[FoxHunt] DF on 27 MHz

[Dale Hunt, WB6BYU]

KD7JYK wrote:
> 
> What is the most accurate way to DF a 27 MHz signal?  Size of an array isn't
> an issue so long as it is accurate.  I've been trying to locate three
> signals since last november and with nearly 120dB of attenuation and a 2'
> whip you'd think I am sitting on top of the source, but I can't see it
> despite it being strong enough to hear 15 miles away and being in a low
> density rural area!
> 
> Kurt



    First, what sort of attenuator are you using?  A standard
  switched resistor attenuator is rarely useful beyond about 60dB
  or so - there is too much pickup through the case of the receiver.
  What happens if you take off the antenna entirely?  If the signal
  is full scale with no antenna connected, then adding attenuation
  in the antenna lead isn't going to help at all.  The answer in
  that case is an "active" or "offset" attenuator, which is capable
  of over 100dB of usable attenuation.  If it is a CB'er running
  high power you may want to use a 20 or 24 MHz oscillator so the
  receiver is tuned far from the transmitter frequency.  (Double
  shielded coax and/or ferrite beads on the coax cable will help
  to reduce pickup from that source.)

     For an antenna, about the only practical antenna for close-in
  sniffing is a loop.  They aren't perfect, particularly if the 
  signal is horizontally polarized, but you can make one about 
  6" to 12" in diameter with a tuning capacitor.  There may be
  plans for one in the Moell/Curlee book.

     You might also try a TDOA if the signal is a continuous
  carrier and you have an FM receiver, but getting short antennas
  carefully balanced takes some work.  (You could use a pair of
  dipoles mounted on a tower, but it is a bit difficult to
  build a 18' tall rotatable antenna for mobile or hand-held use.)

     I suspect that an offset attenuator and a loop will make
  a huge improvement over your current equipment.  Let me know
  if you need help designing a loop.

      Good luck!

         - Dale   WB6BYU