Wrong desigend GSM GPS Tracker
The two yellow bars on the screen of the vector network analysers mark the – 6 dB return loss target for GSM 900 and GSM 1800. In the lower GSM 900 band the antenna is far away from the -6 dB. The GSM antenna is not an antenna. It is a piece of metal inside of plastic. Even at the higher GSM 1800 band the -6 dB return loss is not covered.
The space for the GSM antenna is huge. The gap between antenna and ground plane is big enough to reach a good antenna bandwidth. Nevertheless, I assume that the designers of the antenna never ever measured the return loss. If a customer will open the enclosure then they will find something that looks like a reasonable antenna. If you have a closer look, then you will find a wire soldered by hand to the antenna shape. First of all it is a piece of wire and no coaxial cable. The manual soldering gives the option that the antenna will not gives the same result always. A shifting of the soldering point left and right will gives different results during production.
GSM GPS tracker conclusion
The wrong designed GSM antenna inside the GSM GPS tracker, was manual soldered by a wire and not by a coaxial cable. If the antenna will change its parameters all the time by manual soldering, how it pass an European R&TEE certification? As soon you change a mayor part of a radio device as soon the CE certification will be lost.
If you plan an own mobile device like a GSM GPS tracker or a Point of Sales System then my IoT M2M Cookbook is a good starting point. It will explain you in detail how to integrate an embedded antenna, how to develop a proper power supply and a lot of more.
Anyhow, if you still have questions and need consutting, then do not hesitate to drop an email to harald.naumann (at) gsm-modem.de