I know you want watts, and in this case since you are adding just one stage (the amplifier) it is a little bit annoying that the amplifier's gain is in decibels. The reason for that is because when you have a lot of things to add, for example…
SDR + amplifier - coax loss + antenna gain = Effective radiated power
…then decibels becomes very handy. The trick I use is to start with my power (watts) in decibels too!
Think of a plain decibel
dB unit like an increase/decrease ratio. But there are also decibel-with-reference units, like
dBm (power relative to a milliwatt) and
dBuV (voltage relative to a microvolt).
You can think of numbers with these "reference decibels" as absolute values — e.g.
0 dBm is the same as
1 milliwatt. That's because it's the reference milliwatt (the
…m part) with no increase or decrease (the
0 dB… part).
Let's say your SDR is 0.1W. That is
20 dBm, because 100 mW (0.1W) is a 20 dB increase from the
…m reference unit of 1 mW. I made myself a little "dBm ↔︎ watts converter" that's available here https://utils.ipcalf.com/#dbm and you can type in your actual SDR watt in the right hand (watts) box and the dBM will show up in the left box.
I'll continue with a 100 mW transmitter for example, into an amplifier that has 35 dB gain for example. You can replace with your actual numbers. We know 100 mW is the same as 20 dBm. So now we can do your math:
SDR output (20 dBm) + amplifier gain (35 dB) = to antenna (55 dBm)
So you get
55 dBm to the antenna. But the FCC safety limits and some band restrictions are given in watts on most charts. So we don't want
I put that back into the left box (dBm) of my calculator and it shows
316.2277660168379 on the watts side. So our example SDR plus example amplifier together output about 316 watts total!
Or to get Effective Radiated Power, we keep it in dBm for a little longer:
20 dBm (SDR) + 35 dB (amplifier) - 5 dB (coax loss) + 3 dB (antenna gain) = 53 dBm (ERP)
Converting to watts again,
53 dBm is approximately
200 W (199.5…). But it was very handy to add in more steps while it was still in decibels. You can even add more
dB things, like your predicted path loss and a receiver antenna gain to predict whether communication is possible…