How to test the power of RF signal

[zeller]

Hi please help.

I have designed an oscillator which oscillates at 27 mhz(from readings from an oscilloscope on the antennae of the oscillator).Now I want to find out the POWER of this signal i.e in watts so that i can know how far it can travel in space.I only have Digital oscilloscope.How do I find out?

Thanks.

 

[mister_rf]

Simple power measurement:

http://www.qsl.net/vu2msy/homebrew/RF_Power_Measurement_VU2BD.pdf

Power Measurement with Simple Techniques

**broken link removed**

---------- Post added at 17:13 ---------- Previous post was at 17:12 ----------

**broken link removed**

 

[young.microwave.eng]

dear zeller
do you have any peak detector available at your Lab?using peak detector you can see the signal in oscilloscope
this adaptor available for different frequencies and is not expensive.because you have oscilloscope available then this is the right method for you I think.then using the table of voltage to dBm you can find the power of your signal in logarithmic format

 

[FvM]

The power measurement methods mentioned by mister_rf are all assuming a power amplifier/transmitter with 50 ohm output impedance. I guess you are referring to an oscillator according to your previous thread https://www.edaboard.com/threads/202858/#post872261

This oscillator never achieves 50 ohm impedance, nor the "antenna" (a piece of wire, I assume) is matched to it's output impedance. In this case, different power numbers exist.
- the power delivered into a 50 ohm load respectively a specified nominal load
- the maximum RF power that can be supplied by this circuit, has to be determined by varying the load impedance (load pulling)
- the power radiated by an antenna (E.R.P., effective radiated power), has to be determined by a field strength measurement

 

[zeller]

The power measurement methods mentioned by mister_rf are all assuming a power amplifier/transmitter with 50 ohm output impedance. I guess you are referring to an oscillator according to your previous thread https://www.edaboard.com/threads/202858/#post872261

This oscillator never achieves 50 ohm impedance, nor the "antenna" (a piece of wire, I assume) is matched to it's output impedance. In this case, different power numbers exist.


- the power delivered into a 50 ohm load respectively a specified nominal load
- the maximum RF power that can be supplied by this circuit, has to be determined by varying the load impedance (load pulling)
- the power radiated by an antenna (E.R.P., effective radiated power), has to be determined by a field strength measurement

As always, your posts is very useful!!! So I would need a field strength meter.Is it easy to design? Can this tell you how far the signal is expected to propagate?

Thanks.

 

[mister_rf]

The link between two points includes the origin transmitting antenna, propagation medium = earth’s atmosphere and the destination receiving antenna.
The key factors are transmitter power, antenna gain at the transmitter, propagation or link losses, antenna gain once again at the receiving station, and finally receiver sensitivity. Taken together these factors makeup the balance of power plus gain required to compensate for losses in the link so that sufficient signal strength is available at the receiver to allow data reception. Also radio waves at high frequencies range from about 3 MHz up to about 30 MHz propagate by combinations of line of sight, refraction and reflection. Propagation characteristics tend to vary from day to night, even summer to winter. This is because the dominant propagation of short waves is dependent upon the earth’s ionosphere, and as the ionosphere’s characteristics change through day, night, summer and winter, so do its propagation characteristics. Propagation ranges thus can vary from line of sight (50-70 km) during poor conditions or up to thousands of kilometers during optimal conditions. These long distance propagations are possible because of ionized particles in the ionosphere causing the RF signals to refract or reflect back down to the earth’s surface. And when the earth’s surface is highly reflective multiple reflections between the earth and ionosphere occur allowing long distance propagation. If you want to calculate the maximum distance you need to have some data also on the receiver side, not only for the transmitter.
More details here:
Radio propagation - Wikipedia, the free encyclopedia

 

[FvM]

So I would need a field strength meter.Is it easy to design?

It's easy to get a relative measurement, allowing e.g. to tune an antenna. But a calibrated measurement, that tells about radiated power isn't easy. A special problem at 27 MHz is, that you are most likely operating with electrical small anatennas of a size < quarter wavelength. So you have difficulties to match both the transmitter and the measurement antenna.

I'm also not sure what you want to achieve. Assuming you have a transmitter power of a few mW and manage to send it to the antenna, you should be able to receive it at least over several 100 m up to km.