Start with a visit to K0OV's web site:www.HomingIn.com
His book on transmitter hunting contains plans for
building both switched and sliding-tube attenuators.
One big problem with using standard attenuators for
fox hunting is that there is an effective maximum of
about 60dB or so, regardless of how well the unit is
built. At this point, the signal leaking in through
the case of the radio will overpower anything coming
in through the antenna jack. When the S-meter is
pinned without any antenna, no amount of attenuation
in the antenna cable will give you a usable signal.
The currently popular solution to this is called the
"Offset Attenuator" (or "Active Attenuator".) It
consists of an oscillator and a mixer: the receiver
is tune to the receiver frequency plus or minus the
oscillator frequency, so it has some buffering from the
direct transmitted signal. The response of the system
is reduced with a pot controlling how much of the
oscillator drive is applied to the mixer.
Not only can these work VERY WELL, they are relatively
simple and inexpensive to build, often with junkbox
parts. And you can reasonable performance even without
any shielding. There are many such circuits available
on the net - and most are pretty similar. K0OV has one
on his site, others have been posted by WB2HOL and
WB8WFK (from memory). The local ARES group sells some
boards that I designed 10 years ago, and at least one
ham sells ready-made units.
(We may have linked some circuits from the equipment
page of www.ardf-r2.com?
Most such circuits use a single diode in the mixer.
I use a double balanced mixer, and seem to have far
fewer problems with image mixing of strong local signals
than the single diode circuits. But not everyone has
such parts in their junkbox, or is confortable building
their own using coils of wire wound around their finger.
If you follow the links and still have questions, feel
free to email me and I'll do my best to help out.
Happy Hunting! - Dale WB6BYU (at arrl.net)