Some people are wondering why no one has been able to track MH370 through cellphone data. We know the NSA has been storing cell metadata for tracking. Isn’t this the same scenario?
A brief lesson in physics is needed. Don’t be scared. No math.
Think of an antenna as the launching point for radio waves. If the antenna is a single element, those radio waves will depart at equal levels in all directions. A car’s AM/FM whip antenna is an example of this.
Remember the days of rooftop TV antennas? They had many elements. That wasn’t just to make them more unsightly!
Antennas may also include reflective or directive elements or surfaces not connected to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. – Wikipedia
That’s exactly what’s going on at the cell site.
Why bother sending signals where they’re wasted? With a directional antenna you can redirect power to where it’s needed. In most cases that means concentrating nearly all a cell tower’s power parallel or slightly down toward the ground. Pointing down slightly is necessary because of the Earth’s natural curve.
The extra gain in the antennas beams, used to increase signal strength to us on the ground, is taken from what would radiate upwards!
I’m sure I’m not the only person to ever power up a cellphone in flight to see if I could hit a tower. I’ve never been successful. This is why.
7 thoughts on “Why Cell Phones Don’t Work (Well) From The Air”
So does this mean that cell tower signals are in many ways “line of sight” like TV, AM, and FM signals?
They are line-of-sight. However, because of these directional antennas they see much better in some directions that others. Power is concentrated where it’s needed–near the ground.
Cell phones are actually radios that opperate at 2w. Ever notice you have good service in the winter, but it gets worse in the summer. Its called foliage attenuation.
Some of the broadcasting signal will be lost to trees.
Adam – That’s not totally true. One of the amazing parts of our cellular system is the dynamic adjustment of phone power. If the cell tower is getting a strong signal from your phone, it commands your phone to drop the power! This allows for small cell site footprints where necessary to increase capacity.
Max power for handheld cells is around 1/4 watt. Two watts was the norm when we had ‘base stations.’
Very good description of antenna theory, Geoff. The other half of the explanation is that the cell sites must be able to receive the tiny signal radiating from the cellphone. In a perfect scenerio, a signal of several millwatts at an approximate frequency of 1 gHz is attenuated greatly and proportionately to the distance. The atmosphere will also reduce the signal strength. Now factor in the effects of this tiny signal bouncing around an aluminum cylinder( the airplane) and the path must go through a thick layer of plexiglass ( or whatever the windows are made of). Add to that that the signal will be directed horizontally not downward towards the potential receive tower.
I have a better chance of winning Powerball!
I hate auto correct. It should read, milliwatts!
There’s been discussion of this over on PPrune. Supposedly the reason that passengers were able to get cell calls out during 9-11 is that the phones worked differently (and were not nearly as directional) back then?