# Cell Phone signal in a metal building

#### GeorgeCurl

##### Junior Member
I'm new to this forum and want to ask a simple(?) technical question.

My question is: How to determine how big of a hole to cut in a metal building to allow cell phone signals to get to the folks inside? Obviously, the real practical solution to the problem is to install a repeater rather than cut a hole in the building but some folks were sitting around with a beverage or two and the question came up. You might know how these conversations develop.

So there are really two questions: How big a hole? but more importantly, how would one determine the answer to the first question?

My thoughts are: Cell phone frequencies are roughly 300MHz to 3GHz, therefore wavelengths are about 1meter to 10cm and expect the hole needs to bigger greater than the wavelength to prevent diffraction/interference. Therefore an approx. 1m hole (square??round??) might do it. (How important is it to be pointed at the cell tower?, no clue!)

Hope you can help,
George, neither a physicist nor engineer

#### WelshBloke

##### Lifer
I'm new to this forum and want to ask a simple(?) technical question.

My question is: How to determine how big of a hole to cut in a metal building to allow cell phone signals to get to the folks inside? Obviously, the real practical solution to the problem is to install a repeater rather than cut a hole in the building but some folks were sitting around with a beverage or two and the question came up. You might know how these conversations develop.

So there are really two questions: How big a hole? but more importantly, how would one determine the answer to the first question?

My thoughts are: Cell phone frequencies are roughly 300MHz to 3GHz, therefore wavelengths are about 1meter to 10cm and expect the hole needs to bigger greater than the wavelength to prevent diffraction/interference. Therefore an approx. 1m hole (square??round??) might do it. (How important is it to be pointed at the cell tower?, no clue!)

Hope you can help,
George, neither a physicist nor engineer

Seriously though, no idea. Guess it would depend on the strength of the outside signal and the internal layout of the building.

#### GeorgeCurl

##### Junior Member
OK, point taken. Building is an empty metal quonset hut, 10 meters tall, 50 meters by 30 meters and no significant internal structure. Would like to make calls from bar stool in the center of the building.

Thanks.

#### PsiStar

##### Golden Member
1.) Based on the slot antenna, 1/4 wave length slot of the lowest freq. I kind of forget how wide, but just a few cm. Probably best to find out what side the nearest cell towers are on & make the slot at the end. This is assuming that there is enough signal because this is creating another antenna (the slot) that just re-radiates the direct signal. The slot should be vertical & higher the better. Multiple slots would be better at different locations.

2.) Related is to find some kind of yagi antenna (with gain) & point at that nearest best cell tower. Route short length of coax thru wall to a 2nd antenna which could be just an omni. Like above, vertical polarized.

3.) Then there are things like this from ATT

#### bobdole369

##### Diamond Member
Cell phone frequencies are roughly 300MHz to 3GHz
In the USA - typically 700-900mhz, and 1800-2000mhz. 1700mhz, and 2500-2700 mhz.

Are you seriously keeping the entire quonset hut airtight besides this slot, and the whole exterior completely swathed in a conductor?

#### LiuKangBakinPie

##### Diamond Member
300mhz that cant be right. Switch to a UMTS 900 carrier as it has generally better penetration than 2100 depends on the cable used by the operator at the signal stations. Simple way to see if your on 900 hold the phone close to the radio speaker. If you hear interference your on 900.

#### Lemon law

##### Lifer
It seem to me, no poster on this thread has come up with a logical answer.

But our OP lives inside of a mental building, it does not matter if its a cube, a rectangular solid, or a Quonset hut, the OP is still surrounded by metal, and metal is cell phone frequency opaque. Light does not go through typical metals in any way, why should cell phone frequencies?

Therefore our OP has to think out of the box or what ever shape he is inside of.

And mount a small antenna above and outside of the building, drill a tiny hole big enough for co-ax cable to go through, ground the antenna, and use the coax outside to feed a wireless router or repeater inside the building.

#### Red Squirrel

##### No Lifer
I find that metal only degrades signal, and does not block it.

Put your cell phone in the microwave and try to call it. It will ring. In fact when I do that, I only lose 1 bar. So the shield that is made to keep the microwaves out only degrade the signal, don't actually stop it. Hopefully it's more effective against the micro waves though... lol

#### stevech

##### Senior member
change carriers to one with a cell site closer to you and who uses a lower frequency. In the US, the best is Verizon.

#### wirednuts

##### Diamond Member
wait, so that att microcell is NOT a repeater? its an actual mini cell tower??! it says you plug it into your broadband internet connection at home and it creates a cell service hotspot!

that is WAYYY freakin cool if it works as stated...................

#### PsiStar

##### Golden Member
wait, so that att microcell is NOT a repeater? its an actual mini cell tower??! it says you plug it into your broadband internet connection at home and it creates a cell service hotspot!

that is WAYYY freakin cool if it works as stated...................
It doesn't do too bad, but there are caveats I guess you would call them. I have a friend that has 1 at his home. Going in/out of range of the microcell (out/in range of the actual cell tower site) is not always seamless. Meaning my phone worked, his phone worked, but not always for his wife's!

#### Venom20

##### Senior member
I find that metal only degrades signal, and does not block it.

Put your cell phone in the microwave and try to call it. It will ring. In fact when I do that, I only lose 1 bar. So the shield that is made to keep the microwaves out only degrade the signal, don't actually stop it. Hopefully it's more effective against the micro waves though... lol
This made me chuckle somewhat. Microwave ovens are designed to block waves of 2450MHz from exiting the device. You will notice that the glass on the door has holes in it, the holes are just the correct size to ensure that the microwaves do not exit.

Cell phones operate anywhere from 450 - 2700 MHz, therefore the holes won't stop most signals. Thus, of course it will work.

