Wikipedia:Reference desk/Archives/Science/2012 December 17

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December 17

Doppler Phase and Frequency shift

Hi,

Suppose I am having a stationary receiver and a moving (say approaching) transmitter, there is a frequency shift due to the Doppler effect, what will be the effect on the phase? Will there be a phase shift as well?

Gulielmus estavius (talk) 19:57, 17 December 2012 (UTC)

The phase difference will continuously change. When the relative distance changes by half a wavelength you will get a π change in phase. You can think of phase as the integral of frequency over time if you are into calculus. Graeme Bartlett (talk) 20:37, 17 December 2012 (UTC)

Strictly speaking, doppler effect IS primarily a continuous phase shift. Moving the transmitter means moving the transmitter waveform - this is readily visuallised as changing the phase. Changing the phase means changing the frequency as the wave has to "catch up" or "slow down" as the case may be. Mathematically, the differential of a continuous phase shift is a step change in frequency - this fact is often used in frequency modulated radio transmitters as phase modulating a fixed frequency is easier than precisely modulating an oscillator directly. Keit 58.167.244.186 (talk) 00:15, 18 December 2012 (UTC)

To add a bit more clarity to the question: If a pulse was transmitted from the moving transmitter, and received at the stationary receiver, the received pulse would have a phase shift, or a frequency shift or both? Gulielmus estavius (talk) 18:18, 18 December 2012 (UTC)

The concept of phase is inappropriate to a pulse. Phase actually is a concept applicable to continous waves. If, say, the transmitter is moving toward the reciever, the pulse width will be shortened, if the transmitter is moving away, the pulse width will be lengthened. This is the simplest and best way to think about it.

However, by fourier analysis, any periodic waveform, including pulses assumed to be periodic, can be shown to comprise of a number of superimposed frequencies that are harmonically related. See http://en.wikipedia.org/wiki/Fourier_analysis. You can calculate the frequncy shift and phase rotation for each component frequency and then recombine them all. The resulting new pulse shape will then emerge, shortened or lengthened as I mentioned above. In practice, such a calculation woulkd be lengthy and not show anything you didn't already know, unless the medium (ie the air or whatever that the wave is travelling through) offers a significantly different attenuation to the different frequencies. If this is the case, as it is for humid air and rain at microwave frequencies, the pulse shape will be distorted. Keit 124.178.37.15 (talk) 02:13, 19 December 2012 (UTC)

Electromagnetic waves and bathroom tiles

Do bathroom tiles or any other tiles have an effect on radio waves? Specifically, would a wifi router placed in tiled room perform worse? bamse (talk) 23:46, 17 December 2012 (UTC)

They will have some effect, but probably not much, unless they contain metals (which is possible, in the glaze). However, it could either magnify or disburse the signal, depending on where the router is placed. StuRat (talk) 00:13, 18 December 2012 (UTC)

Stu likes to dash off answers without first thinking. To "magnify or disburse the signal" is presumably StuRat's way of saying the tiles could affect the direction of propagation of the signal. This can only occur if the tiles contain electrically conductive channels or a suitably graded (as in shaped like a lens) dielectric constant - neither is very likely. However, tiles are ceramic, and as a rule any ceramic that is not specifically designed for radio applications or is designed as microwave oven safe cookware will absorb the microwave frequencies used in routers. The absorbsion does not require the presence of metals as it comes from inconvenient molecular resonance, and many oxides show absorption at microwave frequencies. For example, water does not contain any metal, but absorbs the 2.5 GHz radiation used in ovens and earlier types of router very well.

