BUILD A DF #1: THE BROMSTEAD BOX

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This page outlines how to build a simple DF tool for use in body shielding techniques.   More advanced builders may want to consider building a Handi-Finder or similar project.   These designs are more similar to the Little L-Per in theory of operation and are still relatively easy to build.   Try One DF Building Site and/or this one.   If you want to get even more advanced in building projects, consider building a Doppler DF.   There has been much discussion on the CAP ES Listserver on the use of Doppler Direction Finding equipment for ELT search.   While they will probably not always work for ELTs, they may--on occaision--give some advantages.   If you're possibly interested in building such a device, see these sites.   An original doppler design by Bob Simmons and/or Dopplers by Greany, the Roanoke Doppler.

The principal of the Bromstead box is a radio that is shielded inside of a metal box except via the antenna.   The milli-ammeter measures the strength of the signal coming into the box.   The radio used for the Bromstead Box is a Jetstream Radio from Radio Shack.   The meter needs to be foil wrapped on the bottom side to keep it from being a 'leak' in the enclosure.

The idea for this device was pioneered by Major Leonard Bromstead of Illinois Wing.   "Lenny" a retired individual who made CAP communications his retirement.   When he passed on into the ionosphere above, he was well into his 80s.   That was several years ago, only about 3 months after 73 magazine did a spread on CAP communications.   While Major Bromstead may be gone, his project lives on.

Except for the tuning and volume holes (unavoidable), the case is closed.   The antenna, affixed to the PL259, gets its strongest signal at right angles to source, and two nulls, one when pointing at source, one when pointed 180 degrees away.   Since your body is in the way when you are pointed away, the weakest signal/deepest null are when your back is to the target.   To reduce sensitivity, collapse the antenna one section at a time.  

The Bromstead Box is a decent tool for the "ELT at the airport" problem.   Its sensitivity doesn't give it the range of the venerable L'Per, but the price is right.  

Editor's Note: To forewarn everyone reading this page, I have not personally constructed a Bromstead Box (yet).   The instructions relate mainly to the older version of the Jetstream radio--the one that used the 9-Volt battery.   I intend to construct this device, update the instructions, include updated diagrams, and display photographs of the construction.   I will also include part numbers and sources for everything you'll need.   And now, without further delay, here is a copy of the late Major Leonard Bromstead's instructions for constructing his DF box...

 

ELT LOCATOR

In order to speed up the locating of ELTs that are inadvertently set off on the ground, I put together the instrument described below.   With it I am able to locate the transmitter within a matter of minutes.   This also reduces the annoyance of all parties concerned with a nondistress beacon.

LIST OF PARTS   (Cost: $50 or less)
 

QTY       DESCRIPTION
1 VHF Aircraft radio (JETSTREAM Radio Shack Part # 12-608A)
1 4 x 5 inch aluminum chassis with back
1 0 to 1 milli-ammeter in metal case (2 1/2 or 3 inch diameter) (if meter case is plastic, insulate it and wrap in aluminum foil)
1 Diode
1 Microswitch (normal position open)
1 Antenna (from radio)
1 PL-259 plug (must be insulated)
1 Chassis antenna connector
1 Earphone jack

OPERATION

Close the microswitch with your left thumb and extend the antenna.   Tune for the ELT.   Holding the antenna horizontally, sweep a circle and note the dip on the meter.   Walk in the direction of the dip until you can receive the signal without using the microswitch.   Start reducing the length of the antenna.   When it is all the way in, you are within 25 feet of the ELT.   Remove the antenna and walk around the suspected aircraft.   The needle will peg at maximum about one foot from the ELT antenna.

ASSEMBLY

 

  1.   Drill all holes and cut slot in the end of the case for tuning control.
  2.   Glue aluminum foil in all corners to keep unwanted signal out of the box.
  3.   Glue the microswitch in the upper left hand corner with the button extending through the hole.   Use a drop of red fingernail polish to color the button red if needed.
  4.   Fit a 2 inch piece of thin paneling in the bottom of the case and cut a slot to hold the radio circuit board.   Do the same on top and allow room for the tuning wheel.   Glue it in place.
  5.   Remove the radio from its original plastic case.   Extend the output leads so they will reach the speaker, phone jack, and the meter.
  6.   Remove the cellophane from the antenna and pull it out through the hole in the top of the case.   Save it for later.
  7.   Lay the radio speaker on the corner of a piece of floor tile 2 1/4 by 2 1/4 inches.   Trace around the speaker, then cut out that section.   Use the tile to hold the speaker in its place adjacent to the speaker holes.   Glue the tile and the speaker in place.
  8.   Remove the 9 Volt battery and extend leads for mounting it on the meter side of the case.
  9.   Mount the chassis connector for the antenna on top of the case and centered.   Place the phone jack in front.
  10.   Mount the radio in the slots you constructed.   Ensure that the tuning wheel sticks out through the slot in the end of the case.   Wedge in another piece of paneling from the top to the bottom of the case.   You should leave slits for the batter leads, speaker wires, and the antenna wire.   Glue the panel in place.
  11.   Solder a stiff piece of #20 wire from the antenna to the microswitch.   Run another piece of the same wire from the switch, ensuring that it is approximately 1/2 inch away from the first wire and parallel to it.   Solder the second wire to the antenna lead on the circuit board.   This gives a direct coupling when the switch is closed, and capacity coupling when it is open.   In essence, depressing the microswitch temporarily increases the sensitivity of the device.   If too much signal comes through, wrap aluminum foil around part of the lead going to the radio, thus reducing coupling.   Do not remove the speaker from the circuit or you will have no audio to hear the signal.
  12.   Mount the meter and run the output leads from the radio to the speaker, then to the phone jack, and finally to the meter.   If the meter is backed with plastic instead of metal, it will need to be shielded (covered) with aluminum foil.   Make sure that you insulate the meter from the foil so the meter is not shorted.   Put a small diode between the output lead and the left meter terminal.   If the meter indicates downward, reverse the diode.
  13.   Glue another knob of the same size to the volume control and extend it through the back of the case.   I just cut off an old television knob.
  14.   Mount the antenna in the PL-259 connector.   Ensure that it is insulated from the ground sleeve.
  15.   Tuck a spare battery inside the unit.   Pack the remainder of the open space with foam/sponge rubber to keep everything secure.

This locator will work equally well in an aircraft by using earphones and hold the unit on edge with the meter at the top.

 

This page of the CAP Emergency Services Resources website was last updated 01/06/2009

1998 - 2006 Scott E. Lanis.  All Rights Reserved.