Microwaves

wolsty

wolsty

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#1
I know there's a straightforward answer to this question, but it's evaded me so far.

When I'm sitting in my car on the vehicle deck of a Cross-Channel Ferry, I normally have a strong signal on my mobile 'phone. If microwaves need line-of-sight conditions and won't pass through several layers of steel plate, how can this happen? Is it down just to transmitter power? In which case, why are passengers not cooked by the radiation?

What is the relevance to satellite reception of this phenomenon?

Wolsty
 
rolfw

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#2
I would assume that they are being reflected from the steel surfaces in the ship Wolsty, exactly as they are with a dish.

Rolf
 
wolsty

wolsty

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#3
I'm sure you're right, Rolf. But if the bow doors are closed (they should have been during the crossing!), how do the microwaves get into the hull, which is, in effect, a closed steel box?

Wolsty
 
rolfw

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#4
Nothing to stop them bouncing off of the ceiling of the car deck, loads of surfaces to deflect the signals, they certainly don't go through steel.

rolf
 
Channel Hopper

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#5
Mobile phone frequencies are not quite line of site only, they can also bend or creep to a small extent around objects, filling a lot of void spaces. This along with

1) Dual frequency operation and
2) the straight relections off metal (and hidden metal such as concrete)

means that the main limitation is range owing to the power of the phone and the repeater.

So cross channel reception stops about 5 to 10 miles offshore normally

Its still not regarded as safe to use a mobile phone in either a car unless it has an external aerial, (or I suppose a small steel ship), as the microwaves do get concentrated near the centre of the box by virtue of focusing, so your bodily fluids will still get heated!

The debate on long term damage is still ongoing so use them wisely (foil underpants are available in Japan for protection)
 
2old4this

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#6
Interesting discussion. The question (why does my mobile phone receive refleted signals?) is asked because you've assumed (based on satellite experience) that microwaves always need line-of-sight reception. But in fact that's not generally true, and satellite reception turns out to be a special case.

So what are the differences between satellite and mobile-phone signals?

For a start, satellite transmissions use higher frequencies than those used by mobile-phone networks. The latter are between 900 and 1800 MHz, while even the lowest (C-band) sat. transmissions are at twice that higher figure. And the Ku-band is much higher still (three times higher than C-band).
The higher the frequency of electro-magnetic radiation, the less susceptible it is to refraction ("creeping"). Visible light hardly creeps at all - if it did, you would constantly be able to see "round" objects. This is because visible light is VERY high frequency (about 43,000 times higher than ku-band!).
So that's part of the answer to the question.

The other thing to bear in mind is the relative field-strength of the signals. A mobile-phone cell is no more than 35km radius, so that the user is never further than 35km from the nearest transmitter. But the satellites are at 1000 times that distance. And since signal strength drops with distance according to an inverse-square law, that means what arrives at the earth's surface is very weak indeed.
So whereas a typical mobile-phone antenna only needs to achieve something like a 50-fold gain, a satellite dish/LNB has to amplify the signal by a factor of about 100,000! Such a feat can only be achieved by gathering as much coherent signal as possible and focussing it all to a single point. This means a large gathering surface, and the larger that is, the greater the accuracy with which it must be aligned to the source. Furthermore, once you've plumped for such a focussing system, you are committed to having clear line-of-sight. This is because although refracted or reflected signals may well be incident on the dish, they will be arriving from different angles and be "focussed" at a point not occupied by the LNB.

And one final point: reflection is never perfect. Any reflecting object will absorb some of the incident signal, and will also introduce distortion in the reflected wavefront. Refraction (creeping around corners) will also cause similar losses and distortion. The resulting signal arriving at the gathering device may be highly degraded. So although a reflected signal from a relatively powerful source such as the mobile phone or terrestrial TV transmitters might still be usable, it would never be so for the much weaker satellite signals.

See here for a frequency spectrum: http://www.adec.edu/tag/spectrum.html
Here's an interesting article on mobile phone technologies: http://www.iegmp.org.uk/IEGMP-4.pdf

2old
 
Channel Hopper

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#7
Dear Wolsty

Theres always a small chance the ferry has a small repeater on the roof sending down a decent path for the signals, ensuring that you really can suffocate and phone out at the same time.

The US also has a thid frequency hence the need for triband phones when going across.

How 3G will alter all thats been said before is anybodys guess (2+GHz is a somewhat nasty thought radiating a few centimetres from your flesh and blood)
 
wolsty

wolsty

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#8
I was hoping for an informed set of answers and was not disappointed! Not a simple explanation, but very interesting.

I haven't used my mobile on board ship, but many people do, so perhaps I should stay well clear of their handsets.

A univerity lecturer friend of mine has done some research on the effects of microwaves on human tissue and, whilst he isn't unduly concerned about the effects of 'normal use', he does urge caution, particularly for young people, since his work showed a detectable temperature rise in tissue exposed to microwave radiation at mobile 'phone wavelengths. I didn't enquire too closely about the source of the tissue.

Since my ovum fertilising days are over (I had the snip 15 years ago), I think I'll forego the lead-lined underpants and confine my plans for cooked gonads to a selection from the menus across the channel.

Wolsty
 
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#9
Cooked sweet meats are not really my favourites. I had them once in a Turkish restaurant, they dont descibe what they are in the menu
 
P

p

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#10
From personal expirance my one2one phone worked about 7 / 8 miles out. But in Calias regained recepion infont of the pastport control building each time I been there. And get a free of charge call to home :)

Also all mobiles are digital now but my dads fisherman mate, used to beable to use his phone 20/25 miles out ( motorala analouge brick ) when upgrading to digital his new small nokia only worked a couple of miles out. He wern't impressed with the newer modern technolgy much!

Phil.
 
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