Transmitters' RF power measurement

[Fred Salles]

Hello,

I am trying to understand how are RF power measurements made, how to check that a transmitter has the right RF power output, how to test RF antenna cables insertion loss, etc...

I got myself a little gadget that is supposed to measure RF power outputs in dBm and mW. But it is a cheap stuff and I am getting weird results, like higher measure when adding cables... Before I dismiss it completely, I realised I need to actually understand what is it I am supposed to get.

 

So, for those of you RF insiders, what is the correlation between the rate of a transmitter power and what is measurable at the antenna output? in other words, if I have a transmitter rated for 50mW, what am I supposed to get at the antenna output? 50mW and 16.99dBm or something else?

 

Thanks in advance!

[Patrick Farrell]

Does the device you have measure the conducted power at the output or the radiated power? If radiated, this site https://www.antenna-theory.com/definitions/eirp.php has a formula to work from as well as lots of other useful information. But be careful because distance greatly affects the measurement and won't accurately reflect the conducted power (i.e. 50mW, etc). The FCC has a document that provides even more guidance. https://apps.fcc.gov/eas/comments/GetPublishedDocument.html?id=204&tn=255011

[Constantin]

I think with such a device you can mostly see the difference (if any) between two measurements. The actual numbers won’t be that useful, but you should see a difference when you add a cable or a passive splitter or whatever. If your receiver hooks up to something like Wireless Workbench or Wireless Designer, you can take more accurate measurements with your computer 

[borjam]

Well, those are very different measurements!

 

For an accurate output power measurement you would use a RF power meter connected to the transmitter output. What is the "correct" output power can be a tricky question though. If, for example, you are using an antenna with gain your actual output power should be lower in order to avoid violating ERP (effective radiated power) limits. Or you can (extreme care here!!) use a spectrum analyser together with a properly configured chain of known attenuators.

 

A spectrum analyser also allows you to check a modulated signal in order to detect spurious signals, etc. Some even include specialised modulation analysis software.

 

For other measurements such as insertion loss, frequency response (important for filters!) etc the most usual instruments are either a spectrum analyser with a tracking generator (which is a RF generator which frequency is synchronised with the spectrum analyser window) or a SNA/VNA (scalar or vector network analyser).  The latter instruments (especially the VNA) also allow you to check impedance matching.

 

Which gadget did you get? 

 

Stuff such as VNAs or spectrum analysers used to be extremely expensive but nowadays there are cheap alternatives good enough for many purposes. For example RF Explorer is very well known in this forum. 

 

There are also some SNA or VNA units sold mostly for radio amateurs as "antenna analysers" at prices below 500 euro. And if you want to get more serious Siglent (a Chinese manufacturer who works together with LeCroy) have released surprisingly capable instruments at an affordable price. Even Rhode&Schwarz have followed suite.

 

 

[Fred Salles]

Thanks guys for your inputs. Yes it is conducted power at the output antenna connector that I am trying to measure (not radiated).

The gadget I got is supposed to be a RF power meter, so it is giving me dBm and milli or micro watt. It allows to add an offset, either attenuation for strong power over 1 watt or booster for low ones.

It is a cheap chinese stuff so I have no trust in it but since I do not know what the measures are actually supposed to be I cannot be sure that I need to invest in something better like this one: https://www.bs-rf.com/en/products/rf-power-meter/

 

So apart from guessing it should be reading 50mW (+-17dBm) when the transmitter is set to 50mW, I still have no clue.

Also I got Constantin's point about measuring differences rather than accurate mW, but for now adding longer cable gives me higher mW !! 😂  it does not make sense, yet.

 

 

Borjam, insertion loss is what I am most interested in measuring, and I understood the RF power meter to be exactly the tool for that.

[borjam]

Have you tried making the measurements at an entirely different location? I am wondering whether local interference is messing

with the measurement, either through the cable or the meter itself.

 

If you have ferrite cores you can also try to put them around the coaxial cable and see if they make a difference. That would point to a

cabling problem (or a severe impedance mismatch at the meter input).

 

[LarryF]

[snip] ...but for now adding longer cable gives me higher mW !! 😂  it does not make sense, yet. [snip]

 

The fact that minor cable length difference makes a power difference is a very strong indication that the measuring device is not a 50 Ohm load and is not terminating the cable properly, i.e., there are standing waves on the cable.

Best Regards,

Larry Fisher