BUILD A DF #1: THE BROMSTEAD BOX
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...
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)
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.
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
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