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The math is simplest if you do the calculations in dBm (dB referenced to 1 milliwatt). 1mW = 0 dBm. 1W =?+30 dBm. 10W =?+40 dBm. For HTs and QRP rigs, perhaps the most important number is that 5W =?+37 dBm.

The maximum permissible input to the tinySA Ultra is?+6 dBm (4 mW). If you are applying a?+37 dBm signal, you need 31 dB of attenuation to decrease that to?+6 dBm, more than the 25 dB external attenuator that you proposed, and a tad more than the 30 dB attenuators that are popular surplus items. And you want a bit more in case your HT produces more than 5W; I'd recommend starting with at least 34 dB, which would make you safe for up to 10W.

The internal attenuator in the tinySA Ultra is an active device, and as such it causes some distortion. That will make the device under test (DUT) look a bit worse than it actually is. For best measurement accuracy, you want the internal attenuator to be inactive, which means you need an additional 30 dB of attenuation to get down to the range where it's not used. That theoretically comes at -24 dBm, but the auto mode of the tinySA is a bit more conservative about using the attenuator; you'll get down close to -30 dBm before it turns off.

For my measurements, I use a 30 dB fixed attenuator with a 20W rating, plus a step attenuator. For testing higher power radios I also have a 20 dB fixed attenuator with a rating of 150W that I can add to the chain. It's often difficult to find the rating of surplus step attenuators; it's safest to treat them as having a maximum input rating of?+17 dBm (50 mW). Some fixed attenuators specify an input end and an output end; make sure to heed those, or else you can burn out your attenuator. If you buy a used attenuator, you should test it for accuracy, which you can do with the tinySA itself using its signal generator as the source signal (procedure at ) or with another instrument such as a NanoVNA. The NanoVNA is nice because it can easily produce a display of attenuation over a range of frequencies.

For testing an HT, I would start with the 30 dB fixed attenuator plus a setting of 40 dB on the step attenuator. That would reduce the expected +37 dBm input signal from the HT to -33 dBm. Then I would reduce the amount of attenuation in steps until the internal attenuator turns on, then back off from that until it turns off again. That procedure gets maximum dynamic range and minimum distortion from the tinySA. The actual output of "5W" handhelds varies a bit, but I'd typically end up with about 35 dB on the step attenuator. Once you have dialed in the needed amount attenuation, you can set that in the tinySA so that it will directly display the actual signal levels.

ALWAYS start with at least as much attenuation as you need, adding a bit more for safety if possible. You can then reduce it to the optimum point. If you start with an inadequate amount you can destroy the front end of the tinySA, and you don't want that.