Measuring RF Power Output with DMM or DSO?

[vinlove]

Is there any way to measure RF power output from HF tranceivers with DMM or DSO instead of usual Power Meter?

[ogden]

Is there any way to measure RF power output from HF tranceivers with DMM or DSO instead of usual Power Meter?

Scope: depending on frequency and RF power and scope. What frequency (range) you are talking about?

DMM: no, not without "RF power adapter" which in case of DIY needs to be calibrated/characterized:

[CJay]

As Ogden says, lots of variables but I use a variant of the below:

http://www.ai4ji.com/projects/dummyload.htm

It's pretty accurate up to approx 150MHz and I've run 100W into it for a short period of time.

[radiolistener]

yes, you can use DMM to measure RF power, but you're needs logarithmic power meter module and some attenuators, because transmitters have too high power.

Something like that:
https://www.aliexpress.com/item/32944265316.html

and attenuator: https://www.aliexpress.com/item/32915632953.html

[joeqsmith]

Yes in both cases.   I made something similar to the home made setup shown for my handheld meter.   I used a real Watt meter that was in cal for a reference to characterize the home made one. 

I also posted about this one and made a video showing it in operation.   
https://www.eevblog.com/forum/projects/home-made-pa-and-load/msg635226/#msg635226

[vinlove]

I am trying get back to HF radio operating after a few years QRT.  In that time, my father was critically ill, and I had to fly the other side of the world, and care for him for a few months. But he passed away, and I had to deal with many family matters.   I  returned home, and had to do a lot of work on renovating house ... etc so had no time to operate radio.

Now I am slowly trying to get back to HF, but have no radio gears, so I have been getting one by one from eBay and Amazon looking for bargains.  I managed to get old cheap rigs, and manual ATU and Morse key. But I have no power meter. When I had a look for metres, they are either cheap CB ones, which I dont need, or if ones I want, they are expensive. So I thought how about trying to measure RF power with DMM or DSO? But I had no idea how to do it.

I will be watching those videos closely and try to pick up what I can.  I think there was also some kind of formula that you apply to out voltage measure by DSO on some American Ham operators web page.  I don;t know if it is still there. But will add link here if / when I find it.

I do operate mostly on 80 40 and 20 metres, and sometimes 30m 17m bands too. So freq 3.5 - 17 Mhz in HF.

[joeqsmith]

People still use Morse code?  I thought the FCC dropped all that years ago.   Wait, and you don't want to just buy your 2 meter radio and blab about the antenna you  made?   lol.   Your a bit of an odd duck. lol. 

All joking aside, even into the 50 meter band, I wouldn't think making your own tap would be much of a problem but I would just keep hunting for a deal on a decent SWR/Watt meter if that's really what you want.  With a modern scope, you may be able to add your equation to automatically display the power but is seems like a lot of cost and room used up for what you want to do.

[bob91343]

When my Radio Schlock digital power meter failed I pressed my venerable HP 410B into service.  I put a tee in the coax line and connected the ac probe to it.  Now it's a permanent part of my station.

Of course the meter doesn't measure power but if my SWR is close to 1:1 I can compute power.  Unity standing wave ratio means the load is 50 Ohms so if I read 200 Volts I can pretty much believe I have 800 Watts.  The probe is good to nearly a gigahertz and can withstand the legal limit.

For a more accurate reading I would have to use a device that could deal with mismatch.  But why would I need a more accurate reading?  I know my equipment is incapable of more than the legal limit, and I know that tuning it for maximum line voltage is a good way.

I also have an oscilloscope that shows me the transmitted wave so I can adjust my transmitter audio, whether it be a modern transceiver or an AM boat anchor.

Power isn't really a commodity that you can measure and be done with it; it's a dynamic function that needs some smarts to interpret and use.

[vinlove]

People still use Morse code?  I thought the FCC dropped all that years ago.   Wait, and you don't want to just buy your 2 meter radio and blab about the antenna you  made?   lol.   Your a bit of an odd duck. lol. 

All joking aside, even into the 50 meter band, I wouldn't think making your own tap would be much of a problem but I would just keep hunting for a deal on a decent SWR/Watt meter if that's really what you want.  With a modern scope, you may be able to add your equation to automatically display the power but is seems like a lot of cost and room used up for what you want to do.

Morse has been 90% of my QSOs   It's more efficient way of making contacts with smaller antenna and power other than any reason, and of course, the most traditional method of long distance communication.

I do have a Rigol 1052E DSO already, so if it is possible at all,  I thought why not make use of that measuring power output from tranceivers.   I don't constantly measure RF power of rigs, but once when I got it for the first time, I would like to check the min and max output of TX before using it, just to make sure it is in quoted spec.

[radiolistener]

You can try it with 1:100 probe with 4 pF on the input. Just connect your TRX output to dummy load and use 1:100 probe to measure amplitude on it

then you can use equation:

P = Vpk^2 / (2*R),

where
P - power in Watts
Vpk - amplitude in Volts (half value of the peak-to-peak voltage)
R - dummy load resistance in Ohms

Chinese manufacturers promises 100 V up to 250 MHz for these 1:100 probes, but I'm skeptic about that. I didn't tested it for 100 V RF, but for less than 50 V it works ok.

If you have RF voltmeter which can measure RMS voltage, you can use this one:

P = Vrms^2 / R

[bd139]

100% CW here 

You can measure 25W easily on most scopes in HF spectrum without blowing them up just with a BNC T-piece and a cheap dummy load. Convert peak-peak to watts or be lazy and use [1]. Any higher powers, invest in a 10dB power attenuator. That'll get you up to 250W.



Throwing it on my analyser, SWR is flat up to 55MHz and purely resistive. Above that, no idea.

[1] http://wera.cen.uni-hamburg.de/DBM.shtml