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I've been doing research on radio transmitters, and though I understand how the signal is generated and how data is transmitted, I still can't understand what defines a radio's transmitting power. How is a certain transmitter more powerful than the other?

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Very simply, transmitter is "more powerful" if it puts out more power. It uses a higher voltage and higher current from its "more powerful" power source, and it outputs higher voltage into a given antenna impedance (typically 50 ohms).

So, a transmitter that outputs 7V into the 50 ohm antenna is putting out 1 Watt. And if the transmitter is designed to output 225V into the 50 ohm antenna, then it is outputting 1000 Watts.

There is also some amount of gain you can get from the antenna itself. Some antennas can make the ERP (Effective Radiated Power) 2x or more what the transmitter is sending up the feedline.

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  • \$\begingroup\$ Why don't people use resistors as antennas then? Or pieces of pencil lead? \$\endgroup\$ – Carrot M Jul 24 '16 at 0:58
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    \$\begingroup\$ We DO use resistors (call it "pencil lead" if you wish) to terminate transmitters during testing. So that the transmitter is loaded during testing when we don't want anything to go out into the ether. HOWEVER, the whole point of a transmitter is to TRANSMIT the signal out into the ether so that it can be received by the intended audience. If you simply send the transmitter power into a resistor, then you just have an extraordinarily expensive space heater! So you stick an antenna as far into the air as possible to send the signal out. \$\endgroup\$ – Richard Crowley Jul 24 '16 at 1:04
  • \$\begingroup\$ So if you were to have an extremely long antenna and a high impedance then it'd work? \$\endgroup\$ – Carrot M Jul 24 '16 at 1:26
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    \$\begingroup\$ @RichardCrowley If you have a 50,000 watt AM broadcast station, is the voltage going into the antenna higher (than your 1000 watt example), or the antenna's impedance higher, or both? Back in the 1930's, before transmitters were limited to 50,000 watts, WLW in Cincinnati broadcast with 500,000 watts. How did they ever accomplish that? \$\endgroup\$ – tcrosley Jul 24 '16 at 3:19
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    \$\begingroup\$ Antennas and waveguide is pretty standardized at 50 ohms. If you look at the YouTube videos about WLW, you will see super high voltage and super high current components. Transformers as big as a bus. Capacitors as big as a car. An antenna as big as a football field, etc. Ohm's Law is Ohm's Law. There are no short-cuts. Making the antenna impedance higher just makes it more difficult. It wouldn't surprise me if their antenna was significantly lower than 50 ohms. \$\endgroup\$ – Richard Crowley Jul 24 '16 at 3:34
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A radio transmitter is generally rated by the power level (in watts) that its antenna radiates into the atmosphere. The power required is produced by an electronic power amplifier which can be constructed either of vacuum tubes or solid state devices. Vacuum tubes once ruled this application until solid state devices that could handle reasonable amounts of power (tens to thousands of watts) became available. Vacuum tubes are still used when a combination of high frequency and high power are required (google traveling wave tubes for example).

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