[FoxHunt] Attenuator

[Kuon & Dale Hunt]

>What are the thoughts on a good attenuator...

  Clete -

      That Radio Shack attenuator probably won't help much.
  You will generally need more attenuation than it will provide.

  I find 60dB of attenuation is about the limit I can use with
  most receivers before the signal picked up through the case
  exceeds that from the antenna connector.  After that point,
  any additional attenuation won't help.  60dB may get you
  there if the transmitter is running low power and it isn't
  well hidden, but I've seen receivers wimp out at a quarter
  mile from a 25 watt transmitter - the signal was full scale
  on the "S" meter even with the antenna disconnected.

  The solution is to use a "active attenuator" (or "offset attenuator".)
  This is an oscillator and a mixer circuit.  The oscillator is on
  some frequency in the MF or low HF range - say, 2 MHz.  The receiver
  is tuned 2 MHz off from the desired signal, and the level of the
  oscillator signal applied to the mixer is varied with a pot.
  Because the receiver is not tuned to the transmit frequency, it is
  much more immune to overload and pickup through the case.
  Attenuation levels of 100dB are possible - which is usually enough
  to sniff out a transmitter hidden in a clump of bushes

  The design on the HomingIN web site is a good start, and there are
  others around on the net.  Personally I like to use double balanced
  mixers in mine, but the single-diode ones are simpler and still
  should do the job for you.

  Good luck and happy hunting!

       - Dale WB6BYU