Minimum threshold fm equation
Firstly there is no such thing as an FM antenna - an antenna doesn't understand the modulation method.

E\$_b\$/N\$_0\$ - what practical values are needed? 

N\$_0\$ basically equals kT i.e. Boltzmann's constant x temperature of the object that the antenna is pointed at. It is usually accepted that T = 290 kelvin and hence kT = 4 x \$10^{-21}\$ joules.

It is also generally accepted that for a decent bit error rate (BER), E\$_b\$ should be 100 times higher than N\$_0\$ i.e. 4 x \$10^{-19}\$ joules.

To convert the energy needed per bit to power we multiply by the data rate hence, the received power needed is 4 x \$10^{-19}\$ x bit_rate (watts).

Converting to dBm we get -154 dBm + 10log\$_{10}\$(bit rate) dBm.

This the generally accepted rule of thumb formula when transmitting data. For digitized speech a lower energy per bit can be acceptable, maybe one-tenth of the above power.