Can a Cell Phone Spin Too Fast to Communicate?

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Discussion Overview

The discussion revolves around the question of whether a cell phone can spin so rapidly that it becomes unable to communicate via radio waves. Participants explore the mechanics of cell phone antennas, the effects of rotation and speed on signal reception, and the implications of the Doppler effect in high-speed scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if a rapidly spinning cell phone could fail to communicate due to its orientation and the mechanics of radio wave propagation.
  • Another participant mentions that fast movement can change the frequency of electromagnetic waves due to the Doppler effect, potentially causing communication issues if the frequency shift is significant.
  • Concerns are raised about the relevance of rotation versus linear movement, with some arguing that rotation does not affect the signal unless the speed of individual antenna components becomes significant.
  • One participant discusses the historical context of cell phone communication from high-speed trains and airplanes, noting that while speed can affect communication, it is not necessarily prohibitive.
  • Another participant introduces the idea that the type of antenna structure in the phone (monopole, dipole, or loop) can influence how spinning affects signal reception.
  • References to external articles are made, discussing the Doppler effect and its implications for mobile communication systems, particularly in high-speed scenarios.

Areas of Agreement / Disagreement

Participants express differing views on the impact of rotation and speed on cell phone communication. There is no consensus on whether a spinning phone could effectively communicate, as various factors such as antenna design and the Doppler effect are debated.

Contextual Notes

Participants acknowledge limitations in their understanding of the mechanics involved, and some express uncertainty about the specifics of how rotation affects signal reception. The discussion includes references to specific frequencies and communication technologies, but these are not universally agreed upon.

GreenAce92
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I asked this question in Yahoo Answers but I didn't think that the responses were very good

This is just for my own knowledge

I wanted to know if let's say for instance that I had a rotary device, clamp right on the center of mass of my phone and it spun rapidly... could the phone spin so fast that it is unable to communicate with radio waves?

I don't really know the mechanics/structures of cell phones, for one it doesn't seem like orientation matters in receiving a signal with your cell phones. I am not saying that the radiation pattern of cell phone towers and perhaps the cell phones themselves are circularly polarized rather than linear but anyway...

My phone is quite symmetric so assuming that it could spin in excess of something like 3000 rpms which someone told me is actually pretty slow.

When doing a test to determine the speed of light, we used interference... well it was a rotating mirror that operated at a varying rpms, even though it was spinning rapidly, the location of the reflected beam was predictable or at least the beam struck the same location enough times for me to count it as "stationary", a dot on a wall for instance.

That was a semester ago so I apologize if my terminology is not correct and I haven't been keeping up with my physics lately.
 
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A fast movement changes the frequency of incoming/outgoing electromagnetic waves due to the Doppler effect. If the change is too big, you might miss the frequency band used for communication. This can be an issue in fast-moving trains, so it is certainly possible. But: It needs an actual movement of the antenna. Rotation does not matter, unless the speed of individual parts of the antenna becomes relevant (something you won't get in your setup).

The rotating mirror is not related to this experiment.
 


mfb said:
... If the change is too big, you might miss the frequency band used for communication. This can be an issue in fast-moving trains...

What kind of trains are you talking about. I know that cell phone calls were made from the planes that crashed on 9/11. I'm sure they were going several hundred kph. That's faster then any bullet train that I know of but I'm no train buff.
 


I don't understand, if you hold the cord and start swinging the phone in circle, no matter how fast you swing ( not 3000rpm, try 30000000000...rpm!), the length of the cord don't change, the wave propagate down the the cord don't see the change of length. Why should it not working?

Even on cordless phone, you do introduce a phase shift like doppler stuff when you swing. I don't know about how doppler affect the reception, but if you swing in a small circle, the distance to the cell towel is not changing by much. EM wave do travel out, just with the phase shift. Someone here might know the answer of this.
 


It is hard to find details. This article could have details, but I cannot access it.

http://upcommons.upc.edu/pfc/bitstr...ile devices to Doppler shifts and poor s….pdf
The UMTS signal uses the frequency channel of 2GHz. This
channel introduces some propagation difficulties like multipath effect, Doppler shifts
and fading.

http://www.akamaiuniversity.us/PJST10_1_286.pdf
However, in mobile WiMAX systems, the Doppler spread also has to be considered along with the above mentioned parameters in the design. For Wi-Fi, the subcarrier spacing is about 300 KHz while in mobile WiMAX, the value is around 11 KHz.
WiMAX uses frequencies up to 66 GHz. A spacing of 11 kHz corresponds to the Doppler shift at a velocity of 50m/s or 180km/h - and it gets problematic as soon as the velocity is comparable to that value.

WiMAX might be an extreme example, and not useful for trains. And I don't say communication in trains is impossible - but the Doppler effect can be relevant (even if it can be compensated if the relative velocity is known), and you have to consider it.

Airplanes are a bit special - while they have a high velocity relative to the ground in general, they fly perpendicular to the line of phone/mast for the closest masts.
Some airplanes and trains have special equipment to communicate with cellphone masts.
 


mrspeedybob said:
What kind of trains are you talking about. I know that cell phone calls were made from the planes that crashed on 9/11. I'm sure they were going several hundred kph. That's faster then any bullet train that I know of but I'm no train buff.

Yes, I know there was a problem early on in cellphone evolution where the hand-off protocols from tower to tower were a little slow, this could break the connection if you moved too fast between towers.
 


GreenAce92 said:
I asked this question in Yahoo Answers but I didn't think that the responses were very good

This is just for my own knowledge

I wanted to know if let's say for instance that I had a rotary device, clamp right on the center of mass of my phone and it spun rapidly... could the phone spin so fast that it is unable to communicate with radio waves?

I don't really know the mechanics/structures of cell phones, for one it doesn't seem like orientation matters in receiving a signal with your cell phones. I am not saying that the radiation pattern of cell phone towers and perhaps the cell phones themselves are circularly polarized rather than linear but anyway...

My phone is quite symmetric so assuming that it could spin in excess of something like 3000 rpms which someone told me is actually pretty slow.

When doing a test to determine the speed of light, we used interference... well it was a rotating mirror that operated at a varying rpms, even though it was spinning rapidly, the location of the reflected beam was predictable or at least the beam struck the same location enough times for me to count it as "stationary", a dot on a wall for instance.

That was a semester ago so I apologize if my terminology is not correct and I haven't been keeping up with my physics lately.

For the spinning case, it would depend on what type of antenna structure was used in the cell phone. If it is a monopole or dipole structure, and the spin axis is parallel to the antenna elements, then there are no issues. If the spin axis is perpendicular to the elements, then you will get a modulation of the received signal that may interfere with reception. If the antenna structure is a loop, then the directions of the spin axis that cause problems are reversed compared to the linear monopole and dipole antennas.
 

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