Can a small piece of metal sheet by used as an antenna?

I have seen many things used as antennas, bed springs, fire escapes, even a tree. This last thing was a US Army experiment from around 45 years ago.

Yes. sure. nail, knife, can, ... , all can be used.

coat hangers as well... seriuosly!

most metals can be used so long as they are cut to required dimensions according to frequency you want to receive or transmit