# Tagged Questions

149 views
+50

### Sequence of E and B field in radio waves and in single photons

In antenna technology we distinguish between nearfield and widefield. In the nearfield the electric and the magnetic fields are shifted by 90°. If you look closer you can see that there are two ...
69 views

### Electromagnetic waves in an antenna

There is a few questions that need to be answered in detail but in an easy way... What does it mean to describe the 'plane of polarisation' of electromagnetic waves? Why will some antenna have rods ...
31 views

### Wave frequencies and barrier width?

I know a fact that says a wave can go through barriers thinner than its length. This is why for example FM radio can be picked anywhere while antenna TV needs direct sight to the transmitter. Is this ...
101 views

### How is extremely low frequency (ELF) radiation collected by a submarine antenna?

The U.S. Navy Project ELF managed to generate extremely low frequency (ELF) radiation at down to $\approx 76$ Hz (implying a wavelength of $\approx 3,945$ km!). I was curious, what kind of receiving ...
45 views

### Why do (most) metals cause problems in a microwave oven? Which metals (if any) don't? [duplicate]

Most of us have seen microwave ovens with metal racks or shelves, which challenges the common notion that you can't (safely) put metal in a microwave oven. What's going on here? Is it a matter of ...
127 views

### Is there a differential equation that can represent a circuit with an arbitrary voltage source connected acrorss an antenna?

An RLC circuit with a voltage source can be characterized by the differential equation: $$LC\;\ddot{I}\left(t\right) + RC\; \dot{I}\left(t\right) + I\left(t\right)-C \;\dot{V}\left(t\right) = 0$$ ...
467 views

### Radio antenna producing waves in the visible spectrum

If a radio could produce waves in the visible light spectrum, what would the result be? This is a thought experiment that I've pondered for a few years now. I realize there are a few/many real-world ...
100 views

### Astronomical-wavelength radio (AWR) transmissions between cosmic plasmas?

My son asked me if electromagnetic waves longer than radio exist. I told him that even though physics permits such waves, there are no antennas long enough to radiate or detect them. However, on ...
1k views

### Radio antennas that are much shorter than the wavelength

From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
185 views

### The length of an antenna is twice the amplitude of the wave

I have seen it remarked in some problem sets that if you have an electromagnetic wave traveling in the $x$-direction with it's $y$-coordinate given as $y(x,t)=y_0\sin (\omega t +kx)$ and you want a ...
47 views

Given a RF sender and a receiver, suppose the obstacles between them are fixed, both their material and dielectric properties, does the path loss caused by their shadowing vary? If yes, what causes ...
2k views

### How do EM waves get detached from an antenna?

How does an electro-magnetic waves get detached from an antenna and spread to the space? While an antenna receives an EM wave, which quantity of the EM wave (electric or magnetic) is used for ...
100 views

### Getting a given wavelength radio signal given an antenna with real-world constraints

Supposing you are given a transmitting antenna of whatever type of metal is most commonly used these days, and supposing that you are applying an AC current with the intent of transmitting a 1 m ...
10k views

### Why is Near Field Communication (NFC) range limited to about 20cm?

Near Field Communication (NFC) operates at 13.56 MHz. Near Field is the region situated at a distance r << λ λ = c/f ...
410 views

I'm trying to understand how an antenna is able to understand, but have a few quesions: I understand that for an antenna to distinguish between 2 (or more) signals, they have to be in different ...
270 views

Radio signals are being transmitted in a frequency of $8.4 \times 10^9 s^{-1}$ and being received by an antenne that is capable of receiveing power of $4 \times 10^{-21} watt$ ($1 watt = 1 J ... 2answers 96 views ### What is difference between the miltary radar in 1940's from commercial antenna that is for the use of TV What is difference between the miltary radar in 1940's from commercial antenna that is for the use of TV? I have read article from some of the WW2 history website that call the German radar the ... 1answer 94 views ### Is this claim from historician true for physicist point of view? "The original Naxos I had a vertically polarized antenna, with poor results as the British radars initially used horizontal polarisation. (This seems to have been a case of the German designers being ... 2answers 1k views ### How to capture electomagnetic radiation/waves? If I wanted to find out what kind of electomagnetic waves "travel" through my room at which frequency, what kind of equipment would I need? Suppose I want to view frequencies from 0 Hz to 6 GHz. 1answer 748 views ### What are the optimal shapes, if any, for fractal antennae? A DIY fractal antenna project is described at http://www.htpc-diy.com/2012/04/diy-flexible-fractal-window-hdtv.html Mainly idle curiosity, but I was wondering what the principles behind these might ... 3answers 2k views ### Some questions about car radio and cellphone antennas 1-Why the antenna of the radio of cars is located outside the car and not inside? 2-If the answer to 1 is because that cars are like Faraday cages then how come my cell phone can receive signal ... 2answers 453 views ### Near-field around parabolic antenna? Having a discussion at work about the$H$-field around a big parabolic antenna. All of the safety tests done around the antenna only mention$E$-Field. They state in the radiating near-field the$E\$ ...
3k views

A while ago, I was hiking near the Hollywood Sign in Los Angeles. When I got to the sign, I found out it was fenced off; with several signs and a security camera promising prompt enforcement. As I was ...