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How to Make a RF Sniffer

sniffer_thSometimes noise generated from within the motor compartment of a tractor or auto can be very difficult to trace down. One of the best ways is with the use of a RF Sniffer.  A simple one can be made easily and quickly.

Here are the steps to construct the sniffer:

  1. Use an 18' length of either RG/58 or RG/8x stranded center coax. The 18' length should provide enough length to allow reaching all areas needed.
  2. On one end, solder a PL-259 connector.
  3. On the other end trim back 12" of the outer rubber covering and the shield. This will leave 12" of foam exposed.
  4. Trim off 10"of the foam, leaving the center strand exposed.
  5. Twist the strands of the wire together to form a tighter wire.
  6. Using a broom handle or similar sized round tube, wrap the exposed wire around the handle forming two loops.
  7. Tightly twist the excess wire back around the beginning of the wire, to prevent loops from undoing.
  8. Remove the broom handle from the windings.
  9. Spread apart the loops so that they are not touching each other.
  10. Construction of RF Sniffer is now completed.


When completed, it will look similar to this picture:


A more enhanced version that will eliminate most interference or radiation of coax line can be constructed by adding a 5 or 10 watt, 50 ohm resistor in line and soldering the other end to the coax shield in this picture:


Here are the steps to actually use the sniffer:

  1. Connect the end of the coax with the PL-259 connector to the radio.
  2. Turn radio on, and the volume up enough to be able to hear the noise when you are outside of vehicle.
  3. Start and run vehicle engine.
  4. Move the loop end of the coax around the motor compartment, over motors, etc., being careful not to touch any thing. CAUTION: Be careful around fan motors to avoid fan blades.
  5. When the noise level increases, the item that the loop is over is the part causing the noise to be generated.
  6. This part may be needed to be filtered, grounded, or shielded ( depending on the part ) to cure the noise problem.
  7. Repeat process to locate all the sources of noise.
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