how 2 test tx power of a transmitter

[jeff6000]

cane someone tell me how 2 test the output power of a transmitter

 

[MikeMl]

What frequency? What is the expected power level?

 

[languer]

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.

You could also use an RMS sensor (Linear Technology - Parametric Search for RF RMS Detectors or Detectors | RF / IF ICs | Analog Devices) to convert RMS power into voltage.

If you want power and frequency content, a Spectrum Analyzer is more in line for that.

 

[tytower]

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

 

[abeza]

Power Measurement of a transmitter

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.

 

[tytower]

Thats if yo have an oscilloscope handy of course !

 

[Nigel Goodwin]

Thats if yo have an oscilloscope handy of course !

And a very expensive one that works at the frequencies required.

 

[abeza]

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

 

[stewart_bakeruk]

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

Good enough for government work...

Stewart

 

[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

 

[Nigel Goodwin]

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

Presumably you're either not a Radio Amateur, or you're only a recent one

The light bulb technique is an old tried and tested solution for Radio Amateurs - I seem to recall you were even tested on it in the RAE.

 

[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

 

[Nigel Goodwin]

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

Like I said, it's a tried and tested method of measuring transmitter power, you obviously need to select the specific lamp carefully.

 

[abeza]

As you indicated, with the proper lamp selected it would do the job.
abeza

 

[stewart_bakeruk]

Presumably you're either not a Radio Amateur, or you're only a recent one

The light bulb technique is an old tried and tested solution for Radio Amateurs - I seem to recall you were even tested on it in the RAE.

Absolutely, now you can guess my age !

I did say a "rough" method. As the original question did not give the frequency, power or accuracy required it was just a suggestion.

To make any really accurate measurement of RF power is not easy, I have tried...

S.

 

[abeza]

Stewart,

Dummy Load Construction

I built it, and I was very satisfied with the results of the measurement.
Not that expensive to build.

abeza