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atb
# Posted: 2 Feb 2007 13:54
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Apart from Mobile Phones, DECT, Wifi, and WiMAX, there's a possible new contender for a whole new network of pulsed RF... fill-in transmitters for satellite radio like Sirius and XM (2.3GHz) and XM in the USA. Apparently these rely upon either a couple of geostationary sats, or a fleet of hundreds of low-orbit 'birds'. But because of the problems of maintaing a good view of the sky in built up areas, apparently there are thousands of terrestial fill-in transmitters.

http://en.wikipedia.org/wiki/Sirius_Satellite_Radio
http://en.wikipedia.org/wiki/XM_Satellite_Radio

These use OFDM (see http://www.radioeng.co.uk/ofdm.html ) like DAB and DTV in the UK (and DRM soon, perhaps).

From what I've read so far, it seems that an OFDM transmission consists of hundreds/thousands of narrowband carriers at once, each of which are modulated using schemes such as Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK).

I thought that in practise this probably means that the total power output is fairly constant, with much less obvious pulsing than other single-carrier systems? Does anyone suffer ill effects or hear nasty things on detector equipment around DAB/DTV sites?

http://en.wikipedia.org/wiki/OFDM says that Wifi and WiMax use OFDM, so I guess that answers that question - it also says that transmitters need to be very linear, which is a give-away that the total power is not constant.

Anyhow, there's a possibilty that a European version of Sirius/XM may arrive in the next decade, and that means even more 'electrosmog' in cities with fill-in sites.

abacus
Member
# Posted: 2 Feb 2007 23:10
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I have a theory based on the received idea that microwaves excite water molecules,as in microwave ovens.This may be inaccurate.Anyway,the theory is that microwaves emitted from phones and transmitters,if they do not get picked up electronically travel and are absorbed by material.When water molecules absorb microwaves they become excited-their energy levels are increased,analagous to being heated conventionally.As water is ubiquitous,it's excitation can take place in whatever substance it is incorporated-plant,animal,mineral-in response to the absorption of microwaves.Thus in it's many forms,it provides a 'sponge' which can absorb stray microwaves.
In this theory,the energy levels of water molecules-in whatever form-are cumulatively rising.Based on the above received idea-which may be inaccurate-which says that microwave energy provides a highly effective means of heating ,i.e. increasing the energy levels of water molecules,then the water molecules in the enviroment are being effectively excited over and beyond the electromagnetic radiation from the sun and the heat from the earths core by the generation and transmission of microwaves and consequent absorption of excess microwaves.
Before this technology,there was the rise and fall of temperature,the increase and loss of energy,with day and night.The heating up and cooling of the enviroment.
Water vapour constitutes an effective cooling mechanism by which energy can be absorbed.If water molecules are already pre-excited,and effectively so, through the absorption of microwaves,on an ongoing basis,then their cooling capacity is reduced.Thus the heating effect of the sun is increased through the reduced cooling capacity of water molecules which have been effectively excited by microwaves.This is an ongoing and cumulative effect.The more water molecules are excited,the less their cooling or heat absorption capacity ,meaning that the greater the heating effect of the sun.That's the theory.What do you think?

atb
# Posted: 28 Jun 2007 12:01
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Another new tech : a 60GHz wideband digital system for sending home entertainment signals around the house.

http://www.eetimes.eu/200000922

I know wires and cables can be an unsightly nuisance, but surely they're easier, 'greener' and safer in the long run than some over-engineered wireless 'solution'?

Henrik
Admin
# Posted: 28 Jun 2007 13:20
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atb,

OFDM is more of a subband-compositing method where subbands are transmitted at orthogonal frequencies with respect to each other.
With OFDM each sub-band can use it's own modulation scheme and the subbands are separated by Fast Fourier Transfomation.
At lower bitrates, there is use of guard intervals which should give it a "pulsy" nature like GSM.

On the 60GHz issue, I've read a translation of Russian research into microwave resonance of biological cells. The Russian researchers found that cells react to microwaves in the 30-60GHz range... I'll post a link when I've dug it out of the pile.

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