# Tag Info

19

It's spherical because the main dish cannot be steered; steering is done by moving the receiver (the big thing hanging over the center of the reflector). A parabolic reflector would produce varying errors when aimed in different directions; a spherical reflector has the same error for all directions. Presumably the receiver is designed to compensate for ...

16

In the early days of radio, the resonance of the antenna in combination with its associated inductive and capacitive properties was indeed the item which "dialed in" the frequency you wanted to listen to. You didn't actually change the length of the antenna, but by changing the inductor (a coil) or capacitor connected to the antenna you tuned the resonance. ...

11

Did you read the Wikipedia article? It explains the signal rather well, I think. At any rate, it is called the Wow! signal because, as the picture shows someone wrote Wow! in the margin. As for the code and why they were excited, I quote the Wikipedia article, The circled alphanumeric code 6EQUJ5 describes the intensity variation of the signal. A ...

10

A human body may reflect and absorb radio frequencies, though not very efficiently. It may as well act as a resonance chamber for certain frequencies. For a signal of 100 MHz, the involved wavelength is 3 m, and so it is possible that parts of your body are acting slightly as a resonant chamber. (for an optimal resonance, you should have 1.5 m diameter, too ...

10

Say if I transmit: $\sin(2\pi x)$ And separately: $\sin(2\pi x\times 2)$ Does it end up as a single wave of: $\sin(2\pi x)+\sin(2\pi x\times 2)$? Yes, that's exactly how it works. This is called superposition. There are electromagnetic waves at hundreds of different frequencies all filling the air simultaneously. The way something like a ...

8

The answer to your question is Yes. More specifically, light is 400 to 800 Teraherz, RF is 3 Kiloherz to 300 Gigaherz.

8

AM radio typically transmits at around 1 MHz, FM radio at about 90 MHz. Measurements of the RF spectrum of lightning strikes show a falloff with frequency of about 20 dB per decade in that frequency range, so with FM about 2 decades above AM, you'd expect AM to have about 40dB higher interference from a lightning strike. In addition to that, FM signals ...

7

Let your carrier signal be $A_0 \cdot \cos(\omega_c t)$ with amplitude $A_0$ and carrier frequency $\omega_c$. Let your signal be a simple wave, $\phi(t) = A_s \cdot \cos(\omega_s t)$. Then the modulated signal becomes $$A_0 A_s \cdot \cos(\omega_c t) \cdot \cos(\omega_s t)$$. In addition, as pointed out by George in the comments, the carrier also gets ...

7

They heat it, by different degrees depending on the polarization of molecules in the tissues and liquids. The molecules try to re-align after the radio-wave field and the movement dissipates as general heat. Think microwaves.. a consumer-grade microwave oven operates at the same radio wave spectrum as your home WiFi network (2.4 GHz) but much stronger. The ...

7

You're not boosting the signal; you're either acting as a reflector (capturing a bit more of it to feed to the antenna) or blocking a competing source, or perhaps a bit of both. By analogy, when you hold your hand to your ear to help you hear something, your hand is acting a reflector for sound waves to direct a little more energy into your ear. It can also ...

7

Radio waves are large wavelength waves, and non metal walls are transparent to the radiation at those wavelengths, depending on the thickness of the walls, because there are no energy level "receptors" to absorb them in bulk by excitation of electronic orbits. The wavelengths start from a centimetre up to kilometre, which is the width of the cycle of the ...

7

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 ...

7

This is something we must all have observed, but I don't know of any definitive study. In the absence of hard data I can think of three potentially relevant effects: Dielectrics, like the human body, deform electromagnetic fields in their vicinity The wavelength of FM radio is around 3m and therefore comparable to the size of a typical human. This means ...

6

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 ...

6

Yes, LED's, luminescence of phosphorous (CRT screens), most of the current screen technology, fluorescent lights. They aren't a thermal source. They heat up due to the electric current but that's not the working principle.

