In almost every large building I enter, my cell phone reception drops. Sometimes I can’t any at all.
At first I was assuming because large buildings tend have more than one floor, the waves will have difficulty reaching cell phones. Especially if I am on lower floors. But I have noticed that I also have trouble making calls or surfing the internet in one story large buildings.
I am wondering if cell phone reception is reduced purposely in large public buildings, for security reasons.
I think it is illegal in many places to intentionally jam cell phone reception, even on your own private property. What if someone needs to call 911?
Why are you assuming the signal will be coming at you from above? Mobile-phone masts are generally pretty much at ground level, on short-ish masts (there are exceptions, obviously) so the signal will be coming more-or-less horizontally in most cases. (Say the mast is even 100ft high, which I’d say is taller than average. If you’re even half a mile away then the line of sight is pretty close to horizontal if you’re standing on the ground.)
I’ve always assumed the reason is simply that large buildings are, well, large, and have more walls between you and the outside world - and additionally, big office buildings are more likely to be of steel-framed construction, which will have a shielding effect (think Faraday cages). That is true whether it’s a single storey or not.
Not to mention the ductwork, plumbing, wiring conduit, equipment room/floor, and probably a few other things I’m not thinking of.
The effect is not deliberate and is being addressed with these kinds of gizmos:
Riva Networks Nano Cell Building Applications
WARNING link is to a PDF.
Essentially these devices are network amplifying repeaters that take a cell signal and allow it to penetrate better into areas that are partially shielded by intervening structures (i.e. walls, support pillars, duct-work, etc…)
It’s illegal to broadcast a signal that interferes with a cellphone.
It’s not illegal at all to put up a barrier that keeps a cellphone from reaching the tower.
I can tell you when I worked in a two story building that housed 3000 people, plus had some prototype factory lines, we had crappy cell phone coverage on certain bands in some locations.
We installed repeaters in key areas to improve the situation.
I’d also note that the building was built for us, and we designed cell phones in it.
Either it’s the walls and other metal, or we intentionally made the signal bad, and then paid even more money to get around the issue.
(Hint: It was the walls and other metal.)
-D/a
How would the phone or the tower know you were in a large building?
One of the most common types of commercial building construction in North America is Concrete Tilt-up. A concrete slab is poured out on flat ground and then a crane tilts it up and the walls are bolted together.
Many large stores, warehouses and office buildings are built this way. Your cell phone signal is being blocked by 8 inches, or more, of concrete.
So, if only they could develop the technology to carry cell phone conversations on beams of neutrinos! Problem solved. 
I work in a new building that has repeaters, or similar technology, built into it. I get great reception anywhere, including in the elevators.
When I was in tech school, one of my buddies theorized that the Air Force designed the dormitories specifically to block cell phone signals for some obscure reason (along the lines of “to dick with us”) .
I pointed out that this would require the Air Force to spend significantly more money on the dormitory design, and we both agreed that it was much more likely that it was just bad luck rather than design that lead to the crappy cell reception in the dorms.
When I was stationed in Korea, many of the buildings had cell phone repeaters mounted on the walls inside. Little boxy blade looking things that stuck out like street signs, with the phone company’s logo on it. We had a couple in the gym, which struck me as mildly funny because who the heck talks on their phone while working out?
I don’t know how common it is in commercial buildings, but in industrial buildings they will often bolt or weld the rebar together at the ends, effectively connecting it all together to form a big grid. This grid then gets connected to the building’s earth ground to form a very cheap but very effective ground system. It gives the building (and the equipment inside of it) excellent protection against lightning strikes, but as a side effect it’s going to pretty much totally kill cell phone reception inside the building.
The technique is called an “Ufer Ground” for those who want more info.