Hi,
first of all: Yes, you can use this kind of AC/DC adapter.
While you are mentioning, that you will need 20meters of cable between your antenna an your Box, I'd like to recommend the ECOFLEX15 cable because of its extremely low damping factor and electrical resistance. I have the same distance between antenna and Box and I'm using the ECOFLEX15 cable. Since I switched to the ECOFLEX15 I have no limitations of reception-range any-more. Well, the EF15 has to markable disadvantages:
1st: Its price. 2nd: It's quite "stubborn" in concerns of Installation, due to its quite high diameter.
Some hints if you are planning a 24/7 operation of your RadarBox:
1st: You may buy a AC/DC adapter of fixed voltage, because they are much smaller than ones with variable voltages. Further their efficiency is usually a little better, because all components are optimized for 12V.
2nd: Do not stingy in concerns of power of the used AC/DC adapter. To use them in the area of 75% to 100% of maximum current rating, will lead to a reduction of MTBF (Mean time between failure). Means: If your preamp needs 800mA use a adapter, which is able to deliver 1500mA or even 2000mA instead of one that is capable to deliver 1000mA or worse 850mA. To proceed this way have several advantages, also in concerns of adapter temperature and electrical interference, which leads to number 3.
3rd. Don not use the cheapest switching mode power supply, because the may cause electrical interferences because of harmonics of their switching frequency.