit. For an actual antenna, where you're ... 8 A matter of thresholds The reality of spread-spectrum is complicated but let's imagine that the WiFi router and cell phone tower have both allocated 1 Watt to transmit to your phone, and it in turn can transmit 1 Watt back in both cases. If the WiFi router is 10 meters away and the cell phone tower is 1 km away, then it's possible to imagine a four order ... 7 This isn't hypothetical. There is nothing that a radio does that can't be done in other parts of the spectrum. Many FM/AM radios operate in the optical range too. Your TV remote control uses IR. Lasers are used for high bandwidth point to point communications. And don't forget fiber optics, these are all radios that just use optics for the communication ... 7 The key to the efficiency of an antenna (whether for transmitting or receiving - the two processes are essentially reciprocal) is resonance, and impedance matching with the source / receiver. The size also matters in terms of the relationship between power and current. A nice analysis of the impact of size of an antenna on the power/current relationship is ... 7 "If that is possible, how do you produce a spherical EM radiation?" A spherically symmetric transverse field is topologically impossible - if it is required to be coherent and linearly polarized everywhere. This is the case for usual dipole or higher multipole radiation, as has already been pointed out in another answer. On the other hand, an incoherent ... 7 When a metal antenna wire is put into the field of a propagating electromagnetic wave with time-varying fields, there will be an electric and magnetic field inside the wire and thus also a current but the penetration is exponentially damped. The penetration depth$\delta\$ is called the skin depth. In treating boundary conditions with metals for ...

7

... to transmit in the longwave region, we only need to make the circuit oscillate in the right frequency That is the point. For lower frequency the electrons - which are accelerated inside the antenna rod -, need a longer distance inside the rod. Would the disturbance of these electrons reach the end of the rod too fast, the power of the antenna generator ...

7

A slight deviation from the actual question -- focus on antennas. Antennas can be capacitive or reactive in how they respond to RF energy either by feeding the antenna used for transmitting or from electromagnetic waves in receiving. A purely resonant "ideal" antenna to a single frequency is neither capacitive nor inductive as one definition of resonance ...

6

So here I am, answering my own question... Long story short -- I found the answer here, and in page 149 of 'Tools of Radio Astronomy' [Rohlfs/Wilson/Huttemeister 5/e 2009], and page 24 of 'Radio Astronomy' [Pawsey/Bracewell 1955] and I will now express that answer in my own words to save the reader a click! Interestingly, one part of this proof comes from ...

6

When you have a capacitor, current flows even though the "circuit" is not complete. This is because it's possible for electrons to bunch up - temporarily - in a conductor and generate a corresponding electric field. That is what happens in an antenna. An antenna is really a combination of an inductor (a straight wire) and a capacitor (when you put a net ...

6

Antenna performance is strongly affected by the presence of the ground nearby. The standard rule of thumb is to raise the antenna to a height above ground of about one half of the wavelength it will operate at, in order to minimize power loss in the ground and radiation directionality effects. At low frequencies- say, ~1 MHz- the wavelength is 300 meters ...

5

(All images in this answer were made by me, for wikipedia! Links here) Let's start with the simplest question: Voltage relative to what? It's the voltage from one line to the other. Now let's look at an animation of a transmission line. This one is terminated with an impedance-matched resistor, so we don't have to think about reflections yet. The dots ...

5

The requirements for transmitting antennae are much higher than for receiving antennas. Transmitting antennas must optimally radiate, so that the signal is not obscured by other stations with better antennas. If a receiver antenna is too short and far away from resonance, all received stations are uniformly weaker. What matters is that the desired signal is ...

5

With a resonant antenna, the reactance (capacitive and inductive) should be zero. Short antennas are usually capacitive so that capacitive reactance is offset using an inductor. Often for an AM radio a loop inductance is included. Also, some antennas are longer than they appear because the conductor is wrapped around the core of the antenna (sometimes you ...

5

This paper contains an important analysis of the different trade-off between bandwidth and energy efficiency. The interesting conclusion from that paper is that the most energy-efficient way to send and receive interstellar messages (over flat spacetime) that maximise the bit-rate requires making the bandwidth of transmission very large. In particular, this ...

5

You can't prove this, it's not true. It is only a very good approximation: it fits with the boundary conditions. For example, in a half wave dipole, by symmetry, the current needs to be maximum in the middle at the feeder inputs and nought at the ends of the antenna (no current flows out into space!). It's also found that calculations grounded on this ...

5

I think that this is a very good question because it makes one think beyond a "standard" explanation. When you study electromagnetic induction you learn about the magnetic flux change through a closed loop, which produces an induced EMF. However, the loop does not have to be a conducting loop. If it was an ideal dynamo with no resistance, friction etc., ...

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

The radio waves or microwaves that are used for communication don't contain just one photon. They contain a bunch. (Maybe someone will do the math for how many photons a standard radio broadcast antenna is producing each second; it'll blow your knee-high off even if you're wearing sandals over them.) Consider for example a frequency-modulated signal. The ...

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