h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Agreed T in T, but we've established that cell-phone radiation doesn't have sufficient power to heat.

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Except that radiofrequency emissions have a way of bending around such obstacles, which is what makes them so useful. You don't have to have a direct line-of-sight to the transmitter in order to receive. An RF source held up to your head would be able to propagate energy waves to your brain via soft areas like your eye sockets, nose, neck, etc.

I think that A) RF probably goes through bone without too much trouble (but I don't know) and B) bending of RF requires diffraction, which requires structures that are large with respect to the wavelength. Microwave is on the order of centimeters so it's borderline for bending around eye sockeets, mouths, etc.

"radiofrequency emissions have a way of bending around such obstacles"

WHAT!!!

Can I have a pint of what you've been drinking!

Sorry, but to set you right...

ELF/VLF signals - will follow the curvature of the earth
anything else travels in a straight line (MF/HF will bounce of the various levels of the ionosphere under certain conditions)

VHF and above _does_ travels in a dead straight line - and the higher the frequency, the lower the penetrative power (Ask yourself, why do handheld GPS devices not work inside buildings and why has no one developed a set-top satellite dish?

as to diffraction... now going back a few hundred years to my schooldays and experiments with a laser and a diffraction grating the you'd be looking at holes of around 15cm for microwave radiation and 36 cm for cell phone signals...

X-rays are stopped by bone, and are much higher powered than any radio frequency. So is visible light. I would guess that, for any radio frequencies that are a short enough wavelength to actually 'hit' our heads (as opposed to us being effectively invisible to them), the bone would stop them.

<<"the higher the frequency, the lower the penetrative power">>

This seems incorrect: frequency is proportional to the energy per photon.

Traveller in Time on an antenna
"Penetration power is not equivalent to power needed to generate the wave. "

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In a vacuum. Do you live in a vacuum?

Put down your crack pipe and ask yourself why your cell phone, your portable radio, and (if you're old enough to remember) your VHF/UHF television with the bunny ears all work indoors. Ask yourself how ham radio operators talk across UHF frequencies thousands of miles from each other where line-of-sight is impossible due to the curvature of the earth. Ask yourself why cell towers aren't built 1,000 feet high in an effort to increase line-of-sight areas around trees and buildings.

Refraction.

But I was really enjoying my crack! Oh OK.

>>>Put down your crack pipe and ask yourself

Ah - we're in for a reasoned arguement then...

>>>your cell phone, your portable radio, and (if you're old enough to remember) your VHF/UHF television with the bunny ears all work indoors.


Cell phones work at about 800-900 MHz - that's low enough to penetrate a brick wall, but try it in a sub-basement and you might have more problems

Portable radios have various bands - the highest of those is general VHF (Total range of the VHF band is 30 to 300 MHz - actual civilian commercial radio station usage is limited to 88 to 108 MHz) easily low enough to penetrate brick - but certainly wouldn't work in a mine. (What happens to your car radio when you drive into a tunnel?)

(oh and UHF TV broadcasts are in the 470 to 806 MHz range)



>>>Ask yourself how ham radio operators talk across UHF frequencies thousands of miles from each other where line-of-sight is impossible due to the curvature of the earth.

Simple - they don't - when they're talking over long distances Hams are using the HF range (3-30 MHz) You can contact someone using UHF frequencies over a long distance and they do occassionally, using a phenomenon known as 'moonbounce' (do I really have to explain that involves bouncing your radio signal of a ruddy great rock in space) and to do this, obviously you need a radio signal to travel in a straight line - and they generally use the 2 m band for this (150 MHz ish)

HF signals on the other hand bounce of the various levels of the ionosphere - during the daytime the best results can be had by using frequencies up in the 10-20 MHz range, whilst at night, the various layers of the ionosphere change and it's generally easier to get a signal over lower frequencies (5-10 MHz and below).



>>>>Ask yourself why cell towers aren't built 1,000 feet high in an effort to increase line-of-sight areas around trees and buildings.

Yet again, line of sight is not required as long as the frequency is low enough... The phrase "Cell Tower" you use is a bit of a giveaway itself - why build towers at all if radiowaves bend? It's because the tower (or high building) extends the range of the transmitter because of the curvature of the earth - but the waves themselves are of a sufficiently low frequency to penetrate bricks and mortar.

When you get up into the 2GHz range, then you start getting problems with penetration of bricks and even thin sheets of metal.

Think about it - a couple of mm of metal is enough to reflect a signal from a satellite - otherwise how the hell would the signal hitting your parabola be bounced back at the LNB sat in front of it. (Oh, and why wouldn't a cup of water placed _on top_ of a microwave over start to boil).

None of this has anything at all to do with refraction.
Sure - refraction 'might' theoretically occur when radio waves pass through differing densities of air, but the actual angles involved would be negligible.

Reflection on the other hand does occur - which is one of the reasons in mountainous areas you often get 'ghost' images on TV sets - caused by a 'double' reception of a signal - once direct from the transmitter and once reflected from a mountain.



