# Measuring RF transmitted Power

I am running a software radio application. I need to measure the power of transmitted ofdm signal. The issue is that FCC has regulations of -50dBM/Hz. So I understand that if i need to send a signal at a bandwidth of 1Mhz, the the maximum allowed power -50dBM*10^6 which is 0.01W or 1mw.So i am trying to measure the power at output of my USRP transmitter. I just brought an oscilloscope and measured the sinusoidal signal peak to peak voltage in the oscilloscope. It was 7.2 v peak to peak. I was unsure how to use v^2/R because i don't know the resistance. when i assumed resistance to be 1, I got the power output to be 39dBM. I then checked online and found that you need to measure power spectrum. So that means I need to know power at frequency of transmission. I need a RF power analyser for the same. How is the power at frequency spectrum related to analog power measured by oscilloscope at the output of antenna? What should i measure and reduce if i need to keep it within desired FCC regulations? Is the power of the frequency spectrum constant?

• This is off-topic for dsp.SE and should be migrated to electronics.SE. – Dilip Sarwate Jun 27 '13 at 14:16
• I wanted to know the about frequency spectrum power. So I asked here hoping there is dsp. How should i migrate it to electronics.SE? Sorry for posting at wrong place. – Karan Talasila Jun 28 '13 at 5:18
• Click on the flag link at the bottom of your question to contact the moderators of dsp.SE and ask them to migrate the question to electronics.SE – Dilip Sarwate Jun 28 '13 at 13:47

You need

1. A RF anechoic chamber (wedges on the walls, floor and ceiling to prevent any reflections)
2. A test device with an antenna and physical setup (construction, mounting, form, shape etc.) that's very close to what you want to ship
3. A calibrated measurement setup, that's typically multiple receive antennas in the chamber or a full 3D setup (vertically movable antenna plus a horizontal turn table)
4. Very good understanding of the polar pattern of your transmit antenna
5. A good calibrated spectrum analyzer
6. Time & patience

This is not an easy measurement to make, so most companies farm that out to certified test houses, who have the equipment, facilities and experience to do this.

• This is overkill for an amateur device that you don't intend to mass produce / sell. – nibot Jun 28 '13 at 14:47
• @Hilmar Ya i want to see my transmission for different power levels. The level of complexity you suggest is for actual device prototyping and manufacturing. At an experimental level, can i not do with a RF power meter or any other simple experimental setup. – Karan Talasila Jun 29 '13 at 6:27
• It really depends on what accuracy you need and what you already know about your system. Antenna efficiency, polarization and radiation pattern change transmitted power a lot. If you now you have vertically polarized dipole you may get away with a measuring just 3 or 4 points. – Hilmar Jun 30 '13 at 19:55