Increase range of 433MHz remote sensor

[vivitern]

I have wireless weather station with 3 remote 433MHz sensors so I cannot change the protocol and program the controllers. Also I'm new to radio but have some basic understanding how it all works.

I'm trying to modify the stock product to increase its capabilities, a sort of DIY project for home.

The sensors and station base are mounted on concrete walls of my home, where maximum distance between base and sensor is about 15 meters (and crosses 3 walls). The Base is able to receive signals from all sensors except one that is the most distant.

Link to the weather station on amazon. Transmitters TH71071EDC (datasheet: http://www.kynix.com/uploadfiles/pdf9675/TH71071EDC.pdf  )run on 2 AAA batteries, 3V.

What is the easiest way to allow it receive the signal? Attach better antenna to the transmitter? Or completely replace the transmitter module?

I understand that I didn't provide much information about the circuits because I'm not sure if it matters in my case. I can post images of the inside later if needed.

UPDATE

Here is a receiver module of the station Base.

The Sensor box is harder to open because it looks like sealed. I'll try to open it later and now hope the receiver can give some ideas.

[Lurker]

I think this kind of question better on electronic forum, something like this :

http://www.sonsivri.to/forum/index.php

but if anyone here has experience then , please answer