In my application, the device is going to be placed on top of a metal disc, less than a centimeter away from the pcb!
Now I have prototyped up the device using a nRF24L01+ breakout board with only a chip antenna, and I can communicate with the device from a few meters away (obviously from a receiver with line of sight to the PCB, not from underneath the disc). However, if I want to enhance my design and enhance the reliability of the wireless link what kind of antenna should I use?
Should I switch to a lower RF frequency band?
From what I have read around the internet, a patch antenna with a ground plane underneath might be a good antenna to use, because the effects of the metal disc underneath the whole pcb will be reduced since our design involves a ground plane underneath the antenna anyway. This seems to take up a lot of space as opposed to a IFA antenna etched directly on a pcb.
However, being quite new to all this RF theory, I basically don't really have a clue. Any ideas?
cksa361, I am confused by your bounty because I am not sure what else you need answered except for the comment you left me, so I will answer that.
When you pick between different chip antennas you will normally not find one that is significantly better than another in every way unless a new process was developed. In most cases you will have to pick the one that better fits your application, and to you this will seem significantly better.
The larger one, Mica, has better radiation characteristics(more power out for the same power in). This means that for two transmitters radiating the same power, this antenna will get greater range. This also means the Mica antenna will receive from a source at a great distance much better.
The Taoglas is smaller. This is self explanatory, if space is an issue, or if the odd size of the mica causes problems, the Taoglas wins.
You will need to tune the matching circuit for the Mica one. For the Taoglas it seems they have a set matching circuit. This problem is approached in the question about measuring output impedance. This can be challenging if you do not have the right equipment. A mistuned antenna will severely hurt your range, if you cannot tune you may find the Taoglas will be easier to use.
Edit: I misread the datasheets, they both have reference designs. If you are going to vary your layout from the reference design (ie. You do not have room to do the exact say layout) on either antenna you will need to re-tune the circuit, as was linked in the paragraph with the strike-though.
I hope this helps.