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Post Time: 04/12/2007 11:01:07 |
SHneuro
Total Forum Posts: 10 |
Can you please give me some idea of the electrical field strengths emitted by a range of mobile phones in use (V/m) (right next to the phone, as experiences by the side of the head to which it is held), as my monitor only goes up to 6.5V/m, and mobile phones exceed this?
Is there a monitor which measures only microwave frequencies, but ranges from 0.1V/m to much higher levels, so that mobile phones, and WiFi can be measured and compared?
Thank you.
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Post Time: 04/12/2007 11:31:03 |
alasdairP
Total Forum Posts: 173 |
Electric fields from mobile phone handsets are in the range 75-300 V/m, but the actual numbers are very variable and are not particularly relevant. A tiny (few mm) movement of the position of the phone will change the level considerably as the field levels depends on the 'loading' you head presents to the signal.
In fact, close to your head the magnetic field component (which creates the SAR induced power into your brain) is more important and this does not have a mathematical relationship to the external electric microwave field. The currents inside the handset from the battery also create several microtesla low-frequency (ELF) magnetic fields throughout your brain. 0.4 microtesla ELF magnetic field is strongly associated with a doubling in childhood leukaemia.
So, compare handsets by their SAR values (published at a few internet sites).
Only use the phone when you have all signal strength bars showing. A modern mobile phone can control its power over a range 1000:1, so in a poor signal area 1000 times more power is being emitted than when you you the same handset close to a network basestation.
Volts/metre (such as the COM measures) is really most suitable for far-field exposures - that is where you are at least about 30 cm from the transmitter. Closer than that, very specialised and expensive laboratory equipment has to be used to asses the power in the signal.
Most meters that measure much over about 6 V/m are not good below about 0.5 V/m.
EMFields sells the Gigahertz HFE35C meter which is good at lower levels.
Aaronia (in Germany) sell a variety of meters and anlysers which are very good value for money, but which have had their accuracy queried and we find can be confusing to use and interpret. Still, they do have ones with the range you are interested in. Their HF Detecor 3 Pro A3 sells for only 200 euros and will basically do what you want. |
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Post Time: 06/12/2007 09:42:24 |
SHneuro
Total Forum Posts: 10 |
Thank you for your reply, and suggestions of monitors.
I thought that SAR (specific absorption rate) was measured in W/Kg, and that there is a relationship between W/m2 and V/m? I'm a bit confused by whether the SAR is related to the electric field strength. How is the magnetic field strength used to calculate the SAR? Can you please tell me whether the electric field and magnetic fields are linked, i.e. does one increase as the other increases? You mention that the magnetic field strength is more important. Are there studies suggesting that this has more of an effect on the body than the electric fields?
Thank you for your help.
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Post Time: 06/12/2007 18:00:00 |
alasdairP
Total Forum Posts: 173 |
You are correct in that SAR is the absorbed heat load in W/kg, in Europe it is the max in any 10g of tissue. In the USA it is the max in any 1g of tissue, which is about twice as strict.
In the near field (about up to 30cm - it varies from 1/6th the wavelength up to several times the wavelength, so at 900 MHz it is in the region 5 to 60 cm, mostly in the distance up to about 10 cm from the handset) that dominant factor is the H-field (magnetic reactive field) that creates the SAR in the user's head. In this region it is not appropriate to use any field-strength measurement to predict the SAR - the only satisfactory way is to use a suitable SAR system (like DASY 3) that uses a small power probe and special liquid that simulates brain tissue.
When you get into the far field - you are fully into to it by about 5 times the wavelength - 1.6 metres for 900 MHz and 0.8 metres for 1800 MHz and 0.6 metres at 2.4 GHz - then the electric and magnetic fields do have a mathematical relationship with each other and it is adequate to just measure the E-fields in order to work out the power density.
You would have a reasonable idea if you measured E-field from the phone when held it at arms length, but that would not relate to the SAR in your head when you hold the handset against your ear.
Basically, our advice would be not to have any microwave RF source closer to your body than about 1 metre away and to minimise your exposure. If you HAVE to use a handset, then use an air tube hands-free kit. If you HAVE to hold it against your head, then SAR is a reasonable indicator.
However SAR is not even an ideal indicator for this, as the tests specify max SAR when the handset is working at maximum power. Some of the early Nokias (e.g. 2010) had very high max SARs, but low typical SAR's because their antennas were very efficient at radiating the signal and thus allowed the handset to usually work at quite low power. Some of the low max SAR handsets had poor radiating antennas and so, in typical use, produced higher SARs in the users head than the apparently high Nokias.
As usual in these matters, nothing is simple.
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