what is dBm of 15uvolt/meter loss at 54 Mhz?

Answer 1

At an impedance of 50 Ohm (assumed, you didn't say), it would be (minus) -83.47dBm
Not sure what the 54MHz have to do in this context....
If in doubt, download this Excel file which does all the relevant conversions: http://www.autoid.org/2002_Documents/RF_Calculator.xls