One of the problems, if you want to use the attenuator to get close to
a strong transmitter, is that typical ham receivers aren't shielded all
that well, and you'll find that the attenuator is unable to give you as
much signal attenuation as you'd like. (Disconnect the antenna from the
receiver entirely, and the signal's still strong!) A way around that
is to build a mixer in a well shielded box, mixing the signal with some
fairly low frequency to offset it from the transmitted frequency. You
listen on the mixer output freq. For example, if you mix with 10MHz,
you'd listen to 455MHz at 465 (or 445). Then you have a knob to
control the efficiency of the mixer in some way, and you get an output
that can be attenuated smoothly. We used to use such things here, and
they seemed to work well for the small effort involved in putting them
together. I don't think I still have a schematic around, though.

If you build a step attenuator, be aware that at 500MHz, it's tough to
keep signals from leaking around your attenuator stages. Be sure to
keep leads extremely short. Use switches with low inductance...tiny
DPDT slide switches can be pretty good. And don't try to do more than
about 20dB in any one stage. Unless you have an exceptional receiver,
you'll probably find that you can't use more than about 80dB total
attenuation effectively, if that much.

Another useful tool if you're looking for a nearby well-hidden
transmitter is a field strength meter. It's possible to build one with
logarithmic response, so you can see 60dB or more range easily on a
meter scale. Some of the Analog Devices, or Linear Technology, or
National Semiconductor dB-linear RF detectors should work well. You
can detect inputs below a millivolt with them. Add a tuned circuit on
the RF input to keep other signals out. If you build the FSM so it's
well shielded and you can swap between say a quarter-wave whip and a
tiny stub for antenna, you can cover quite a range of field strengths.

Too bad you're not closer...I'd give you my old doppler RDF. But you
have some very good RDF teams in Australia, I know, and maybe some of
them could help you out with ideas and construction help if you need
it.

Cheers,
Tom