Whether or not the tiles have any noticeable effect depends on other factors. For instance, even if the tiles "shadow" the signal, it may still reach the shadowed area by reflection off other metal structures. The moisture in timber can absorb the signal. If you have brick internal walls (common where I live), the bricks, another form of ceramic, will absorb some of the signal. In other words, the bathroom tiles are just one of a multitude of structures in a building that will reduce the signal strength. That's why the rule of thumb for the undegraded range of typical home routers is given as (typically) "8 m or two walls". Keit 58.167.244.186 (talk) 00:33, 18 December 2012 (UTC)

Keit likes to dash off insults without first thinking. StuRat (talk) 00:49, 18 December 2012 (UTC)

At any rate, the moral is that it's quite pointless to try to figure out which objects will have what affect, as you'd need to know the exact composition of everything in the room and then run some complex software simulations. Just move the router around and check the reception at each point. If you've ever dealt with rabbit ears, it's a similar process. StuRat (talk) 00:49, 18 December 2012 (UTC)

Speaking of which, does anyone know to what degree can you image the inside of a house simply by moving around the perimeter and/or watching the variations in signal from a moving source such as a cell phone? I imagine there must be some "top men" who have worked on that one. Wnt (talk) 15:53, 18 December 2012 (UTC)

Since there could be any number of materials and objects, some of which reflect the signal, amplify it, or absorb it, it would be a very difficult problem. I believe there have been methods developed for detecting airplanes by how they affect cell phone transmissions, but an airplane in an empty sky is a far easier problem than a crowded houses. StuRat (talk) 19:45, 18 December 2012 (UTC)

The output from a cellphone is not very suitable - not in spectrum, modulation, or in sufficient power. I would be very hessitant in saying it would be difficult though. If using a more suitable energy source, this is readily possible. Just as doppler effect and time domain reflectometry can be used to image the body's internal structures (http://en.wikipedia.org/wiki/Doppler_ultrasound#Doppler_sonography), time domain domain reflectometry (http://en.wikipedia.org/wiki/Time-domain_reflectometry) can be, and is, used to image the internal structures of a building, as where there are transitions in the dielectric constant (http://en.wikipedia.org/wiki/Dielectric_constant), a tiny amount of energy is reflected. The fraction reflected is very small, and no way large enough to concentrate the signal in the way apparently expected by StuRat in his use of the words "magnify" and "amplify", but it is still measurable, particularly if the spectrum and pulse shape of the energy source is chosen appropriately. In fact, since the variation of dielectric constant in building materials and air doesn't vary as much as the acoustic properties of the internal structures of the body, it can work quite well.

Such building penentrating radar devices are available commercially. They are used in law enforcement (looking for evidence concreted in for example, searching buildings without breaking in, in order to justify a search warrant) and in civil engineering (examining building structures for hidden defects). Looks like StuRat did a quick post on something he knows nothing about again.

In a previous career posting, I was responsible for buildings owned by my employer. One of the buildings showed some concrete spalling and cracking - a sign of overloading (or concrete cancer), even though what was in the buiding was not especially heavy. I watched the consulting engineers use one of these radars to figure out why. It showed up the steel reinforcement buried deep within the concrete quite well, along with coke cans and other rubish the builders had apparently tipped into the pour during construction. It also showed where the concrete floors transitioned into the floorspace below. If I manage to remember what it was called, I'll post the manufacturer's website.

Ratbone 60.230.205.13 (talk) 01:13, 19 December 2012 (UTC)

Looks like you posted on something you know nothing about yet again, Ratbone. See [1]. It's not an individual cell phone power we're talking about, but the summation of the power in all the cell phone towers in the area. StuRat (talk) 20:49, 19 December 2012 (UTC)

Your link returns Error 404 Page Not Found. And you have again failed to read what was posted. Wnt clearly mentioned a cell phone, not a tower. Ratbone 121.215.50.184 (talk) 02:03, 20 December 2012 (UTC)

Link works for me. And I expanded on Wnt's comment by indicating that while a single cell phone can't map out a house, a cell phone network can map out planes in the sky. StuRat (talk) 05:19, 20 December 2012 (UTC)

"As irregular as the light curve of Alpha Ceti"

