I have a small collection of "software defined RF lab" equipment, notably a HackRF and a VNWA3. I am almost done building a 35W HF amplifier and once I've done some other testing, would like to look at the spectrum of the final output as a whole.
The most natural tool for this job would be a spectrum analyzer, of course, but any of my SDR receivers and/or the VNA should be able to handily gather spectrum data as well. The trouble is: all of my equipment is rated for input in the ballpark of 0dBm, i.e. milliwatts or less. It looks like even a "real" spectrum analyzer like the entry-level Rigol DSA815-TG has input rated around +20 dBm (100 milliwatts).
In preparation for another project where I want to diagnose a ~5W signal, I bought a handful of SMA attenuators. The larger dB-drop ones have heat sinks and are rated at 5W — so I should be able to connect my "device under test" for this other project directly to my equipment through the attenuators.
But how would one go about measuring the spectrum of a 35W signal? Or 1500W and beyond for that matter! I could be mistaken, but I get the impression that that inline attenuators are NOT a common method past a certain power level.
How are high-power signals typically measured in a professional RF lab setting? How might I go about characterizing my signals with more typical amateur operator equipment? Say, between my amp and a dummy load and an SDR, what's a good way to connect/couple them so that I can get a known — or at least "known-to-be-safe" — signal level into some spectrum analysis software?