Measure transmitter power - Page 1
Forum section: electronics / rf-microwave

author: Faranight
author_rank: Unknown
timestamp: September 19, 2022, 07:46:57 am
content: Hello.Is there an easy way to measure a transmitter power in a home lab? I'm working on a hobby project that will use wireless communications on 433MHz ISM band, and I'd like to make sure the transmitter isn't exceeding maximum allowable power to abide by the local regulations. Basically I'd need a way to measure up to 100 mW or slightly beyond that. I noticed my Siglent SDS2104X+ has an option for 50 ohm input impedance. Can I use that to some degree? I don't wanna fry my scope.

author: radiolistener
author_rank: Super Contributor
timestamp: September 19, 2022, 08:17:02 am
content: you're needs to buy powerful 20 dB attenuator and use logarithmic detector calibrated with some known power level source. Just add attenuator value to the measured value in dBm and you will get transmitter power.Also you can use oscilloscope through attenuator for measurement, but your model is unable to measure amplitude at 430 MHz. You're needs some model with at least 600-1000 MHz bandwidth. Since oscilloscope has not a flat frequency response, such amplitude measurement will not be reliable.In any casedon't try to connectyour transmitter into oscilloscope or other measurement equipment without attenuator! Your equipment will be burned out.

author: DimitriP
author_rank: Super Contributor
timestamp: September 19, 2022, 09:18:46 am
content: The scope doesn't have enough bandwidth to be accurate at 433MHz, but there is always a way!(100mW is less than 2.5V rms across a 50ohm load )You could order one of thesehttps://www.ebay.com/itm/165475431192and while you wait for it to arrive,take a look herehttps://k4eaa.com/dummy.htmland see if you can do something with a resistor or two a diode , a capacitor and a DC voltmeter(or you could use the scope for that!).

author: radiolistener
author_rank: Super Contributor
timestamp: September 20, 2022, 10:56:35 am
content: yes, oscilloscope can be used to measure Voltage on logarithmic detector output. But usual DMM will be more comfortable

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2022, 03:42:44 pm
content: K4eaa design above is not suitable for 433MHz.100mW at 50 ohm is 3.162Vpeak.Take two 100ohm smd resistors in parallel, solder them to a sma connector (directly on the connector), and use for example 1N5711 diode plus a blocking cap, like 1nF ceramics. With an ideal diode you should read those 3.162V DC (10Meg dmm input), but there will be some loss at the diode, like 0.5V at that frequency (best you calibrate the range).

author: radiolistener
author_rank: Super Contributor
timestamp: September 21, 2022, 06:52:12 am
content: some diode may works bad on high frequency, and there is Voltage drop on diode which is different for different diodes, so it's better to use logarithmic amplifier.I was used power detector on diode, it worked well at 145 MHz, but has some issues on 430 MHz. And if you don't have reference power source for testing you will never know if your diode works as expected.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 21, 2022, 11:02:47 am
content: Quote from: radiolistener on September 21, 2022, 06:52:12 am100% agree and I would go so far as to suggest that's a common problem with anything I make at home. In the video below where I show my Watt meter, I mention several times how I would have no way of knowing what the absolute accuracy is.From the OP's questionQuote, the obvious answer is to buy a Watt meter. Then again, the OP leaves out a lot of details.

author: LM21
author_rank: Regular Contributor
timestamp: September 22, 2022, 05:49:10 pm
content: There are cheap spectrum analyzers which can easily measure power levels with an attenuator. Without attenuator you will break it.https://groups.io/g/tinysa

author: MathWizard
author_rank: Super Contributor
timestamp: October 11, 2022, 05:06:52 pm
content: So when the OP say 100mW, is that the max power expected ? And what inside a typical DSO or SA would get damaged by that ?I'm about to make ~2MHz colpitts osc., as part of a AM transmitter. I could calculate the power of the BJT and collector resistor, but yeah I have never tried anything with radiated energy. Or EM fields in this way.