What is this referring to? Full sentence: "Or was the mentality of an average girl in her first year of high school supposed to be as irregular as the light curve of Alpha Ceti?" from The Disappearance of Haruhi Suzumiya. --107.207.240.46 (talk) 23:55, 17 December 2012 (UTC)

Did you read our article on Alpha Ceti? If so, do you still need the allusion explained? μηδείς (talk) 01:34, 18 December 2012 (UTC)

Perhaps I'm dim, but having seen the above I did just read Alpha Ceti, and Alpha Ceti in fiction and I still don't understand the allusion. Tonywalton ^Talk 01:48, 18 December 2012 (UTC)

Don't think there's anything particularly notable about the light curve of Alpha Ceti [2]. Maybe the reference is meant to be Mira? Peter Greenwell (talk) 01:51, 18 December 2012 (UTC)

Or perhaps Beta Ceti, which is variable and is the brightest star in the constellation, so the author could easily have assumed it was called Alpha. --Tango (talk) 02:07, 18 December 2012 (UTC)

Apparently it's just a typo[3]. The author probably mistook it for either Alpha or Beta Ceti; we can only guess.Dncsky (talk) 03:51, 18 December 2012 (UTC)

At a guess, and without any familiarity with the work, I imagine the sentence should be read for meaning as "...supposed to be irregular?" Light curves can be irregular in the case of variable stars, but Alpha Ceti is not one. So a literal reading would be "...supposed to be completely regular?", but that strikes me as unlikely. Rather, I suspect the author was going for the first meaning I suggest, with the Alpha Ceti phrase chosen for atmospheric, rather than astronomic, effect. — Lomn 01:56, 18 December 2012 (UTC)

Our article on Minkar refers to the likelihood that it will become a Mira-type variable. That seems clear enough if you have a basic knowledge of stars. It's unclear whether the OP even realizes Alpha Ceti is a red giant. Some clarification would be helpful, otherwise we're going to end up speculating all over the place. The OP should explain in his own words what he understands and doesn't at this point. μηδείς (talk) 03:07, 18 December 2012 (UTC)

Doesn't the reference Peter gave settle it? [4] "MENKAR (Alpha Ceti) ... The star has a variety of characteristics that rather set it apart. It is a light irregular variable, changing its brightness erratically by about six percent. (It is technically known as an "Lb," or "giant irregular," variable). Menkar is also notably deficient in carbon, having less than 20% the amount found in the Sun. More interesting perhaps is that it is also observed as a source of radio waves, the radiation coming from a cool wind that blows from the star's surface. Though its exact evolutionary status is uncertain, it is surely dying. The best estimate is that it began life with about three times the mass of the Sun, has quit fusing hydrogen to helium in its core (as the Sun is doing now), has already passed the stage in which it fuses the helium to carbon, and is now entering the stage in which the carbon core contracts. Menkar is then on its way to becoming another great unstable variable like its constellation-mate Mira." I don't know the story, but perhaps the author was looking for something like a six percent variation rather than a werewolf. Wnt (talk) 03:52, 18 December 2012 (UTC)

Werewolf? μηδείς (talk) 04:52, 18 December 2012 (UTC)

Did you read our article on werewolf? If so, do you still need the allusion explained? Alternately, do you recognize just how useless this response is? — Lomn 05:03, 18 December 2012 (UTC)

Our article on The Disappearance of Haruhi Suzumiya? Was she a werewolf? μηδείς (talk) 03:30, 19 December 2012 (UTC)

I mean that if the girl had the personality of Mira she would be varying from catatonia to the most intense sort of mania. Perhaps the author's deliberate intent was to represent a girl with noticeable, but not overwhelming changes of mood. And Lomn, remember Nietschze and monsters. Wnt (talk) 15:52, 18 December 2012 (UTC)

Please remind me never to date any woman named Mira... ;-) 24.23.196.85 (talk) 02:11, 20 December 2012 (UTC)