I did some thinking here, plus some math and testing, then I found an excellent youtube from W0QE describing all that was asked here :) The only reason NOT to check his presentation would be the time - two videos (Part 1 and Part 2) are 30 minutes each, and are well worth time spent!
For some of you too busy to check them out, here is the digest version :) Even if we assume that my digest will tell you everything you need to know, still go and check Larry's posts!!!
Let's assume you want your tap to cover some extreme cases - from 1W to 1500W. You also want to get signal from the tap that you can take to your instrument (tinySA for example) that's 50ohm. This will drive the need attenuate your signal by 40dB (too much and your low power will be to small, too little and your legal limit will fry your instrument). Without too much explanations (see Larry's Part 1), you will end up with resistor divider 2500ohm/50ohm. Given that
1500W@50ohm->270Vrms,?your 2500ohm will need to be 30W. Too much :)
If you scale down your expectations to use tap for up to 100W, you will deal with 70Vrms, and resistor will be 2W, more manageable.
NOTE: all the numbers are for measuring where SWR is very low. Increase SWR, voltage goes up.
Larry is showing examples where you can use Cap divider (or inductive) so you eliminated power dissipation only related to resistors, but both have quite frequency dependent "behavior".
In Part 2 he explains inductive coupling (the current transformer), impact of stray capacitances, sizing, ... I especially liked the part with "how to minimize stray capacitance" and different design approaches to ""extend the shield"
He also touched on impact of SWR, and that applies to all cases (R/L/C/inductive).
Through both parts he is EXTENSIVELLY using SimSmith - if not familiar, don't be scared, he's explaining what's being done.
If you like his videos, check other of his stuff, i just finished his video on designing CMC (Common Mode Choke) and finally understood some of stuff that was always in front of me, but never clicked!
I just need to find more hours in a day and to check all that's on his youtube channel :)
So, to SUMARIZE:
- resistive taps are quite OK, but come with significant challenges related to needed power rating of a resistor used if you are constrained with 50ohm output?
- capacitive and inductive dividers are not practical
- inductive coupling, when done correctly, is the best design to cover wide range of power and frequency
- neither (divider/coupling) should be used to measure power if SWR is not very low
?
