Science Questions

Do modern mobile phones put out a weaker signal?

Sun, 3rd Apr 2011

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Question

Carl Keogh asked:

Hi Dr Chris ,

 

I was listening to one of the "up all night" or dr karl podcasts that you were hosting and a user was asking why do we turn off mobile phones in hospitals and airplanes.

 

I have a story that may explain why this was necessary , but I assume that todays phones dont output the same levels of power.

 

Back about 18 years ago , I was commissioned to install and upgrade Dimms inside of the equivalent of EftPos machines for a bank in Western Australia. This involved going out on site country locations , replacing the Dimm with a new dim and ringing the technical support centre to program it up and get it going .

 

Unfortunately we had what seemed to be a large quantity of faulty dimms , I would ring into the tech support to enable the new chips and they would fail , but when the chips went back they were perfect and worked fine on the benchtops.

 

We eventually worked out that the Motorola Dynatac that I was using was causing enough interference so that the "hardwired"(they used physical POTS phone lines) eftpos machine failed when it was communicating back to the central techs .

 

Also any person that was on a mobile phone call ( luckily they weren't so popular back then) would cause the eftpos machine to fail while it was in transaction back to the bank.

 

So .. yes I can see why they would not have wanted mobile phones being used in hospitals and or planes back then. But I assume todays phones put out a much weaker signal .

 

P.S. This was back in the analogue days when the range of the mobile phone towers was so much greater because of the higher strength I assume

 

Carl

Perth , Western Australia.

Answer

We put this question to Professor Mark Beach, University of Bristol...

Mark -   Yes.  The transmit power of the mobile devices is actually dropping. Although you may think your battery life is quite poor these days if youíve got a smartphone, itís not to do with the RF energy coming off the phone.  Itís more to do with the display technology and the fancy things to do with the applications.  So transmit power is going down and as I said earlier, we need to get it down in case there are any concerns on the health side, but also conserving energy and being more green.s that range.

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Carl Keogh asked the Naked Scientists: Hi Dr Chris , I was listening to one of the "up all night" or dr karl podcasts that you were hosting and a user was asking why do we turn off mobile phones in hospitals and airplanes. I have a story that may explain why this was necessary , but I assume that todays phones dont output the same levels of power. Back about 18 years ago , I was commissioned to install and upgrade Dimms inside of the equivalent of EftPos machines for a bank in Western Australia. This involved going out on site country locations , replacing the Dimm with a new dim and ringing the technical support centre to program it up and get it going . Unfortunately we had what seemed to be a large quantity of faulty dimms , I would ring into the tech support to enable the new chips and they would fail , but when the chips went back they were perfect and worked fine on the benchtops. We eventually worked out that the Motorola Dynatac that I was using was causing enough interference so that the "hardwired"(they used physical POTS phone lines) eftpos machine failed when it was communicating back to the central techs . Also any person that was on a mobile phone call ( luckily they weren't so popular back then) would cause the eftpos machine to fail while it was in transaction back to the bank. So .. yes I can see why they would not have wanted mobile phones being used in hospitals and or planes back then. But I assume todays phones put out a much weaker signal . P.S. This was back in the analogue days when the range of the mobile phone towers was so much greater because of the higher strength I assume Carl Perth , Western Australia. What do you think? Carl Keogh , Sun, 6th Feb 2011

Modern cellphones can put out quite a lot of RF power (up to 2W) though they don't need to if the cell they are communicating with is nearby. Adaptivity prolongs battery life between charges. I am not sure how this compares with the old "bricks" but, nonetheless, mobile phones can and do still put out a lot of RF energy and this can and does disrupt electronic equipment. If you have a phone near a TV or a computer with a sound system you will often hear a familiar sound resulting from disruption of the equipment's sound system by your mobile connecting with a local cell; it can be more severe if the cell is distant or shielded because the phone tries harder and ramps its power up. It is not at all obvious that sensitive equipment could not be made to fail to work correctly with such interference. hospitals and airlines, with advice from equipment manufacturers, have taken a cautious approach here and I think this is right. The only way to verify this is by a lot of practical and exhaustive testing with each system. Screening all sensitive circuits from RF interference is not trivial and neither is testing and verifying such equipment in a worst case scenario. Most hospitals have relaxed such restrictions now - at least by a "normal" bed. Some airlines also permit use of some electronic equipment and take a more relaxed view of mobile phones, although I do wonder whether the attitude is one where they can steal advantage over their more cautious competitors at the expense of taking a fairly low risk (if high stake) gamble; I guess they would not suggest that this is what they are doing though. graham.d, Mon, 7th Feb 2011

Modern phones reduce power till the link to and from the cell tower is just adequate ( with a very small margin) and continue to adjust power depending on just how the tower tells it it is being received. Thus if you are close to the tower, and are not shielded by walls, buildings, your head/hand or an accessory that absorbs energy ( those twinkly lights that are powered by the RF the phone transmits) then the power output drops to a very low level.

On the other hand, at extreme range, the phone will transmit with full power, up to 2W with modern phones, some of the old Nokia bricks would go up to 5W at full power ( great in a fringe area, especially with a wired car kit that had an antenna used instead of the phone one) in an attempt to maintain connection.

In a plane the phone will be searching for a cell tower, and will transmit at full power, not a good idea in a good approximation to a Faraday cage, especially if there are another 200 doing the same Put 400W of any type of energy in that volume and you will be easily able to measure the temperature rise that results. SeanB, Mon, 7th Feb 2011

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