Is it possible to design a GSM/GPRS antenna that has a gain of 3dBi? if not, what is the maximum gain possible with a PCB GSM/GPRS etched antenna?
I'm thinking of a PCB sized 11cm long and 5cm width. FR4 of higher grade material for PCB can be used if necessary. I only want to know if it's possible.
Best Answer
Yes, of course it's possible. Just keep in mind that antennas are not magic. A perfectly isotropic (spherical reception pattern) cannot possibly be better than 0dBi. Any antenna with more than unity gain is doing so by having less-than-unity gain in certain directions while having greater-than-unity gain in others. You know, the gain pattern.
I like to think of antennas as something analogous to the reflector cup in a flashlight - it's not actually increasing the amount of light a flashlight's bulb is putting out, it's simply concentrating it in a cone. Antenna's are similar.
So, can you create an antenna that has ~3dBi gain for cellular frequencies? Most definitely. You could create a a PCB antenna that has in excess of 11dBi gain (by making a yagi style antenna using pcb traces) if you wanted. The pcb would be huge, and the directionality would require you have it pointed directly at the cellular tower, but it would work, sure.
As for something practical, a simple dipole/whip style antenna, but on a pcb, is easily done and should get you 2dBi easily, though at the cost of a toroidal radiation pattern. This radiation pattern is well suited to cellular network use, however, as long as the orientation is fairly constant. This is the same radiation pattern as most wifi routers and receivers have.
A 1/2 wave dipole (just google pcb dipole to get an idea of the fairly abundant geometries one can use on a pcb to implement a dipole) ought to give you 2dBi-2.1dBi gain easily. At 1800MHz, a typical GSM frequency, this would require (at the most efficient receiving geometry but also least efficient in size) about 8.3cm of inline copper trace length. In other words, your pcb would need to be, at its longest, slightly longer than 8.3cm.
The easiest way to get 3dBi gain would simply be a 2-element collinear array. These are simply two 1/2 wave dipoles in the same plane. This would require a pcb roughly 17cm long unfortunately. However, you would get double the gain vs a single dipole, which equates to about 3dBi.
So yes, it is not only possible, but fairly trivial depending on how large you're willing to go. I am sure there are more efficient geometries with similarly acceptable radiation patterns, any of which could be implemented in the form of a PCB. And at 1800MHz, dielectric loss from FR4 is small enough that it can simply be ignored on these scales.
In theory, you could simply scale up any of the readily available 2.4GHz 3dBi pcb antenna geometries to the wavelength of 1800MHz (or whatever) and get similar results.