When we use a "non-inductive resistor" to check the output impedance of say an unun, say 50ohm 1:1, what resistor type is actually non-inductive, or does it not really matter as long as the leads are short and frequency is low? I know there's non-inductive resistors with bidirectional windings, and some other non-inductive types but which would be the proper (or most ideal) type(s) to use? I'm thinking of getting some hence the question!
For measurements the best resistor is a surface mount one - an 0805 resistor is non-inductive to a few GHz. Even better is to use two reststors, usually 100 ohms each, one each side of the coax.
A 1/8 W or 1/4 W carbon film resistor is also fine for HF, its leads are the main source of inductance and they're shorter than the rest of your unun circuit.
The whole non-inductive resistor story comes from the design of dummy loads which need high power resistors. These are often wire wound and hence high inductance. There are thick film high power resistors available for terminating high power lines.