1.) Materialsa.) 6.7" of 22 gauge copper or steel wire
The antenna will require 6.7" of 22 gauge copper or steel wire which is commonly found at craft stores and used for flower arrangements. 22 gauge is perfect because it will bend and stay in place. You can coil it up for storage and point it out straight when in use.
Why such an exact length? Because antenna length is directly tied to the operating frequency (433.92Mhz) in which the device was designed for. There is a specific formula used to calculate the length which is what I used to derive 6.7". You still may get improved performance with a longer or even shorter antenna, but it won't be maximum efficiency.
Ideally the wire should be non-coated. If it is coated (as mine was) you will just have to sand the paint off one end for soldering.
If you need to buy something I would look for 22 gauge non-coated copper wire, something like this should work great:http://astore.amazon.com/johspro-20/detail/B000SN7J7Qb.) One Antenna Tube
In addition to making the project look very professional, the antenna tube protects your wire from breaking off, reduces strain on the PCB, and helps keeps water out (of the antenna hole). While any standard hobby antenna tube can be used, this Instructable is based on one from Dubro Racing, Model 2338 (Red, with cap). It can be seen and ordered here: http://astore.amazon.com/johspro-20/detail/B000BP4JC4
Finally, it is assumed that you already have a thermometer that you are willing to hack (which has obvious risks such as breaking the device if you're not careful). If you want to purchase a thermometer I highly recommend the same one I used (Maverick RediChk Model ET-73) here: http://astore.amazon.com/johspro-20/detail/B0000DIU492.) Tools Needed
a.) Soldering Iron (along with flux and solder)
b.) Sandpaper or some type of file (an emery board would even work)
c.) Wire cutters
d.) Drill (or Dremel Moto type tool) and small drill bit (1/8")
e.) Very small (jewelers sized) Philips screwdriver