Since there are lots of variations on the hunt, we are going to begin with just a basic hunt. Our area of operation is shown (approximately) in the MapQuest aerial map at the right. It will run from Sahuarita Road on the north, to Elephant Head Road on the south, to the mountains on the east, and the mines to the west. The transmitter will be hidden somewhere within this area.
Initial ground rules for the hunt are as follows:
1. Transmitter will not be hidden on private property.
2. Transmitter location will be accessible by 2-wheel drive vehicles.
3. Transmitter will be within 100 feet of the road.
4. Transmitter frequency: 147.5 simplex
5. We will meet at the SAV Building at 1400 on Sunday, 20 April for instructions.
6. Directions to a meeting spot following the hunt (i.e., where we can EAT!) will be placed by the transmitter.
7. "Clues," if necessary, will be given on the 145.29/.27 repeaters.
8. Time limit: TBD
Equipment Needed
To participate, you will need the following equipment:
1. At least one 2 meter VHF radio (144 - 148 MHz). You may want to keep one transceiver tuned to the hidden transmitter frequency and another tuned to the 145.29/.27 repeater.
2. Directional antenna (see Antenna Workshop below).
3. Attenuator to reduce the signal strength as you approach the transmitter location (see below).
4. Area map, preferably a topographical map..
5. Compass, protractor, pencil/marker.
6. Proper shoes and gloves for short walk in the desert.
7. Water.
8. Snacks.
Many Internet sites have information on transmitter hunts or radio direction finding in general. The site noted below is very informative and has antenna and attenuator designs that are both inexpensive and simple to construct. Using the "Tape Measure" antenna design, about a dozen GVARC members participated in building the antenna at a workshop conducted on 24 March at Gene Schouweiler's home. See the pictures below for the finished product...
Useful Web Site
Please visit Joe Leggio's outstanding Web site (link below) for information on a variety of Radio Direction Finding Projects.
http://home.att.net/~jleggio/projects/rdf/rdf.htm
The antenna and attenuator some of us are using is on the site.
Antenna Construction
Under "Antennas" click the link to Tape Measure 3 Element Yagi - RDF Optimized Beam.
We built a
variation of this antenna at the Antenna Workshop in 24 March. The main
change is that we used 3/4" Schedule 40 PVC instead of the 1/2" PVC shown in the
article. This change eliminated the need for the six 1 1/2" hose clamps
(at $1.49 each) specified in the design. Instead of the clamps, pieces of
1/2" PVC were ground down slightly and short sections cut. These sections,
seen in the photo below left, were used to wedge the tape measure elements into
the PVC crosses and tee. This modification also changed the way the driven
element hairpin match and coax are attached (below left), thus eliminating the
need to solder the connections. The finished product is shown below at
right.
Attenuator Construction
Under "Attenuators" on the Web site click the link to Active Attenuator.
An attenuator, connected between the RDF antenna and the HT, is needed to reduce the strength of the signal entering the HT as the hunter gets closer to the hidden transmitter. The active attenuator design available at the link above does the job better than just inserting a potentiometer between the two. According to the article, it can provide over 100 dB of attenuation as needed.
All of the parts can be purchased new at Mouser Electronics, www.mouser.com, and other places as well. The total cost was less than $40.00, including $12.88 for a 1 MHz crystal and $7.24 for a 5" x 4" x 3" aluminum enclosure. If you want to save a little money, a 2 MHz crystal is available at Mouser for $1.38. It should work in this design, but testing by GVARC has not yet been accomplished. Parts for the attenuator have been ordered. Construction and test results will be published here within the next couple of weeks.