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4

The radio spectrum is limited. There are is only a finite range of frequencies over which we can transmit so you if have two people communicate by directly transmitting to each other over two frequencies, no one within the maximum transmission range of either user can use that frequency. If the maximum range of those users is very large, then that means very ...


3

While your concept is theoretically correct, it doesn't work well in practice except for rough locations. The problem is interference. Radio waves get absorbed, reflected, diffracted, and in some cases even refracted. All those mean that there isn't just a single point source from the receiver's point of view. The various components of the signal ...


2

On the BBC in London, in the 1980s, if a BBC radio station had to report on a live sports fixture, and it was not a live broadcast of the whole game, just a half-time report, or full-time report, the reporters usually phoned the London studio from a public pay-phone (land-line) and were switched live into the on-air feed. For a soccer match, broadcast with a ...


1

Crystal radios don't have an intermediate stage: the envelope of the RF is detected. Wikipedia has a good description. While you're there check "superheterodyne receiver" for a discussion of why modern radios always have IF stages.


1

Your premise is wrong. Governments and private industry did indeed expend great efforts to bring radio to the public beginning over 100 years ago. This was quite successful and until the invention of television, most homes had radios in them for receiving news and music. Those radio facilities covered both local broadcasting (within 50 to 100 miles) and long-...


1

I would see what variable resistors were available and using those ranges do a spreadsheet to calculate results of cycling through the resistor range with known values for L and C. Then you can decide input voltage etc - will they be able to touch it? Safety?? 12V or 120V or 1200V?


1

Well, in 1985 cellphones were first used. The handset weighed less than a kilogram without the charger, but battery life was only 2-3 hours. At the same time military field radios varied in size from the size of a toaster up to the size of a backpack, with the battery being the largest part. So I suppose the thing in the picture is possible. There was a ...


1

For people who live near an AM radio transmitter, getting demodulated AM audio bleeding into another circuit is a common experience. The most common cause is poor grounding of the equipment that is picking up the radio signals. The easiest way to defeat it is by making certain that the chassis ground of the equipment is well-connected to the ground terminal ...


1

Your equation is correct for your definition of $d$. However, your definition of $d$ seems incorrect. $d$ is supposed to the be distance between the transmitter and the receiver, that is the distance on a straight line from the transmitter to the receiver. In your diagram you have (I think incorrectly) defined $d$ as the horizontal distance on some plane, ...


1

The old analog signal TVs had a wide frequency range allocated to each channel. Channel 2 for instance occupied everything between 54 MHz and 60 MHz. Because of effects of temperature and other conditions on the hardware (e.g. resistors and capacitors), the transmitter and the receiver could drift slightly from the exact frequency. To compensate, the TV had ...


1

a chunk of material will absorb electromagnetic radiation if 1) its characteristic impedance is close to that of a vacuum (to minimize reflections) and 2) if its impedance contains the right amount of ohmic resistance, so as to dissipate the radiation into heat and not re-radiate it. that said, to interact significantly with EM radiation of any wavelength ...


1

Typical distance sensors depend on time-of-flight. If you're trying to hack the distance from return power level, you will not be happy (to put it mildly). It's next to impossible to calibrate any system that way. As to direction, it should be obvious that you cannot possibly do so with a single receiver. As a minimum you'd need two sensors aka two ...


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I believe there has been some research done on robot localization based on Wifi signal strength. For example: http://robotics.usc.edu/~ahoward/projects_wifi.php and http://www.cs.cmu.edu/~mmv/papers/10icra-joydeep.pdf. But I believe accuracy is very rough (maybe meters of error). For more precise radio localization (cm level), you can use ultrawideband (UWB)...


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