You'll also need to know what type of tech your cell phone uses. For instance a GSM type vs a CDMA type. They use different wavelengths that change the properties of the waves. To skip all the technical crap, you'll notice that CDMA tech is typically better at penetrating materials but does not travel as far; whereas, GSM tech travels further but is less likely to penetrate.

So if you are looking for penetration, you'll be better off using CDMA type. You can look up which carrier uses which tech at wiki (for USA).

#### bobdole369

##### Diamond Member
But our OP lives inside of a mental building, it does not matter if its a cube, a rectangular solid, or a Quonset hut, the OP is still surrounded by metal, and metal is cell phone frequency opaque. Light does not go through typical metals in any way, why should cell phone frequencies?
I submit the idea that our OP doesn't "really" live inside a metal building. They simply aren't built that way (as in inside a faraday cage of sorts). Every house has some sort of wrap in order to prevent moisture, and most phones work inside of that. Perhaps it's metal siding? Metal of a foil type doesn't really block UHF and microwave completely. It attenuates it but it isn't radio silence inside. In most cases MF/HF/VHF might even excite a metal exterior becoming a radiator.

how would one determine the answer to the first question?
Find the wavelength of the frequency in question. Also you can use a spectrum analyzer to see whats in the air - move that SA outside and see whats there to determine how much the building attenuates the signal.

#### PsiStar

##### Golden Member
There are "pole" buildings or metal tool sheds sheathed in galvanized steel panels & can be completely closed. These are used in light industry with office space & farming for storage of equipment. They are quite common & are an effective RF shield. In other words this is a pretty common problem.

#### LiuKangBakinPie

##### Diamond Member
Open a window. Problem solved

#### PsiStar

##### Golden Member
Open a window. Problem solved
"Air waves"? Unless the window has security wire it won't make any diff.

#### Greenman

##### Lifer
From a purely practical standpoint, the hole needs to be pretty big. I've run into this problem building homes, we sometimes wrap the exterior with foil faced tyvek. Walking in through an 8' x 8' opening kills the signal within 2' of the opening. This was in an area with marginal signal strength. Phones would only work if we were right at an opening.

#### Selenium_Glow

##### Member
OK, point taken. Building is an empty metal quonset hut, 10 meters tall, 50 meters by 30 meters and no significant internal structure. Would like to make calls from bar stool in the center of the building.

Thanks.
I think instead of one big hole, it's better that we cut the roof into slots, whose thickness and length are big enough to allow the largest wavelengths (700 MHz to 900 MHz) through. The rest of the signal should permeate into the building via diffraction of electromagnetic waves. So, I will make a very rough guess and say that the slots could be around half a meter long and about 1 cm - 2 cm wide. This is complex, but it should guarantee that signal reaches most areas within the building.

...

I can't make it sound any simpler than this... :|

#### wirednuts

##### Diamond Member
It doesn't do too bad, but there are caveats I guess you would call them. I have a friend that has 1 at his home. Going in/out of range of the microcell (out/in range of the actual cell tower site) is not always seamless. Meaning my phone worked, his phone worked, but not always for his wife's!
yeah i did some research and it seems sprint has a device like that too. thing is, they charge \$5-10 a month to use it. why!? i just dont see why sprint would have to have a charge on there for it. i have read though, if u keep complaining of dropped calls and you jump through the hoops that they want you to try first you can get the mini cell service for no charge.

for what the op wants though, its already been suggested. by FAR the easiest/practical thing to do is install an external antenna feeding an internal repeater.

#### LiuKangBakinPie

##### Diamond Member
"Air waves"? Unless the window has security wire it won't make any diff.
Yes it would.

#### PsiStar

##### Golden Member
Yes it would.
no... for security wire to be ... uh ... secure. It will still be in the way.

With no security wire, the typical glass panes of a window will have no effect unless an antenna is place directly on the glass. Then the dk of the glass will affect the electrical size of the antenna. If this is not considered in the design of the antenna, then it will be detuned.

#### rcko

##### Junior Member
Just so you guys know, the premise of needing to use cell phone in a "complete" metal box is not off-the-wall.

I work at an outdoor industrial plant. We have a lot of temporary office workers coordinating maintenance activities and didn't have enough office desks for them in our control room.

So we trucked in a "ConEx" container (standard metal shipping container) that had been retrofitted inside with cubicles. The walls and doors are 4" thick metal. There are no windows. The doors are always closed and must weight about 400 pounds.

To give you an idea, the box I "live" in for 9 hours every day is blast rated to withstand an 11.25 psi outside pressure differential, and an impulse of 337 psi-ms.

Very little sound, ZERO cell phone reception, and limited utility radio. The company has strong radio repeaters around the plant, so enough gets through that we can hear people calling us over the radio, but our signals don't reach anyone else from inside the box. No AM or FM reception either.

The nearest cell tower is less than a mile away.

#### PandaBear

##### Golden Member
In theory like they said you can cut slots of certain length (1/4 of wavelength?) to allow wave to get in, but in practice you need lots of it or much bigger to get good reception as you are likely not having a direct line of sight to the tower.

If it is a small "building" it would be easier to setup a micro cell or FEM to cell and call it a day.

Metal DO block frequency if designed right. Most wireless testing are done in vaults that is completely sealed from outside

#### PsiStar

##### Golden Member
@rcko ... fresh air? There must be an air exchange + vents/holes in the box somewhere. What happens when someone farts?!?!!!

But yes, what PandaBear says ... place the device ants. near where the vents are.

#### wirednuts

##### Diamond Member
i think its ideal to have an all metal building that does a good job at rf sheilding. that way, all you need to do is install a good \$500 amplifier and run the little antenna outside. effectively, you will filter out most unwanted noise and have a super strong signal inside the building.