GSM chip antenna design mistakes in detail
I. Size of the ground plane
The length of the distance marked with I is 56 mm only. Most GSM chip IFA antennas or PCB track antennas will radiate perfect on an estimated length of the PCB of 90 to 100 mm.
Just compare it with the length of the simulated and field tested antenna in the IoT M2M Cookbook. By cut the length to 55 mm the radiation will be poor. If you have a closer look in the data sheet of the ceramic chip antenna, then the supplier is so fair to mention the antenna efficiency on a length of 55 mm. It will shrink from 65% on reference length of 95 mm down to 20% on 55 mm at GSM 900. At GSM 850 the antenna efficiency is already down to 14%. Just to quote the IoT M2M Cookbook: 40 x 100 mm is not the same than 100 x 40 mm. The antenna is placed on the wrong edge of the PCB.
The distance of the antenna to the plastic enclosure is not mentioned. The data sheet of the cermaic chip antenna tells 2 mm. Worst case the distance is closer than 2 mm. If it is closer than 2mm, then the antenna efficiency will shrink again.
II. Ground beside the chip antenna
Place a candle on the table and put next to a vase of flowers and look at the result. There will be a shadow. Exactly the same happens with the antenna. There is a shadow. At the antenna will happen further. The ground plane in the near will detuned the antenna. Ground surfaces in the near of the antenna are sometimes used for matching. In this case, the ground surface leads to detuning.
III. No space for matching circuit
There is a need for up to five passive components (capacitors and inductors) to match the antenna, but there is no space on for it on PCB.
IV. PCB track with 50 Ohm
The PCB track looks very slim. Maybe it is too small to match the 50 Ohm. If this is true, then a lot of energy will be reflected to the GSM module.
Analyse of GSM PIFA antenna summary
Worst case the design contains summary losses like:
- 20% antenna efficiency instead of 69%
- ? loss by antenna to close to plastic enclosure
- ? loss by refection of RF energy in the feeding line
- ? loss by no tuning with matching circuit
Maybe the antenna will radiate nearly nothing. The best and cheapest result you get, if you select the embedded chip antenna or PCB track antenna first and design the product around. Even better is, to give the freedom on a reasonable size for a ground plane and distance to plastic enclosure.
If you still not sure which antenna to select and why, then do not hesitate to drop an email to harald.naumann (at) gsm-modem.de