Cell Phone technical question

Never heard any more about it. Indeed, the phones presently in the airplanes get so little use that one outfit went out of business and we wound up with "dead" skyphones in the airplane. Most passengers don't really want all the yacking going on anyhow, its a way to get a couple of hours of peace and quite or watch a movie.

We are finding that electronics are very useful nowdays. Magazines are expensive and heavy, and a hassle to restock every month or week. None of this is good for the airline, so we have cut back on the printed matter and are pushing our new entertainment systems with many channels of music and movies at every seat. Its lighter and cheaper, less hassle and the passengers love it.

Charles

RF will not pass directly through metal, HOWEVER, it will propagate along the surface until it finds a discontinuity in the metal, such as a window, non-conductive gap between rivets, gasket around doors, etc., and then "fringe" around the edge and re-radiate on the inside. For signals like cell phones, which have a wavelength significantly smaller than the dimensions of the windows, the RF will also propagate directly through the glass & then bounce around inside.

Mark B is correct, there are two reasons cell phones are not currently allowed in aircraft. The first is the possibility for interference to the navigation systems, some of which operate on frequencies relatively close to the cell phone bands. I have personal knowledge of other aircraft systems being upset by RF from a cell phone. The second is that from the sky a single phone can see a number of towers, & can cause all sorts of havoc bouncing around from one tower to the next as the airplane moves (they move a LOT faster than a car).

Technically, "pico" is a prefix meaning 10^-12, or one millionth of a millionth. In this context, however, it's used as a generic term meaning a cell that has extremely small coverage, i.e., just the inside of the aircraft.

The "pico-cell" solves both problems by minimizing the power output of the on-board phones. Cell phones use power management to extend battery life, and only transmit enough power to reach the cell they're connected to. In the case of an aircraft cell, the signal would only have to propagate a small distance (10's of feet) and the phone would turn the power down as far as it could. This both prevents the airborne phones from interfering with the aircraft navigation and minimizes the probability that a ground based cell will "see" the phone.

There is, however, another problem. The aircraft can't relay the cell phone signals directly to a cell tower on the ground, but has to have an independent communications channel to connect into the terresterial phone system. If the aircraft were to try to connect directly to the cell system to relay signals, it would create all the problems that individual phones do.

There's also a potential channel capacity limit. The current phones installed on aircraft don't get a lot of use, and a relatively narrow band communications channel can be used. However, if a bunch of passengers are all trying to yak at once, a much wider bandwidth channel will be required.

Besides that, nothing bugs me more than trying to sleep on an airplane and having the person nest to me yakking on the phone. People are rude enough will cell phones as it is...imagine being stuck for six hours next to some airheaded teenager jabbering with her friends.

Hope this explains some of the issues. Let me know if anybody has other questions.

Keith

Additionally, you don't even have to be talking for the phone to transmit. The cell system has to know where you are in order to route incoming calls to your phone. It does this by having the phones "ping" the local system occasionally, and passing the information back to a central routing computer somewhere. If you leave your phone on in an area with no service, it'll run the battery down in a few hours "pinging" at maximum power trying to find a cell site to connect to.

If you need local communications from a low flying aircraft, handheld radios are probably the best. Just make sure what you use in the aircraft is type accepted for use in that environment (not sure what the requirements for private aircraft are, but even hand-held equipment has to be certified for military & commercial aircraft).

Keith

The miniature cell sites that are proposed for in the aircraft would communicate with the cell phones in the plane but would not be able to pick up a cell phone on the ground unless the one on the ground were right outside the aircraft window. The miniature cell site on the plane would relay the signals via satellite to ground stations which would tie into the telephone network on the ground which would allow access to a particular cell site that was within range of a particular cell phone on the ground that you wish to communicate with.

With that being said, there are small portable cell sites available that will allow two or more cell phones in a particular area (each within range of the portable cell site) to communicate with each other. They operate just like the larger cell sites that handle the everyday cell phone calls except they may or may not be connected into a phone network. These types of systems were used in desert storm.

