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How a wrong PCB track GSM antenna could look like?

Q: I read your super-interesting page on 0 € GSM antenna and I tried to sketch down a similar one for the M2M device I’m designing.
My PCB is 42×65 mm, so when lying down tracks (79 mm for 900 MHz, 41mm for 1.8GHz) one area surrounds the other. Perhaps this is not a problem, as the GSM module will not transmit on both frequencies at the same time. In this design I used top side as a ground plane; I could use bottom but I fear a via hole would create problems. Could you please give me some suggestions on my design?

inverted F antenna spiral

inverted F antenna spiral

A: I assume that you have not read all the short articles that are listed here:

http://www.gsm-modem.de/M2M/m2m-faq/nothing-will-interfere-your-embedded-antenna/ and further links below. The designs show several common mistakes that I have seen in the past and I assume to see in the future again.

1. Small ground plane for the embedded GSM antenna

The type of the embedded GSM antenna does not matter. A chip antenna, a PCB PIFA flying over ground plane, an IFA embedded in the PCB – all of these antennas are monopoles. The counterpart of such embedded monopole antennas is the ground plane of the PCB. If the PCB is too small, then the antenna efficiency will shrink. Moreover, the bandwidth will shrink as well. The bandwidth will be so small that you will not cover the whole GSM 850 or GSM 900 MHz band. In urban canyons or at country side where you maybe have only GSM base station visible, your M2M design can’t register to the GSM network because worst case the operating frequency of the GSM base station is on one of the lowest or highest channels in the GSM band. The small ground plane will mainly effect the lower GSM band. For the GSM 1800 and GSM 1900 MHz band the size of the ground plane is maybe ok because GSM 1800/1900 needs the half of the ground plane in comparison to GSM 850/900.

2. Embedded GSM antenna with too short distance to ground plane

A very common mistake is to reduce the minimum distance of the embedded GSM antenna to the ground plane. The reason for this fault is based on the idea to make a small M2M design. Also the distance between the embedded antenna and ground interferes with the bandwidth and the impedance. Any antenna planned for a distance of X mm far away from ground plane and used on a distance Y will change its parameters. Let us use an easy to understand example. We all learned how a parallel plate type capacitor will work. If you do not change the size of the plates and move the two plates closer together the value of the capacitor will rise. If the capacity will rise the impendence of the capacitor will be lower because Xc = 1 /(2 x ? x f x C).

3. Embedded GSM antenna with too short distance to the enclosure

The diagram and the text do not mention the distance to the enclosure. It does not mention the material of the enclosure as well. I assume a plastic box is in use. Plastic has an Epsilon R of 2.5 to 3.5. Air has an Epsilon R of 1. If you replace the air to some kind of plastic the higher Epsilon R will interfere with the antenna. For that reason you will find a recommend distance of 2 mm to the enclosure in the data sheets of several GSM chip antennas. If you use a homemade GSM antenna design you have to follow the physics as well.

4. No clearance between GSM module and embedded GSM antenna

A lot of developers forget that they will need some space to place 3 to 4 and sometimes 5 SMT capacitors or inductors on the PCB between the GSM module and the embedded GSM antenna. These components are part of the antenna matching circuit. With such an antenna matching circuit you can optimise several parameters of the embedded antenna.

5. Embedded GSM antenna one two layer PCB

I assume that the PCB has two layers. The picture already shows an integrated IC with 44 pins (the MCU ?). The PCB will hold the SIM card connector as well. Then there will be some capacitors between the SIM card connector and the GSM module. If all necessary components will be mounted on a two layer PCB then PCB will need a lot of via holes. If in the end the PCB will have much more holes than a Swiss cheese the rest of the mass layer on the PCB will be no ground plane layer for the PCB anymore. A four layer PCB will reduce this risk and will minimise the risk on EMC as well. Think about that any PCB track at MCU or other digital signal line will be a small not planned antenna and welcomed quickly.

6. Conclusion

If there is no need to make a small M2M design just make the PCB bigger. Double the size and spend more space for the matching circuit and the embedded antenna as well. If you have no experience with embedded PCB track antennas it makes sense to take a GSM antenna out of the shelf and to make a one to one copy of the reference PCB. Even this reference PCB needs a customised matching circuit because the reference PCBs are designed and optimised without a plastic enclosure. State of the art RF engineering offices offers you the optimisation of the matching circuit by using a network analyser. Some of this companies offer the test in an electronic chamber as well.

Updated: 2013-07-21 — 9:36 PM

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