Hi Nince,
I wasn't referring to the cold temperatures cutting down the transmission, but rather the metal walls of the cold room (which is typically around 2-4 degrees Celsius). Not much I can do to make the signal go through the walls I presume. Acu-Rite rates the tower sensors at 23m (about 90ft) when used in a cold room environment (or similar). This cuts transmission distance by 1/3.
Hence, I'm looking for a way to simulate what remote sensor thermometers do where you have the thermometer outside of the area to be monitored all the while having the wired sensor itself in the monitored environment.
This morning I was getting 1 bar on the interface, but I'd like to have a little more buffer than that so I don't lose the signal when I'm away.
OK. One bar may be enough. Since I assume this is all indoors, I suspect it should be a more stable environment. If you look at the alarm settings on MBW, you should probably set it to alarm you if signal is lost from the sensor for more than an hour as a safety measure.
As for modifying the tower sensors, one difficulty is that their internal wiring seems to vary even though they look identical from the outside. So you'll have to inspect the internals yourself carefully. If you need help identifying things, photos would be a big help.
If you want to maintain both accurate temp and relative humidity readings, it would probably be easiest to try to bring the antenna through the wall. Trying to bring both the humidity and temp sensors through the wall would be more involved since they are usually discrete devices in the tower.
The downside of mucking with the transmission antenna is that the FCC frowns on that sort of thing, so be advised. Having a ham license might cover you, though, as the 433MHz transmitter falls within the 70cm amateur band allocation in the US. The reason consumer devices are allowed to use this frequency is due to the very low power of the transmitter.
Anyway, what I would try is to replace the coiled antenna with a length of coax to feed through the wall, then cut the insulation off the end of the coax exposing the inner conductor (which will now act as an antenna) for 13 inches. That will give you a 1/2 wave antenna which you'll likely want to mount vertically outside your cold room.
Why 1/2 wave? I'm not exactly sure. The coiled antenna stretches out to 13 inches and the strait wire receivers are also 13 inches. A 1/4 wave sounds more logical, but I'd start with the lengths Acurite is using in their gear as a start. You could always cut it down to 6.5 inches and see if it makes any difference.
I'm ignoring the impedance matching issues here. Most coax is 50 ohm, but many radio antenna feeds use 75 ohm. I'm not sure how much of a difference it will make. The worst thing that could happen is power reflecting back into the transmitter and damage it.