James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin
catalog seems to say they are.

They can be suitable for some RF applications. If you are thinking of
the wire-ended ones up to 2-3W, they are a flat spiral of just a few
turns of the metal film, on a ceramic core.

Plugging the number of turns, diameter and length into the usual formula
suggests inductances of less than 0.1uH, even for the larger ones. This
means they have inductive reactances rising up to about 10 ohms at
30MHz.

For example, if you wanted to use say a 47 ohm resistor as a dummy load,
the SWR would be quite reasonable up to 30MHz.

However, the inductance of the higher-value resistors goes up much less
rapidly then the resistance does (they use a thinner film of a more
resistive material, but not a lot more turns). The inductance of a 470R
metal film resistor is not a lot higher than that of a 47R. So if you
were to use ten 470R resistors in parallel to make your dummy load, the
inductive reactance would be divided by 10 and the SWR would be much
better.

I have used multiple-paralleled 2W metal film resistors as high as
50MHz, and up to144MHz in situations where the inductance could be tuned
out.

The ones I tested can also be overloaded to red heat for several seconds
without significantly changing resistance when cold - unlike the cheaper
carbon film resistors which burn (as carbon does), or wire-wound
resistors which generally develop a hot-spot and burn out.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek