I would recommend going to the Agilent website and downloading the data sheets for that family of sensors.
As for linearity and how to select, I would let the frequency and power level be the main factors. You might want to get a set of data sheets for the various power sensors, and do a comparison.
Some other vendors include Rohde & Schwarz, and Anritsu, who also have families of power sensors (those are the only other brands I can bring to mind at the moment).
You might also look at the other model families at Agilent, as they have a number of newer models these days (and this is not an advertisement for Agilent). So in that light of neutrality, I would also recommend checking to see what is available from the other vendors.
Back to the 8484A sensor, my GUESS would be that the 8484A was discontinued perhaps because it's technology was supported only by the older models of power meters, and the 8481D may be a little newer (not sure though). I just downloaded the latest Agilent data sheet, and it has comparative specs on quite a lot of their sensors. You can probably do the same as well for Anritsu and Rohde & Schwarz.
If anyone knows some other current brands of RF power sensors that I can't think of, it would be educational to post them here. The only one other brand that just came to mind is Boonton (although I'm not sure they are still making them).
As for linearity and how to select, I would let the frequency and power level be the main factors. You might want to get a set of data sheets for the various power sensors, and do a comparison.
Some other vendors include Rohde & Schwarz, and Anritsu, who also have families of power sensors (those are the only other brands I can bring to mind at the moment).
You might also look at the other model families at Agilent, as they have a number of newer models these days (and this is not an advertisement for Agilent). So in that light of neutrality, I would also recommend checking to see what is available from the other vendors.
Back to the 8484A sensor, my GUESS would be that the 8484A was discontinued perhaps because it's technology was supported only by the older models of power meters, and the 8481D may be a little newer (not sure though). I just downloaded the latest Agilent data sheet, and it has comparative specs on quite a lot of their sensors. You can probably do the same as well for Anritsu and Rohde & Schwarz.
If anyone knows some other current brands of RF power sensors that I can't think of, it would be educational to post them here. The only one other brand that just came to mind is Boonton (although I'm not sure they are still making them).