Like Keith said the idea of the small cell site on the plane is to keep its power low and tell the onboard cell phones to lower their power as well.
In normal cell phone operation the cell site looks at the signal strength of the received cell phone and sends back data to tell the phone to either increase or decrease its transmit power depending on how well the cell site is receiving the signal. This is how the whole cell system is managed. Each cell site is designed for a rather short range because each set of frequencies are reused on every other cell site. There are only so many sets of frequencies available for cell systems. If a cell phone is transmitting too much power it will interfere with the next cell site that is trying to use the same frequency for another cell user.
This is one of the problems with using a cell phone from an airplane. The cell phone literally sees many cell sites at one time and ties up or interferes with many sites trying to use that same frequency for different calls.

To add to Keith�s explanation of interference to aircraft electronics, a cell phone does not even need to be transmitting in order to cause potential interference to aircraft electronics. There are oscillators that are running all the time in a cell phone whenever it is turned on. These oscillators are like miniature transmitters that are necessary for its receiver to operate. In some phones, or other types of electronic equipment, those tiny oscillators can couple to the cell phones antenna or directly radiate from the phone and be picked up by the planes electronic systems. It is a rather small chance of it happening but it can happen.

Regards
Gary

There are two reasons they don't want you using cell phones in aircraft. The first one is that they could cause interference with some of the navigation and communications equipment aboard the plane, particularly if there are a large number of phones in operation. The second is the impact of airborne phones on the cell phone infrastructure. A single airborne phone can be picked up by dozens, maybe hundreds of towers; the towers have to negotiate which one will handle the call.

That said, I have heard of plans to put some sort of cell phone relay inside of aircraft to handle calls. I guess how big of a problem the cell phones are depends on whether or not the airlines can make money on them. I found this quote on web page http://www.house.gov/transportation/aviation/07-15-05/07-15-05memo.html :

"Without a pico cell, airborne cell phones would normally operate at their highest power setting in an attempt to reach base stations located far away on the ground, potentially causing interference to ground-based cellular networks. For example, signals received from an airborne cell phone at 100 miles from a ground-based cellular base station (assuming line-of-sight) will generally be 100 to 10,000 times stronger than signals received from a ground-based handset 100 miles away. Installation of a pico cell on the aircraft would alleviate the problem of each cell phone caller needing to connect with towers on the ground."

I found the attached web page that discusses airborne cell phone performance. It turns out that there are a couple of factors why cell phones don't always work in aircraft. One is the design of the tower antennae, which have a horizontal pattern. The other is doppler shift, which occurs with fast, low-flying aircraft, basically detuning the cell phone signal until it can't be received.

• Airborne Cell Phone Performance

Regards
Gary

I can't tell you how many times i have transmitted signals thru a dummy load antenna on my ham rig on 50w or 100w. The dummy load is just a big 1g coffee can sized load that is oil filled.. By the way.. it's shell is made of metal...

I leave my cell phone in my toll box when i am on my tractor... It shows missed calls on the display when i open the box to grab it.. thus it is getting signal thru the thin metal on the box...

Soundguy

Look up �skin effect� for an explanation of how metal shields rf. The rf will flow only on the surface and penetrate very little. The higher the frequency the less penetration there is.

As to your cell phone showing missed calls while in your tool box tells you that it did not receive anything when it was in the box. As soon as you open the box it re-registers itself and the cell site sends it a message telling you that it missed a call. If you were very close to a cell site with your tool box it may receive a call via leakage around where the lid closes. But if you were to put it into a well shielded metal box with no leakage it would receive nothing.

Screen rooms are often employed in plants manufacturing phones and were used extensively in pager repair facilities where you could be inside with the cell phone or pager and receive nothing from the outside. This was necessary in order to test the units without interference from the transmitter sites.

Regards
Gary

I've been into electronics and rf theory for many years.. While poor shielding is an issue... I can tell you that there is no 100% guarantee on -anything- blocking 100% of -anything- in this world.

Faraday shileds and metal rf shielding .. including foil wrapped coax with crossed metal ground sheathing can still emit low level rf..

I've simply seen too much evidence of rf prop thru metal to completely dismiss any chance of it, or to chalk 100% of it up to leaky shielding.

Skip over to emp and it gets even dicy-er.

Soundguy

Take it from someone who's made their living for more than 30 years as an engineer working on elecromagnetics, including large RF communications systems, and designed any number of shielded devices (I've also been a ham for 36 years). RF will NOT pass through metal of any reasonable thickness. However, as I pointed out in my post above, it will propagate along the surface until it hits a discontinuity and re-radiate inside. I can provide you with any number of professional references if you would like more information on the physics involved.

