I would recommend different approach - instead of buying attenuator that can dissipate all the TX power (let's say 100W) and be able to present 50 ohm to TX and 50 ohm to tinySA, my approach is to by a good 100W dummy load (it's always useful to have, they can be found for cheep quite often), and then have a "special" kind of attenuator that presents high impedance on the input side (several kohms) and 50 ohms to the tinySA. You can easily scale and buy 2kW dummy load and just add additional attenuation to your tap, and get covered for your full legal limit needs :)
Such device (some call it a "tap") then dissipates very small power (well under 1W), is quite cheaper, and easier to (even) build as DIY project
They come as "resistive divider (or resistive leader) - basically two resistors (non inductive), the "bottom" one being 50ohm, the top one easy to calculate, or as inductive - current transformer - but more difficult to "understand" but there are good instructions how to do it
There was a recent thread on this group addressing both?
