K7ITM wrote:
At 50MHz, you should find that metal-oxide resistors will work fine.
You're just making a power-wasting pad, not a precision attenuator.
(Carbon comps wouldn't be appropriate for precision work, either...)
The low-value metal-oxides I've tested on RF component analyzers have
all been fine for non-precision applications out to 150MHz. Try, for
example, Vishay FP69 series 2W parts or TT/IRC GS-3 series.

A couple years ago, I went through my stock of carbon composition
resistors and found more than half of them to be out of tolerance at
DC. They don't age well at all, even while not dissipating power.

The other half of them would probably be out of tolerance too, after
having been soldered.

I just came across an attempt to use carbon comp resistors (obtained
with some difficulty) as RF impedance "standards" - completely hopeless.
Small wire-ended metal film resistors would have been just fine, because
they don't change value significantly when soldered.

Wire-ended metal film resistors can be quite good for RF, but the
parasitic inductance can vary. They are made by cutting a spiral track
into a basic resistive tube, and the pitch of the spiral is changed to
produce a range of resistor values. When the spiral becomes too fine,
the process steps up to a higher-resistance 'blank' and the whole cycle
starts again with about a one-turn spiral.

Thus the number of spiral turns can vary between about 1 and maybe 10.
You have to scrape the paint off to see. The inductance can then be
estimated using the standard formula based on diameter, number of turns
and length, and it varies from a few nH up to maybe 100nH.

The problem is that different manufacturers make the steps at different
resistance values, so resistors of the same DC value may have very
different values of parasitic inductance. If parasitic inductance is
critical, circuits that worked perfectly well with one make of resistor
may have problems with a different make (as Elecraft recently
discovered).

1/8-W SMT resistors are so cheap that you could just use
series-parallel combinations of them to get what you need, too. For
more serious power dissipation, Caddock (and some others) have power
film resistors in TO-220-like packages which are low enough inductance
to be quite useful up into VHF and maybe above.

Yes, easily. The only problem with the TO220 packages is the shunt
capacitance to the grounded tab - and the power dissipation is very low
if the tab is not grounded.



--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek