# Tag Info

22

For an antenna, a gain of 22 dBi means a gain of 22 dB with respect to a theoretical isotropic antenna. If an isotrpoic antenna radiates 1 watt, it would radiate that power uniformly over the 4π steradians around it, so at a power density of about 80 mW per steradian. For your antenna with 22 dBi gain, 22 dB is a power ratio of 158, the emitted power density ...

11

The frequency is indeed out of range of a crystal oscillator. But you don't need a crystal to generate your fundamental directly. In fact, all previous frequency bands used in 2G/3G/4G are also out of range of these crystals. The trick is to use a PLL. Modern CMOS circuit technologies can product fundamental oscillators past 150 GHz* - making one that ...

11

Wired systems rely on current or voltage to carry signals. These can use a common reference, like early telegraph systems that literally used the earth as a signal return. That’s not strictly necessary though: wired systems can use different means to detect the presence of signal, like detecting edges or sensing a carrier signal. Radios use electromagnetic ...

11

Due to the sharp edges, a square wave has a wide spectrum with lots of harmonics. You could send that over the air with an antenna, but: 1- Signal shape will be distorted at the receiver due to limited bandwidth 2- It will use a lot more bandwidth than it needs to. For radio transmission you want to use as little bandwidth as you can, and not emit any ...

7

Maxwell's equations are chock full of derivatives, which is to say that we only care about change not absolutes. A changing electric field becomes a magnetic one; a changing magnetic field becomes an electric one. An unchanging field does neither of these things. If one of the changing fields hits a wire or coil, we get a signal we can process. Beyond the ...

7

Say you have a source of 0 dBm power. If you hook it up to an antenna with 22 dBi gain, then in the direction of the main lobe, your source appears to be 22 dB stronger than it would if it were connected to an ideal, isotropic radiator.

7

tl; dr: RF needs a carrier, cable can use baseband. Both are band-limited, RF systems using carriers more so. A perfect square wave has infinite harmonics. A good square wave has a large number of them. Because of this, square waves use a lot of bandwidth for the information they carry. They’re very inefficient. This limits their use to lower speed signaling,...

7

Space Division Multiplexing sounds like snake-oil, but it's real. It's a way to push several independent channels of data over a shared medium. If you have multiple well-isolated coaxial cables, then you would be right to insist that this was in fact a totally obvious way to move signals, and that there are in fact multiple physical channels available, one ...

5

Technically infrared light is an EM wave, yes. But it behaves a lot more like the light you are used to seeing, just at a color you cannot. You're looking down the barrel of an IR LED. While the light production is from the movement of charge across a semiconductor band gap (you can go look up how an LED works) it does so when simply fed a DC current. ...

5

But when we go to wireless communication, how does a receiver interpret the incoming data correctly? I know that electrical signals differ from EM communication. But why and how is a "1" sent is also received as "1" since there is no reference signal. In many RF systems that transmit data there is a reference. For instance, the ...

4

The value 900MHz and 1800MHz refer only to the carrier frequency. A pure perfect sine wave doesn't transmit / carry any information. Only when some parameter of the carrier changes, is data transmitted. This process is called modulation. The parameter could be something like frequency, phase, amplitude etc. More frequently you change the parameter, more data ...

4

Mesh attenuates in the range 10dB to 50dB. For anything more than that you'd need sheets of 1mm to 2mm thick, made of copper or steel. It is not true that holes smaller than the wavelength block or shield an EM wave. Also, findings of the Faraday cage -not box- for electrostatic fields (familiar to many from Physics classes) are often misleadingly ...

4

The claimed range for low-cost wireless gadgets often represents a triumph of marketing over engineering. The range may be achievable "in free space" with a clear line sight between two units, but not in the real world with structures, vegetation, interference, and the curvature of the earth intruding. Since the transmit power and receiver ...

4

Along with all the bandwidth and other considerations mentioned, the main reason you don't transmit your data directly is that everyone would be transmitting at the same frequency. Take Ethernet. It uses two (or more) pairs of twisted wires to carry two separate channels of information (your Ethernet card sends data on one pair and receives it on the other ...

4

DSSS vs OFDM: these are different standards (a/g/n vs b, which is obsolete). This situation where you'd need to detect the differences never arises: a beacon from an access point is either of these standards, and that defines what the whole network does. Since the channels for these different modes don't even match, there can't be interoperability. A card ...

