First, here's a quick it's not exactly right but it's not horribly wrong review of FM transmitter efficiency, generally.
The manufacturers of commercial broadcast FM transmitters have been working for decades to improve electrical efficiency. Commercial units regularly boast electrical efficiency of up to 75%, meaning if you put 100 watts of electrical power in you get 75 watts of RF power out.
Down at the hobby transmitter level, we don't even approach the opposite of that. A typical hobby transmitter needs at least 1,000 mA of input power at 12 volts to achieve 500 mW RF output.
Our good friend Georg Ohm tells us that P=I*V or Watts equals Amps times Volts. That gives us 1,000 mA * 12 V for 12,000 mW electrical input. That 12,000 mA of current input gets us only 500 mW RF Output.
Referring once again to Georg and remembering our introductory algebra, 12,000 * X = 500 or 500/12,000 = 4% efficient!
Imagine my surprise when I see a potential FM micro transmitter listed on an ecommerce site with the following technical specifications:
• Frequency range: 76~108Mhz
• Output impedance: 50Ω
• RF power output: 100mW
• Supply voltage: 2.7 to 5.5 V
• Supply current: 35 mA
5.5 V * 35 mA = 0.1925 Watts or 192.5 mW. Back to algebra, 100 / 192.5 = 52% efficient!
Or not... 10 mW of RF output power would mean 5% efficient, which is far more likely.
10 mW could be fine for limited local low power radio so the transmitter listed with these specs is not necessarily a poor choice, it's just that it's very difficult to compare transmitter options when the listings are full of grossly inaccurate information.
When shopping transmitters, it might be better to trust and compare supply voltage and current requirements than purported (and highly suspect) RF output.
So, can these ecommerce listings for FM transmitter modules be trusted? Me, I'll trust Georg and algebra and run my own calculations on output power. I won't be right, but I won't be horribly wrong!