I will be as basic as possible. Lets start with the antenna, and we will assume a factory gain figure of 12db on 70cms and 9db on 2 mtrs.  The antenna gain alone will not tell you what power is being transmitted out of the antenna. There are other factors that come in to play. One of these is the coax it's self.  Coax creates a drain or a loss on the antenna system.

This loss is also expressed in dbs or decibels. a given length of coax has a factory calculated loss figure we will assume 3db per hundred ft.

So how does gain work?  Well, the actual power output from your transceiver is 0db.  Before we start we need to take away any loss factors from our antenna gain figure. Example: Antenna gain is 12db on 70cms. From this figure you have to minus the coax loss of 3db this is assuming you have used 100 ft.

12 minus 3=9db. This figure is the total gain minus coaxial losses.

As I have previously stated the output from your transceiver is 0db we will assume an output of 10 watts from the transciver. For every 3db gain you output power doubles. So heres the hard bit!.

Since your antenna has 9db after losses you have 3 muliplier points.  At 3db your power increases to 20 watts and at 6db it increases again to 40 watts. Last not least 9db becomes 80 watts this is classed as the ( ERP ) Effective radiated power. So you see 10 watts for an m3 or an m6 callsign is not that bad and if you want more output get a better antenna. More gain equals more power.!!