Pcb v.s. Coax Antenna

[Ryner]

I need an antenna for a GSM module that I'm using. The antenna should be about the size of a credit-card to fit inside my antenna box.

Question: Which antenna type would give the best signal reception, PCB traced or a Coax cable snaked/zig-zagged?

Question2: What is the best shape for an antenna for maximized efficientcy? eg. squared spiral or snaked up and down.

[Landrew2390]

If I remember correctly, coax uses a grounded shield between the insulation jacket and the dielectric layer.  I know nothing about RF applications, but it seems to me that this layer would prevent the coax from acting like an antenna.  Hopefully someone with some RF experience will chime in on this topic.

[hagster]

Is this an antenna you're building yourself or want to buy?

Either can give similar performance. The biggest thing to consider will be how you want to mount it and what effect nearby components will have.

[Ryner]

I'm planning on making the antenna myself. The location of the antenna is ~15cm from the actual GSM module. The only things that'll be next to the GSM antenna are the Bluetooth and GPS antennas. All of these are supposed to fit under a bike's saddle, roughly a credit-card sized dimensions with a 9mm hight.

[hagster]

Ideally you need some test kit to make sure that it has a good impedence match. GSM is normally on a couple of different frequencies for each country,  so you need to know which is being used where you are. You may be able to design for a single carrier that only uses one band. This would make the task much easier. I would say that PCB antennas are harder to design in general as you need to allow for the dielectric constant of the substrate, which can change quite a bit from manufacturer to manufacturer, or even batch to batch. Coax cable is very standardised, so you are more likely to have similar results to anything you find on the net and want to reproduce.

Antennatheory.com is a good resource if you want reliable information about antennas. There is some pretty misleading stuff on the web about it.

Good luck with it.

[VK5RC]

Remember coax is a cable designed not to radiate (or receive)  so the signal gets to the other end.
 Antennas are all about the frequency you want to receive,  the gsm often uses several frequencies so your  antenna will need to work 'well'  at both,  unless you are limiting yourself to one.  The other big consideration is directionality,  which is also expressed as antenna 'gain",  I suspect for  your  application an omnidirectional antenna may suit best.
I am sure their will be some good designs out there,  I suspect that the main objective is  to fit in a smartphone case.
Also don't forget RF and digital circuitry don't mix easily.

Hope this helps.