Whisky
(Ex Communications Technician - Royal Navy - i.e.: You're talking to someone with more than a vague inkling of what they're going on about).

QSO 555

Shouldn't that be QSA 5 or QRK 5 ?

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I'm merely providing the kind of argument you requested with your pint reference.

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A brick wall, perhaps. But how about many brick walls? At street level in the middle of a busy city, a direct line of sight to the nearest cell tower would pass through many buildings before reaching you. You would have more blackout spots than not, but that does not appear to be the case.

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What happens is the radio gradually fades out, rather than being cut off suddenly, as you would expect if a line-of-sight were required.

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Moonbounce is beyond the capacity of most amateur operators because they don't have the power. Long-distance signals from weaker handsets are picked up due to bending of waves in temperature inversion layers, the ionosphere, and around physical structures like mountains and buildings.

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Good... you've just made my case for me. Is there a physical object in the ionosphere the signals are bouncing off of? No. "Bounce" in this case is the same as what happens to the light waves when you stick a straw in water. Refraction, not reflection.

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You build towers because radio waves don't bend perfectly and retain full power, so increasing line-of-sight coverage increases overall coverage and reduces dead spots. Apparently they're not teaching waveform theory very well in the Royal Navy, so I will elaborate. Go to a huge swimming pool (to minimize interference from reflection) with a still surface, preferably indoors. Put a solid object on the water, then touch the water on one side of it. You'll see motion on the other side as the waves bend around the object... but they're not nearly as big as the ones you didn't block, are they?

In the water this happens because you have a rise in the water level, and molecular bonding/surface tension are going to make one sure that as one uninterfered with line of water rises up, it'll pull up the adjacent molecules which aren't directly affected. Electrical fields travelling through air will do the same thing. You can't suddenly affect an air molecule with an electrical field without it inducing a change in the molecules next to it... and so the signal bends around the object, at a reduced strength.

<<(Ex Communications Technician - Royal Navy - i.e.: You're talking to someone with more than a vague inkling of what they're going on about).>>

The ex-Fire Controlman of the United States Navy in me is unimpressed.

Traveller in Time still impressed by microprocessors above radio frequency
"Waves in a pool can at best be compared to Ultra Long Waves on the radio band.

In the city center you have lots of reflections, walk inside a room and see the level of cell phone receiver sway from peak to nil. Also notice the number of relay towers in a city center is way higher then needed for the number of users.

Entering a tunnel gives indeed a gradual decrease of reception, partly due to reflections and partly as the structure (metal parts) act as secondary antennas. (Was a measuring project at school) "

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A brick wall, perhaps. But how about many brick walls? At street level in the middle of a busy city, a direct line of sight to the nearest cell tower would pass through many buildings before reaching you. You would have more blackout spots than not, but that does not appear to be the case.


As TinT says - reflection



>>>What happens is the radio gradually fades out, rather than being cut off suddenly, as you would expect if a line-of-sight were required.

Reflection again - radiation gets into the mouth of the tunnel and through the relatively thin covering nearer the entrance then bounces of the walls getting absorbed as it goes... the further you get into the tunnel the more EMR is being absorbed by the walls rather than reflected off them

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Is there a physical object in the ionosphere the signals are bouncing off of? No. "Bounce" in this case is the same as what happens to the light waves when you stick a straw in water. Refraction, not reflection.

No, there's a ELECTROMAGNETIC field up their - hint IONosphere... hence having an effect on the ELECTROMAGNETIC radiation that is a radiowave.

Yes, refraction may occur in certain circumstances, but it's infinitesimal in its effects in comparison with reflection.

Oh, and by the way - people have managed an EME transmission using a 20W transmitter and a morse key - but you're right - it's not all radio hams that try it - it's mostly the geeks (it's far easier to use HF - you don't need to worry about directional antennas unless you're really aiming for distance - but if you're wondering how much power it would use, just think about how little power your average satellite telephone uses , double it for the return journey through the atmosphere and then double it again to counter the signal absorbed by the surface of the moon and you're along the right lines for a theoretical minimum.

To put the 20W power level in context - our HF transmitters used to kick out hundreds of watts at a time - and you _really_ didn't want to spend too much time working next to the antennae.

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That still doesn't explain range extension utilizing thermal layers, nor the phenomenon of UHF signals knife-edging over and down the sides of mountains.

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You need a better quality return signal for voice than you do for Morse. They'd have to boost the power output significantly to make moonbounce work for them.

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Our toys were rated in the megawatt range.

Must admit - the only time I've ever heard of thermal layers being used to extend range is in Sonar never radio - can't see it working frankly.

What thermal layers are you talking about, what frequency range and what effect?

Oh and in response to "Our toys were rated in the megawatt range"

We're not going to degenerate into 'Mine's bigger than yours' are we

This should answer your question regarding thermal layers: http://en.wikipedia.org/wiki/Radio_propagation#Tropospheric_ducting_and_enhancement_or_refraction_via_inversion_layer

Scuttlebutt had it that my S-band radar's range could have been extended greatly by improving the software to account for ducting.