Measuring antenna current/received power with SDR - Page 1
Forum section: electronics / rf-microwave

author: maelh
author_rank: Regular Contributor
timestamp: April 07, 2022, 11:24:23 pm
content: Hi,I have an antenna made of a ferrite rod/coil and a capacitor that is tuned to 77.5kHz. Connecting it to the High-Z input from my RSPduo I can identify the DCF77 signal well, which also gets more or less weak depending on how the antenna is oriented.So far all as expected.I would like to determine the power that is received over the antenna, as in electrical power in watts (or probably microwatts). Can this be done with SDRplay or another software? Ideally I would like to know the (max) voltage of the signal as well. The spectrum and waterfall display show only dBm which vary depending on the selected RF gain. I'd like the real power and voltage the antenna receives before amplification / processing by the SDR.How can this be done?

author: El Rubio
author_rank: Regular Contributor
timestamp: April 09, 2022, 02:27:46 pm
content: dBm is a power measurement. You can convert it to watts, but the problem is how accurate does it have to be. Does the SDR have a calibrated receive scale? A spectrum analyzer should factor in the gain of internal preamplifier and display accordingly. If so, you can factor in antenna gain & cable loss to determine the signal strength hitting your antenna. If high accuracy isn’t required, just search “ dbm to watts calculator” and you should find several choices.

author: cdev
author_rank: Super Contributor
timestamp: April 09, 2022, 03:53:20 pm
content: If I were you I would use a simple tuned circuit and high impedance RF voltmeter or oscilloscope to measure the instantaneous RF voltage into some load.No you could notand should not use any SDR as they are designed to adjust their gain according to the received signal which will make the readings fairly useless, I'd suspect.You could use a simple signal strength meter made with a diode, which would still be nonlinear due to the nonlinearity of the diode.. whatever it is. They all have some nonlinearity, also lack sensitivity at lower signal levels.Detector circuits are designed to attempt to overcome these problems. Impedance transformations are used to present a detector with a high enough voltage to overcome the diode's threshold.Quote from: maelh on April 07, 2022, 11:24:23 pmLinrad allows dbm measurements if the radio is pre-calibrated.Its pretty good for this, actually. For time signals like DCF77 (and WWV and WWVH) you can use a roll of coax as your antenna, tuned to the DCF77 frequency with variable and or fixed capacitors.. like mica caps.This is described on febo.com in the projects area.

author: cdev
author_rank: Super Contributor
timestamp: April 09, 2022, 03:56:57 pm
content: I would expect you might want a bigger antenna to get more voltage..Quote from: maelh on April 07, 2022, 11:24:23 pmAnd more linearity, which is important in minimizing errors so your numbers are meaningful, not just relative. (due to AGC you cant fully eliminate)

author: Roger Need
author_rank: Regular Contributor
timestamp: April 09, 2022, 11:26:34 pm
content: Quote from: maelh on April 07, 2022, 11:24:23 pmYou can measure the signal power from your antenna using a RSP receiver and SDRuno software from SDRplay. The dBm level displayed in SDRuno is the power measured at the antenna terminal connector of the RSP. It is accurate to +/- 1 dBmregardlessof the RF and IF gain settings. This is accomplished by using gain calculation tables built into the SDRuno software that determine the overall gain of the receive chain and then calculate the signal level at the antenna input.So if you observe -90 dBm in SDRuno you know that the output from your ferrite antenna is at this level. It is 90 dB below 1 milliwatt which works out to be 0.001 nanowatts. Using standard formulas you can convert from dBm to voltage or current if required. When doing these calculations you need to use 1000 ohms as the impedance of the HiZ portRoger

author: cdev
author_rank: Super Contributor
timestamp: April 11, 2022, 01:28:46 pm
content: I just want to encourage people to build your own antennas and - yes, you can extract small amounts of power from them, and even power small electronics. I have done this many times, and I dont live anyplace special. Just in an area where there are some radio stations miles away.I am powering a cheap temperature humidity meter from WalMart off of the RF right now.With a tuned mag loop one can get quite a bit of it. More than a volt. This was from a station in Cuba that was coming in very strong here. Using a one turn magnetic loop. And a small signal diode.