How to measure RF Power

  • Very carefully! with the aid of something like a Ebay 30 dB directional coupler, a 30 dB attenuator and a 50 Watt dummy load good for 2.4GHz. The directional couplers seem to be very good are are covered by a few posts on the forum, they cover 800 to 2500MHz and are cheap to buy, I picked up a 50 Watt dummy load for around £30 UKP and you can get a N connector'ed attenuator for similar money, so probably not a cheap option but one which could save your power meter.


    Amp through directional coupler to the load, -30dB port to another 30 dB attenuator to your power meter, so if you assume 43dBm with your 20 Watts you will get a signal at the power sensor of +43 - 60 = -17dBm

    Adrian

  • I would be very doubtful of getting a good VSWR at 2.4GHz by using resistors in parallel, at these frequencies dummy load resistors tend to be co-axial in nature. Have a look at this page http://millimeterwave.free.fr/ATT_DISPONIBLES.htm and see if anything takes your fancy, it really depends on if you can deal with 7/16 connectors. Also you will need more that 40 dB in total to not damage your power sensor. A quick look on Ebay (UK) and I see some 50 Ohm loads 25 watts for about £11, they will only be rated for intermittent use I would suggest, but they say they are good to 3GHz or even some resistors that can be bolted directly to a heatsink that will take a lot more power if cooled correctly, so please do have a look around various outlets before you decide.

    I am probably guessing wrongly, but wonder if you will be trying to test a Pyrojoe 20 watt amp or similar, if you can give your self some margin in case it is 25 or 30 or even more Watts output.


    What ever you do be careful at these frequencies and powers as you are taking microwave frequencies now and soft tissue damage is always a possibility.

  • Many thanks Adrian for your time explaining all this, and George for the excellent links providing also very useful theoretical & practical information.

    My idea is to measure the output power of the Khune PA with an economical AD8318 based RF power meter.

    But I have to be careful and stay focused, because the link of George gave me already some other new project ideas....Hi!

    Have a great weekend

    Daniel

  • Hi!


    For power measurements I'm using a rf probe published years ago on the following article:

    http://lea.hamradio.si/~s53m/phare/Mikrovalovna%20sonda.pdf


    The article is in Slovene language but it is not complicated to make so can be understood from the schematic and building scheme. Mine is working up to 6cm band. The author states it should work on 10GHz as well but don't have any source on 3cm to try it. One thing to know is that seems the disk capacitor has a resonance on 13cm and for this reason the probe is showing abt. 20% less. On other bands is quite accurate. It was the case also with mine so after build is good to check it against a known source. After you know the % of error on 13cm than thats not a problem for measurements. The max power is limited by the dummy load resistors and is abt. 0.5W. For very short measurement mine survived up to 1W... For more it is good to have some attenuators at hand.


    One warning if somebody decide to build it. Don't buy the BAT62-03W diode on eBay from Chine. They are fake at last from the seller I got it. 1st got the diodes from China and could measure power up to abt. 440 MHz corrcetly. On 23cm was showing only abt. 30% of the true power and almost nothing on 13cm. After bought from https://www.rf-microwave.com/en/home/ and got it working as expected.


    73's Andrej - s57rw

  • Don't forget thermal methods. How much a load heats up with RF compared against a second identical load fed with DC.

    Hi!


    Agree with you to consider differente between DC and RF. In the article the author recommends max 0,5W but for very short time. Anyhow not clever pushing the limits. If you burn the resistors it is a real hassle to desolder the box for replacing not speking about burning an expensive rf final after… For this reason got some attenuators I use when applying more than abt. 1/4 W.


    73's Andrej - s57rw