3

That's madness. If you're using Windows then try this: set wsc = CreateObject("WScript.Shell") Do WScript.Sleep(5*60*1000) 'Every 5 minutes ... wsc.SendKeys("{F13}") '... press the F13 key. Loop Save it in nosleep.vbs on your Desktop and run it when required. F13 is chosen so that it doesn't interfere with anything else. To check if it's ...

3

The Shannon–Hartley theorem. If the signal to noise ratio is identical for both systems and they are both presumed to use the same modulation method, the data rate (C) is proportional to B, bandwidth and nothing to do with the carrier frequency.

3

Generally with this type of problem (1 receiver & multiple transmitter antenna elements, or 1 transmitter and multiple receive antenna elements), you take one of the paths and call that your reference. The total phase shift through that path is many multiples of 360 deg, because the path is longer than one wavelength. But by making that path your ...

3

It is useful to think of observable (real-valued) waves as the sum of two complex waves: it doesn't require a nonlinear operation to drop the imaginary part, like $\cos x=\Re(e^{ix})$ does. it doesn't require an (arbitrary) definition whether an observed sine wave has increasing or decreasing phase (because $\sin x=-\sin -x$). it explains mixing ...

3

The hole exists for the propagation of the EM waves coming from inside the remote correct? Yes. The EM waves coming from the remote are infrared (IR) light. While it is possible to make plastic that is opaque to visible light but transparent to IR, it's cheaper to just make ordinary opaque plastic with a hole in it. The surface you see inside the hole is ...

3

Just to give one common example. Many digital radio systems use a system known as frequency modulation, in which a 0 and a 1 are encoded as slightly different frequencies (the technique is sometimes called shift keying). The broadcast signal is thus a continuous stream modulating regularly (at roughly the data bit rate) between these two frequencies. The ...

3

The physics of antennas plays a large part. To understand this, you must consider the Fourier transform of your signal. For a simple binary NRZ code, where 0 is 0V and 1 is some voltage, Most of the Fourier energy is contained within the frequency range 0-B/2, where B is your bitrate. But antennas are frequency-selective: at the high end, an efficient non-...

3

You could also use machine vision + AI to detect what books are there, based on the writing on the spine. If they're tagged with an ISBN number, even better. That's basically what Amazon Go does, but with a wider set of items. Your problem set is simpler, since you’re only recognizing books and text. Using a visual method has the additional bonus of showing ...

2

A signal is a sequence of symbols transmitted sequentially over time. A symbol is any distinct state of the communication channel. For example, in a binary (two-state) channel, the states could be two voltage levels, two different values of current, or even two different frequencies — i.e., a simple modem. Such a channel can transmit one bit of ...

2

The environment for MIMO does not consist simply of N transmitters and M receivers, it also consists of R reflectors of signal creating multipath signal dispersion. These reflectors between the Tx and Rx, and close to them, consist of things like buildings, walls, trees, filing cabinets, cars, people etc. Any communication that takes place other than on an ...

2

Add capacitors before the motor. Put wire mesh or screen around the motor and wires to act as a Faraday shield. Make sure your ground game is on point too. Try connecting a wire directly from the RX to the negative terminal of your power supply. Or do that with the motor, or both.

2

Yes, wireless Internet is a thing, it's called WiFi. And satellite-based Internet is a thing, too. There are many providers of such services. But keep in mind that Internet in general requires two-way communication. It is not a broadcast service like radio and TV. Even watching or listening to streaming audio and video services requires two-way communication....

2

Since the goal of the CP is to remove or prevent ISI and ISCI, you would need a form of DFE with feedback from several neighbouring subcarriers of the previous symbol. That can be expanded into a multi-symbol MMSE with feedback. So yes it's possible.

2

What you show is an infrared remote, with an exposed IR LED. IR remotes are basically like fancy LED flashlights, sending pulsed light from the LED to the receiving device. If your eye could see the IR, it would look like a rapid flicker. Some remotes drive a visible LED with the same signal as the IR to show activity. Some IR remotes hide the LED behind IR-...

2

I'll offer a bit of frame challenge here... The answer to "what is the difference between wired and wireless transmission?" is: there is no significant difference. But that's not the question you're asking 😁 Your question actually is: what's the difference between wired/wireless transmission, and optical transmission. There is no big difference ...

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