Placing any antenna inside a partially enclosed metal box is fraught with issues. Rugged (metal laptops face this problem for their Wi-Fi and GSM antennas. My comments follow.....
1. An antenna placed inside a metal enclosure can suffer detuning due to the proximity of metal to it. You have inductive and capacitive coupling to consider. There is the issue of enclosure and window aperture resonant frequency to also consider. A detuned antenna will lose efficiency. With a high VSWR you could suffer issues with the PA stage.
2. An opening in the side of a metal box provides the required RF path for an internally mounted antenna provided any detuning effects caused by metal proximity are found to be acceptable. Such a RF window will creTe a directional antenna pattern. The closer that the antenna is to the window aperture, the better will be the radiation pattern, but it will be something like a ballon shape emerging from the aperture and so not a normal dipole or monopole radiation pattern.
3. RFI needs to be considered when placing a transmitter antenna inside a metal box in close proximity to other electronics like a MCU. The RFI from the GSM signal can cause unpredictable behaviour if good RF immunity and screening principles are not applied to the design. Think about the resonance of PCB tracks at the transmitters operating frequency !
3. The rugged laptop manufufacturers addressed the problem of antenna placement in two ways. Those equipped with GSM radios had an external antenna attached to either an RF connector or mounted in a plastic “radome” on the laptops lid. The other approach was to place the GSM antenna inside the physical boundaries of the laptop, but outside of the metal casing. That is to say, the metal casing had plastic encased areas on the outside of the metal case that contained antennas that are not badly detuned by the proximity of metal. Common antennas are the “F” type that is found providing the Wi-Fi antenna in most laptop lids. It looks like a weird piece of bent metal and has a coaxial cable connection a specific feed point.
4. For reliable performance I would recommend the use of a suitable GSM antenna on the outside of your metal enclosure. Use the metal enclosure as a benefit that helps to keep the RF energy away from any sensitive electronics.
The external antenna could be a simple screw on connector dipole or a surface mounted F Type as used in laptops. The F type would not have such even coverage as a dipole design and would need a plastic casing for protection. That is an additional cost and complication but it results in a low profile antenna that is relatively robust.
5. I suggest you look at a few FCC approval documents for rugged laptops like the GETAC M230, Itronix XR -1 and General Dynamics GD8000. The internal pictures clearly show how the manufacturer has addressed the antenna location problem on these metal cased laptops.
Hope this helps
Fraser
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