6

Here's a somewhat technical document talking about insterstellar beacons. A beacon would act as a "searchlight", sweeping across the sky, so that the time spent on each target star would be rather short. With a beam aperture able to illuminate 1% of the sky and working at 0.5 Hz they calculate that 6.9 GW power will be "visible" as far as 6000 light years. ...

6

A guitar string produces harmonics because it vibrates in a non-linear fashion. An electronic oscillator can be made to generate a much purer form of vibration (near sinusoidal) than a mechanical device such as the guitar string. Hence its harmonic level, while not zero, is much lower. For example, the harmonic distortion of a guitar string is probably on ...

5

It depends on what kind of radio telescope you're talking about. If you're talking about a single dish, with say, a horn feed at the focus, it behaves like a regular 'dish antenna' by focussing radio waves to the focus where it is amplified, processed in some manner (such as downconversion), digitised, and so on. A single dish can also have a feed array at ...

5

While I agree with Alfred Centauri's answer, I am not sure it gives a direct answer to your specific question for the following reason. If there is a receiving antenna somewhere, there is always some reflection, however minute. If the antenna is connected to a circuit, the conditions of reflection will change, and the transmitter can "notice" that. In ...

5

From a physics perspective, the fundamental reason for this is something called the bandwidth theorem (and also the Fourier limit, bandwidth limit, and even the Heisenberg uncertainty principle). In essence, it says that the bandwidth $\Delta\omega$ of a pulse of signal and its duration $\Delta t$ are related: $$\Delta\omega\,\Delta t\gtrsim 2\pi.$$ A ...

4

There is no realizable filter that blocks all frequencies except one. The fact is that frequencies above and below the tuned frequency are passed but with greater and greater attenuation the farther away from the carrier frequency. More importantly, the actual information is in the side bands, not the carrier. If the filter only passed the carrier, ...

4

Here is the spectrum of the human voice saying "oh": If you translate the sound directly to light, adjusting the frequencies so they are visible by translating all the audible frequencies to all the visible spectrum, how would it look like? Well, it looks kind of similar to two of this: that is the spectrum of a Wolf Rayet star. You can find more here. ...

4

As an intermediate step, consider a sinusoidal source driving an infinite transmission line with some characteristic impedance $Z_0 = 50\Omega$. The source "sees" a real impedance of $50\Omega$ and so, power is delivered to the line and, since the TL is infinitely long, the power is transported down the line, via an electromagnetic wave, without reflection. ...

4

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 ...

4

1) Normally, the antenna isn't the only component that distinguishes between the various competing signals received. The antenna does have a bandwidth and will attenuate signals outside that band. A typical antenna on a cell phone mast for example, may receive in the range 1.8GHz - 2.4GHz (just an example, you would have to look up manufacturer data ...

4

In the microwave band here are multi-element detectors, but at longer wavelengths the telescope is a single pixel. Yes it does take a while to build up an image, but radio pictures aren't usually very large - not the millions of pixels of an optical/IR image. One big advantage of radio telescopes is that you can combine telescopes 1000s of km apart to ...

4

There are ways that EM waves can be used to reconstruct a representation of matter and these techniques vary widely based on the method. Each method is very unique and its own set of issues. UV-D's reference Measurements Using Optic and RF waves provides a good overview. It explains some of the applications, the sources for their EM waves and ...

4

First, no, "radio propagation" is not "via atmosphere". Different wavelengths get absorbed, reflected, or simply passed by different parts of the atmosphere. There is no one general rule. Many of our radio communications within the atmosphere are pretty much like they would be in free space, for example. Second, all radio waves propagate infinitely in ...

3

This is a resonance in the circuit--- when you have a bunch of different frequencies driving a resonant system, the response is only strong for those frequencies which are close to the natural frequency of the resonant oscillator. You can see the same phenomenon in mechanical systems. If you have a mechanical mass on a spring, and you apply a force which ...

3

All electromagnetic signals that leave an antenna have an amplitude, i.e. there is power propagating as they spread. power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently ...

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