Something like your tool box probably has on the order of 10 - 20 dB shielding at cell phone frequencies (reduces power by 10 to 100 times), and unless you're on the very fringe of the cell coverage the phone will work just fine with that kind of leakage. Tool boxes were never intended to be RF shields. A typical metal skinned aircraft will have somewhere from almost no shielding up to maybe 30 or 40 dB shielding depending on the frequency and orientation of the source. Composite aircraft structures have a lot less.

Coaxial cable leakage has nothing to do with RF passing through metal, and everything to do with the fact that the woven braid passes back and forth from the inside to the outside, leading to a condition call "porpoising" where some small amount of RF is conducted from the inside to the outside of the shield on the individual braid conductors. Coax with a solid shield, like semi-rigid, doesn't leak through the shield itself but can leak at connectors which may not have a perfect conducting path. Good quality woven shield coax will have on the order of 60 - 90 dB shielding (i.e., coupling to an external field will be on the order of one millionth to one billionth of the level of coupling from a single open wire).

In the case of your dummy load, there are likely two major leakage paths. First is joint where the lid fits onto the can. In a standard paint can, there's a coating around the groove to help keep paint from sticking, which means there's either a very poor or no connection between the lid & the rest of the can. If you don't believe me, try soldering the lid onto the can & see how much quieter it gets - just drill a couple small vent holes in the top so the can doesn't explode when the oil gets hot & expands.

Second, take a look at the cabinet on your radio itself. I have yet to see one that was actually an RF tight shield. Any currents floating around on the surface of the radio cabinet or chassis can couple into the radio circuits...

Remember, its discontinuities in any metal enclosre that let RF through, not leakage through the metal itself. Another example is shielded rooms used for things like EMI testing. A good shielded room, with good tight seams & a good quality door, can maintain on the order of 100 - 120 dB shielding effectiveness (120 dB shielding effectiveness means that the RF field on the inside is one trillionth of the strength of the incident field on the outside). That would be a little tough to do if the RF was passing directly through the metal...

Your observations of RF leakage make sense if you understand the physics involved.

Keith

It was the statement the other poster made that there was -0- rf thru metal.. then his next message actually posts a calculatiopn to see how much rf gets thru metal.

I'l agree that it is a ver small number... however my original message stated that it would be a function of signal strength nd metal thicknes.

If you had a huge transmission source, and an epsilon thin metal shield.. wll.. the calcs show there would be something thru then...

Sure.. it's a non practical argument.. .. but.. it's possible.

I'd go with the propogation leaks and surface conductivity.

( pant can is darn hard to solder without stripping first.. but makes a darn good pan for an oil cooled dummy.. )

de ke4rrd

73's

Soundguy

I'll go dig thru my arrl books.. I may just have that antenna handbook somewhere.

Thanks for the info.

Soundguy

Yah, but..

We were discussing how cell phone signals got through automobile & aircraft skins, not some theoretical condition which has no relationship to the question at hand...

and...skin depth is only one factor in how well a material shields against elecromagnetic radiation. You also have to take into a account the reflections from the surface due to the impedance mismatch between free space & a conductive surface, and another set of reflections due to the impedance mismatch when the wave exits the conductive material. By the way, at cell phone frequencies, skin depth is measured in micro-meters (one millionth of a meter, or about one 40,000th of an inch).

With the exception of a few special materials, for all practical purposes RF will not propagate through metal.

Soundguy: "I've been into electronics and rf theory for many years.."

Then perhaps I could suggest a textbook or two on Fields & Waves that will explain the physics of how electromagnetic fields interact with conductive materials. You might also look at any recent ARRL handbook - as I recall, there was some information in recent versions on shielding. (I didn't go dig mine out, so it may be something like the RFI or antenna handbook I'm think of also).

Keith

There is such a thing known as �transfer impedance� that is the measure of signals getting thru the shield but with solid shields this only happens at very low frequencies like near DC. I.e. lightning.

One skin depth at 400 MHz is around .0003 centimeters. One skin depth is where the intensity has dropped to 37% of full strength. A second skin depth (another .0003 centimeters) drops the strength to 37% of the first.
That tells you how quickly the signal is attenuated as it tries to go thru the metal.
Even a very thin piece of metal will not let enough signal thru to the other side to be able to measure.

Do a Google search on �skin effect and skin depth� and you will see why rf does not penetrate a metal surface.

Regards
Gary

Charles