The best option to improve the signal is to try to move the Network OWL to a higher / closer location with a more direct radio path to the Transmitter / Sensors. Same for the Transmitter.
The Network OWL is normally easier to move - using a longer Ethernet cable if necessary (can be as long as 100 metres!).
Another option is to use a Powerline Range Extender or a Wireless Range Extender (just Google these terms).
See here for some ideas
A number of customers have successfully used this little unit http://www.tp-link.com/en/products/details/?model=TL-WR702N
You need to use it in Client Mode (http://uk.tp-link.com/article/?id=395) for this application.