GSM antenna placement - Page 1
Forum section: electronics / projects

author: alireza7
author_rank: Regular Contributor
timestamp: February 10, 2019, 01:41:10 pm
content: HiI want to place gsm antenna in my system.My system consist of a metal box.I have two choices :1- Take antenna out of metal box by a coaxial cable2- Make a cut (a 5cm*5cm rectangle) in metal box right in front of antennaChoice number two have lower manufacturing cost for me but i'm not sure with signal strength .Can you please help me to understand if choice number two is appropriate or not?

author: Fraser
author_rank: Super Contributor
timestamp: February 10, 2019, 02:23:06 pm
content: 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 helpsFraser

author: nali
author_rank: Frequent Contributor
timestamp: February 10, 2019, 02:32:11 pm
content: You can get some pretty low profile body mount antennas, just google "puck antenna" for example, which you can mount on your case. They're pretty common in M2M applications e.g. vending machines or where a system is inside a metal enclosure such as a roadside traffic lights junction controller.Take manufacturer's claims with a pinch of salt though! YMMV.

author: Berni
author_rank: Super Contributor
timestamp: February 10, 2019, 02:34:54 pm
content: If you have a choice and don't have experience with RF design go for option 1 and save yourself a good bit of hair pulling.The antenna inside the box needs to be specifically designed to operate in there, otherwise the box will detune the antenna and turn a 900MHz GSM antenna into a 800MHz one for example. Such an antenna will also be pretty directional, tho GSM tends to bounce around enough to eventually find its way due to cell towers being so numerous.But the biggest thing to watch out for is the high transmit power of GSM. When you close 2 or 3 W of RF transmit power into a box you can not only completely jam the operation of any electronics that come in the way but possibly even damage some sensitive analog circuitry. Ever wonder why the internal circuitry of cellphones is completely covered in multiple shielding cans? This is the reason.

author: Fraser
author_rank: Super Contributor
timestamp: February 10, 2019, 02:41:33 pm
content: The FCC documents for the rugged laptops showing the antenna placements are here.....XR -1https://fccid.io/KBCIX270-WL3945/External-Photos/External-photos-806407https://fccid.io/KBCIX270-WL3945/Internal-Photos/Internal-Photos-806409M230https://fccid.io/MAU018/External-Photos/NOTEBOOK-PHOTOS-733293https://fccid.io/MAU019/Internal-Photos/Internal-Photos-742027The GD8000 is basically the same as the XR-1 in terms of antenna placement but it is a later laptop design so I include it here.https://fccid.io/KBCIX-GOBI2/External-Photos/External-Photos-1192252https://fccid.io/KBCIX-GOBI2/Internal-Photos/Internal-Photos-1-of-2-1182238