You need antenna and the type of antenna really depends on how you judge you application and requirement. Attached is the real good manual which can help you choose antenna depending
upon your requirement, also mentioned is the accessories which will be required if you go
for antenna other then ANT795-4MR/4MS.
You can go with omni directinal antenna (ANT795-4MR/4MS) if requiment does not demands for
keeping access and client inside the panel.
Full manual is http://support.automation.siemens.com/WW/view/en/22681042