How many channels on a mobile phone base-station?

[logbook]

I was wondering if anyone has any facts regarding the number of channels available from a single mobile phone mast/base-station in say London (UK). The question arises in terms of the exposure to RF signal power. We keep being told that a mobile phone puts out more power to the user than the base station does, because the base station is so far away. But if there are 1000 channels running simultaneously on the base-station then the "small amount" of absorbed power received by the "passive non-phoner" just got 1000x bigger.

 

[IRstuff]

No, even then, there is the 1/r^2 field expansion factor, which means that you would need to be within about 3 ft of a transmitter to get the equivalent radiated energy for 1000 active channels.

TTFN

 

[logbook]

You missed the "passive non-phoner" concept. If you use the phone for 3 minutes that is one amount. If someone else lives in the field for 12 hours a day, that is a somewhat different concept.

 

[IRstuff]

Sure, but do the math again.

Let's say that you do 3 minutes of calling per day. 12 hrs is 240 times longer, whose square root is a bit over 15. So you'd need to live within 45 ft of a transmitter for 12 hrs a day to get the equivalent exposure from 3 minutes of calling per day.

TTFN

 

[logbook]

I’m not sure why you took the square root of the exposure time. If we just consider this as a total RF energy input then isn’t it just power times time?

Did you take the 1000 channels into account as well?

 

[IRstuff]

The total RF energy is somewhat omni-directionally spread.

So, if you consider the two cases of phone with 3 minutes exposure vs 720 minutes exposure, you'd need to farther away in the second case by the square root of the ratio of the exposure time to get the equivalent exposure.

Likewise, the 1000 channels could be equated to an equivalent of 1000 times more radiated power than the phone, so you'd need to be 33.3 times farther away from a transmitting tower to get the same exposure as from your phone.

I used 1 inch as the phone distance, which is clearly higher than in reality, but it gives an additional bit of conservatism.

Relatively speaking, people living near radio towers probably have a bigger risk. A cellular tower is limited in output power to keep its transmissions within the boundaries of its cell, while a radio transmitter is design to maximize transmission distance, which means maximizing transmitted power. Diito for any ham radio operators in your neighborhood.

TTFN

 

[logbook]

Thanks for the useful discussion IRStuff.

I would use slightly different numbers. The interaction with the eyes, for example, would be at around 6 inches with the phone held in the usual way.

Say 3 minutes / (0.5 feet)^2 = 12 energy units.
Against 1000 channels * 720 minutes /(100 feet)^2 = 72 energy units.

Given the reduced power output from the transmitter for nearby phones, the two values could be similar, although the 1000 channels is pure-guesswork. I would love to know the real facts. Maybe its 10,000 channels?

Since nobody regulates buildings near transmitters, it might be unfortunate if a building was near some metal clad structure than made nasty standing waves in which certain houses got immersed. I wouldn’t want to live in an anti-node!