Selection of antenna and simulation

Hi,

I have a project where a antenna is needed

Requirements:
The antenna must fit in this enclosure: http://www.newageenclosures.com/cart...ail&p=120&c=11

Attention points:
PCB with ground plane inside enclosure
Batteries under PCB (=metal)

Requirements
High efficiency
Operational on 868MHz ISM band

Is there anyone who can help me ?

This will be an electrical small antenna (l < λ/4). Some suggestions in the TI application note AN058 http://www.ti.com/lit/an/swra161b/swra161b.pdf

Popular solutions are wire coil or chip antenna.