Anechoic chamber in 20~40GHz

We are making an anechoic chamber in 20GHz~40GHz. The question is that, do we need absorbers at all? It may sound crazy, but because of the high frequency, we can put the antenna under test and the feeding horn very close, say, 50cm from each other; and we can measure it in a big salon, and I hope in this high frequency the reflections do not make to much contribution. Is this very crazy?

You probably don't need any if you are that close. Many of the reflections in an anechoic chamber come from the floor, roof, and the back wall. If you are close enough you probably won't have an issue. If you are far enough away from the back wall the reflections will be way down. Try testing something and move further away until you see a noticable difference or try using gating to filter out reflections.

I would be tempted to get some of this "microwave absorbing sheet"
http://www.lessemf.com/fabric.html
and drape it down from the ceiling. Just keep the direct angle of incidence at something other than 90 degrees, and you will probably sop up any reflections from the rest of the room.

These are low frequency my friend. Our experiments are around 25 GHz and up. Thanks for the info though.