It will definitely be influenced by power level, signal content, and ultimately; what are you trying to achieve (i.e. power measurement, feedback control, automatic leveling, saving the world from unwanted emissions, etc). But normally a wattmeter (**broken link removed**) is useful to tell the RMS output power for most applications.
A SWR meter suited to your chosen frequency usually will give you a good idea . They have a small wire antenna that screws onto the top for such measurements. Cost about $30 for common ham radio frequencies but can be made to suit any frequency . You would find a lot by googleing.
They work by picking up the transmitted RF as voltage on a meter around the transmitter and as you get further away less voltage is received. So a normal receiver can be used also and at some distance it will cut out and not receive the transmitter. The SWR meter is handy in that it gives you an idea of how quickly the signal falls away as you move out
Use a oscilloscope to measure transmitter power.
1. Connect a 50 ohm load to the output of the transmitter.
2. Connect the oscilloscope across the load.
3. Record peak reading of oscilloscope.
4. Calculate the RMS value (Peak * 0.707).
5. Power in watts = RMS squared divided by load in ohms
Example:
Peak reading = 100 volts
RMS = 100X0.707 = 70.7 V
Transmitter power = 70.7X70.7/50 = 99.9 Watts
The above is based on the output signal being sinusoidal (CW),
Oscilloscope with X10 probe has to be able to handle peak measured voltage.
An inexpensive power meter/dummy load you can build to measure
transmitter power. You need to use a high input impedance DMM,
the DMM will indicate the peak value of the RF voltage. Perform the
Power calculation - Power in watts = ((Peak X 0.707) + 0.4) squared/50. Dummy Load Construction
If you want to make a rough measurement of the output power of a transmitter then:-
a) Get 2 light bulbs of the same power rating say 12v car headlamp bulbs.
b) Feed the transmitter into one.
c) Feed the other from a variable power source.
d) Adjust the power source until the light output from both bulbs is the same.
e) Measure the voltage & current supplying the bulb.
f) Calculate W = I * V
Stewart,
This type of measurement would be too far off, the resistive load of the lamp,
will probable be less than 10 ohms. Can't use a 12 volt lamp, a 100 watt transmitter
into a 50 ohm load is 70.7 volts. You can use a 115 volt lamp, but the resistance offered by
the lamp will be off. Amateur radio operators are concerned about the actual power output from
their transmitters.
abeza
Stewart,
This type of measurement would be too far off, the resistive load of the lamp,
will probable be less than 10 ohms. Can't use a 12 volt lamp, a 100 watt transmitter
into a 50 ohm load is 70.7 volts. You can use a 115 volt lamp, but the resistance offered by
the lamp will be off. Amateur radio operators are concerned about the actual power output from
their transmitters.
abeza
I am well aware of using a lamp as a load to the transmitter,
I was just indicating that if you are concerned about the actual
power the transmitter is delivering to a 50 ohm load, lamps
would not give you a true indication due to not representing a 50 ohm load.
It is ok for indicating the transmitter is functional.
abeza
I am well aware of using a lamp as a load to the transmitter,
I was just indicating that if you are concerned about the actual
power the transmitter is delivering to a 50 ohm load, lamps
would not give you a true indication due to not representing a 50 ohm load.
It is ok for indicating the transmitter is functional.
abeza