35 votes
Accepted

How do you make a spherical radio wave?

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. ...
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  • 14.1k
35 votes

Is every open circuit a capacitor?

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 ...
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16 votes
Accepted

How does the human body affect radio reception?

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 ...
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15 votes
Accepted

Detectability of interstellar messages

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=...
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  • 111k
15 votes
Accepted

Why do we need an antenna of size comparable to the wavelength for surface wave transmission?

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 ...
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14 votes

Why are longwave radio towers so tall?

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 ...
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  • 9,695
13 votes

Why are longwave radio towers so tall?

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

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 ...
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10 votes
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A "figure-8" EM field created by a dipole antenna

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 ...
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  • 116k
9 votes

Is every open circuit a capacitor?

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 ...
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9 votes

Does a receiving antenna radiate the power received?

An antenna has an equivalent impedance both in transmit and in receive. If the load in receive mode is matched to that antenna impedance then there is no reflection from that load back out the antenna,...
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8 votes

Radio antennas that are much shorter than the wavelength

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 ...
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8 votes
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How does a receiving antenna work given that the electric field is 0 in conductors?

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 ...
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8 votes

Why are longwave radio towers so tall?

... 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 ...
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8 votes
Accepted

How come a mobile phone signal is blocked by aluminium foil, but Wi-Fi gets through?

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 ...
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7 votes

How do you make a spherical radio wave?

"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 ...
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  • 9,773
7 votes

Why are longwave radio towers so tall?

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 ...
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7 votes

Is every open circuit a capacitor?

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 ...
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  • 1,537
6 votes

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

(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 ...
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6 votes

How do radio antennas follow electrical rules

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 ...
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5 votes

Radio antennas that are much shorter than the wavelength

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 ...
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5 votes

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

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 ...
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  • 1,537
5 votes

Detectability of interstellar messages

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 ...
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  • 14k
5 votes

Current distribution on a dipole antenna

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 ...
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5 votes

How do radio antennas follow electrical rules

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 ...
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5 votes

How does a receiving antenna work given that the electric field is 0 in conductors?

There are multiple ways in which this is not a contradiction. By “inside a conductor”, we are referring not to the conductor as a whole but rather the interior volume as opposed to the surface. If ...
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5 votes

What is the difference between a radio antenna and a capacitor?

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 ...
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4 votes

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

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 ...
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4 votes

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

The electricity through the coil is probably coming directly from the network, so it oscillates at either 50 or 60 Hz. That would be the frequency your antenna radiates. This is very different to the ...
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4 votes

Rigourous Treatment of Antenna Theory/Physics

I'm an antenna design engineer, so I'll try to give you some pointers. I'll speak on a widerange of topics and hopefully give you (and others) info on where to start. Feel free to PM me with questions ...

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