When you do, you'll probably end up with a series resistor from each source (DC and RF) to the
top of the attenuation control. Careful adjustment of the values of these resistors will
optimize the attenuation range.
First, in DC (or "through") mode, you have to know the forward voltage drop and maximum
current capability of your diode. For a silicon switching diode this is about 0.6V, and the
forward current may be 25 to 75mA. For some Schottky diodes it can be 0.3V and 10mA.
Adjust the resistor so that, with the control set for minimum attenuation, the voltage is
sufficient to turn on the diode and limit the current to a safe range - 5mA perhaps. If
the current is too low the loss at minimum attenuation will be too high, while too high
of a current will destroy the diode, or at least run down the battery faster.
For RF, the series resistor limits the amount of RF applied to the control. Having the
"through" range available means that the minimum attenuation required in the "offset"
mode is less. Ideally you would have about 10dB of overlap between the two attenuation
ranges. By limiting the top end of the range you gain more adjustment at the low end:
there is no sense having to turn the knob down half way before the circuit starts
attenuating. Experiment with this resistor to find a good balance - I generally start
with a resistor that is larger than necessary and solder others in parallel with it to get
the right value.
If the hidden transmitter is too strong and you STILL can't reduce the signal far enough,
add a switch across the input to the attenuation control that shorts the RF to ground
(though the series resistor). The switch leads usually have enough inductance that
there is still some small amount of RF applied, which allows the attenuation to be
increased even further: with one of my attenuators at maximum attenuation I'm only
applying a few millivolts of RF to the mixer. (Using a balanced mixer helps a lot at that
point, too, as it reduces the fundamental RF input so it is doesn't overload the receiver
as easily.)