Rik ON7YD and others,
Here in ZL we have been using wire wound resistors as dummy loads for
some years, and they generally work well on frequencies up to 180 kHz.
Those types with some residual inductance can have "reactance
cancellation" (a low Q tuning arrangement) with polystyrene or
polypropylene capacitors, with 10,000 pF being a typical value. The
aluminium clad type of wire wound resistors are particularly good for
dispersing heat by conduction to a heat sink, as they are made for being
screwed to a flat surface.
However, perhaps the best LF dummy load is an oil jacket "domestic
heater". A 1200 Watt type for 230 Volts AC has a resistance close to 50
ohms. Tests around 180 kHz show that it is very likely to be a low SWR
as an LF load even when clip leads are used on to the mains power cord.
There is of course no problem with long key down testing, as the 1200
Watt rating is nominally the steady state rating. So a 1200 Watt 230
Volts oil jacket heaters should work well straight out of the box :)
Even though the abovementioned loads are good at LF, they seem to be
fairly bad at HF. This could be not only because of the inductive
component, but skin effect may increase the resistive component as
well. So while the ideas for dummy loads are very good for LF, they are
fairly useless on HF.