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In this White Paper I investigate the effect that a metal enclosure has on cell signal strength. A frequent question that we have is how can we get a cell signal out of a metal transformer enclosure without drilling and running wires out? Can a Cell Revolution along with its antenna be installed inside a metal enclosure and actually work? The answer is sometimes, it depends on several factors that I will cover in this paper.
For this white paper some simple experiments were preformed in order to answer these questions and demonstrate the best approach for antenna placement inside a metal enclosure.
In general, a metal enclosure is going to attenuate a cellular signal. The amount varies greatly depending on the variables of each individual situation and location coming with its own unique set of variables. In our case the attenuation was about 24 to 25 dB. Free space path loss to a distant receiver occurs with all electromagnetic signals. This loss causes the received signal strength to decay at a rate proportional square of the distance between the transmitter and receiver. Since cell towers and terrain vary, it is hard to calculate a true value for propagation range; however, an enclosure attenuation of 24 dB as in our experiment would reduce the range by 93.75% of the range without the attenuation. To put another way, 24 dBs of attenuation, due to shielding, reduces the range down to 6.25% of the theoretical range without the attenuation.
Another way to understand a 24 dB reduction is to equate it to cell phone signal strength as represented by signal bars. A typical cell phone indicates received signal strength as follows:
|
Bars |
dBm |
|
5 bars |
-75 or more |
|
4 bars |
-83 to -74 |
|
3 bars |
-95 to -82 |
|
2 bars |
-105 to -94 |
|
1 bar |
-110 to -104 |
|
0 bars |
-111 or less |
A 24 dB reduction would take a -75 dBm 5 bar signal down to -99 dBm, or in the middle of the 2 bar level. The signal outside the enclosure would have to be a strong 3 bar, or better 4 or 5 bars to get at least 1 bar inside the tested enclosure.
Some tips for maximizing the performance inside a metallic enclosure:
1. A larger metallic enclosure is generally better – more likely to have larger gaps, and allows for more space between the antenna and other metal.
2. An enclosure with plastic pipe instead of metal conduit is better
3. Try to locate the antenna as far as possible from any metal inside the enclosure, or from the walls or door. Putting the antenna near a small opening may be counterproductive, if it detunes the antenna. RF will reflect many times inside, and find its way out if there are gaps.
4. If possible choose an enclosure without potentially noisy electronics.
If the enclosure is close to a cell tower, this experiment suggests that can be feasible to locate the antenna inside the enclosure and still establish reliable contact. If the enclosure is at a remote location more than a few miles away from the nearest cell tower, or in hilly terrain, not line of sight with the tower, it would be unlikely to have a high probability of dependable communication. The position and polarity of the antenna inside the enclosure could be optimized to have a better connection. Since there are so many variables from enclosure to enclosure the optimum antenna position inside the enclosure will vary and may evolve some trial and error to get the best performance. Having the antennas whip or radiation element mounted as far away from metal or a know noise source inside the enclosure is a good place to start. Sometime moving the antenna a little can reduce the reflections causing the standing wave to be lowered and the modem to actually increase the output which could enhance the range. When mounting antennas inside the enclosure it is important to remember safety first and never take risks of mounting the antenna where it may come in contact with high voltages.
