PCARS DIRECTION FINDER II
Written by Don Winn AF4Z
Radio Direction finding units have been used to find transmitters for many years. PCARS originally had this club project back in the early 80’s. The club had monthly Bunny Hunts (Fox Hunts) for several years.
The basic unit is unchanged. The PC board was redesigned and the mechanics of the antenna has been greatly improved.
There are units what are more complicated and will provide a more precise bearing of the carrier but they cost a lot more and require more time to setup and operate. This unit will provide very accurate direction point and will depend on how well the user understands the unit as to the results that will be obtained. It would be advisable to spend several hours with the unit and get a understanding of how it works before going out for some serious Dfing (ie: Fox or Bunny Hunting).
Let’s get started!
********** WARNING **********
The Antenna Array can be dangerous. Eye or other body parts can be injured.
DO NOT use this unit without the protector beads installed on the antenna ends.
This unit is designed to be used in the receive mode only. Damage could accrue if you were to transmit through the unit. Set your transmitter to low power mode when using the unit.
To assemble the PCARS Direction Finder II, you will need the following tools:
#2 Phillips Screwdriver
Needle Nose Pliers
Soldering Iron (25-40 watt)
Cable Harness Assemble
· Locate the long piece of RG-58 coax cable.
· Using the RG-58, cut two one half wavelength pieces of coax to a length of 28 inchs. You should now have three pieces of cable.
· Remove 1/2 inch of the outer black jacket on one end of each of the 28 inch pieces of coax. Be careful not to cut through the coax center insulator.
· Trim the shield wires flush with the outer jacket to expose 1/2 inch of the insulated center wire.
· Remove an additional 1/4 inch of outer jacket to expose 1/4 inch of braid
· Remove 3/8 inch of the center conductor insulation.
· Refer to Figure 1.
· Place a 220 ohm 1/2 watt resistor lead around the 1/4 inch exposed shield as shown in Figure 2. Make sure the resistor lead is not cut too short to keep the resistor from being parallel to the cable. Do not solder or connect the other end at this time.
· Connect the anode of a 1N914 diode to the center conductor of one of the two 28 inch cable end by putting a hook in each lead. Keep the leads short. Solder the connection...
· Bend the cathode lead of the diode over to the 220 ohm resistor and wrap it with one turn. Do not solder
· Locate the Black and White #16 gage wires supplied with the kit and cut each into two equal pieces. (White will be 4 inch, Black 5 inch)
· Remove 1/2 inch of insulation from each end and crimp or solder a Solder Lug to one end of the four wires.
· Using a black 5 inch wire, loop the free end of the wire around the shield of the coax with the lead of the 220 ohm resistor.
· Solder the wire and resistor lead to the shield. DO NOT over heat the connection.
· Connect a 4 inch white wire where the resistor and diode leads are joined. Place the wires as shown in Figure1 and solder the connection.
· Inspect the connections and insure they are all soldered correctly.
· Slide a 5/16 inch clear heat shrink over the components and apply heat to the area to shrink the tubing.
· Repeat the same steps above for the other 28 inch coax end except connect the cathode of the diode to the center conductor of the coax instead of the anode.
· Remove 3/4 inch from the free end of the two 28 inch pieces of coax using a small tool such as a small screw driver, un-braid the wires back to the black jacket on one end of each piece of coax.
· Remove 3/8 inch of the center insulation on each of the three pieces of cable where the braid was straightened.
· Place the two 28 inch length of coax as shown in Figure 2 and twist the center conductor wires together.
· Connect the center conductor of the third piece by twisting the center conductor around the center conductors of the two 28 inches length of coax. Solder the three center conductors together.
· Using electrical tape, cover the solder joint to protect it and build the tape up until it has several thickness of tape over the solder joint. The tape is used to isolate the center conductors from the shield.
· Position the braid from all the cables so they can be soldered together.
· Solder the shields being sure not to melt the tape. Soldering for a good electrical connection is all that is necessary. The braid does not have to completely cover the taped joint.
· Slide the 1 1/2 inch piece of black heat shrink over the soldered braid.
· Apply heat to shrink the tubing over the(connection. The final connection should look like Figure 2D.
· Install the BNC male connector on the free end of the long piece of RG-58 coax cable.
· Use an Ohm Meter to verify the harness for continuity. Insure the center conductors are not shorted to the shield.
· Locate the four antenna elements.
· Using a hammer, flatten the 1/8 to 3/16 inch The flattened area will prevent the bead from coming off the element wire..
· Using a match or lighter, heat the flatten end.
· Push the element into the bead half way and rotate 90 degrees.
· Repeat the step above until all four beads are installed. 2 red, 2 green.
· Make sure the beads do not come off the wire. If necessary, epoxy them in place. Do not use the antenna if the beads are not installed. Injury can occur.
· Insert the two 28 inch lengths of coax lines into the center of the 1/2 inch PVC TEE so that the coax with the BNC is coming out the center of the TEE and one of the 28 inch lines go out each side of TEE.
· Cut two 9 inch pieces of 1/2 inch PVC pipe.
· Install one of the 9 inch pieces of pipe on each side of the Tee (DO NOT GLUE) by pushing the 28 inch coax and diode circuits through the pipe.
· Install the other 9 inch pipe in the Tee in a similar fashion.
· Position the antenna mountings boards (brown) into the notch in the 1/2 inch PVC couplers. Install a #8 5/8 inch stainless flat head screw. (See Figure 3)
· Push the solder lugs wires through the end coupler and connect the lugs to the antenna mount board with #10 screws, Flat Washers and Wing Nuts. Do this on both sides. Be sure to install the white wires up and the black wires down and Red Beads on one side and green beads on the other side.
· Push the couplers on the end of the 9 inch pieces of 1/2 inch PVC.
· Install the antenna elements on their mounts using the flat washer and wing nuts. Position the end couplers so the antenna elements and 9 inch pieces of PVC form the letter “H”.
· Insert the BNC connector through the 42 inch piece of pipe and insert the pipe into the TEE.
· Using a ohm meter, verify that the lower two elements (black wire) show continuity to each other and to the shield of the BNC connector case or shield.
· It is not necessary to glue the PVC, this will make it easier to disassemble it for storage. You may wish to glue the 9 inch pieces to the antenna couplers but leave the Tee connections alone.
This completes the antenna construction.
· Install the switch and BNC connector on the front panel.
· Solder a 1 1/2 inch solid wire to each of the BNC solder lugs .
· Install the BNC with the wires toward the bottom of the box. Bend the wires up and cut them off 1/4 inch longer than the BNC connectors. These wires will be soldered to pads on the PC Board later. Be sure the BNC nuts are tight but leave the switch nut only hand tight.
· Mount the 3.5MM phone jacks so the wide contact toward the bottom and the solder connects are at the 12 and 3 o’clock position. (See Figure 4)
· Connect a 2 inch #22 gage black wire from the right (gnd) terminal of the Audio In to the right (gnd) terminal of the Audio Out. Solder the connection at the Audio Oot connector only.
· Connect a 2 inch green wire between the left Audio In and the left Audio Out Jack. Solder the connection at the Audio Out connector side only.
· Install the Meter into the front panel using the double sided tape.
· Install the 9 volt battery clip at the lower botton edge of the box using the double sided tape.
Circuit Board Assembly
· The Circuit Board assembly is easy and straight forward.
· Install the IC Sockets first as shown. Make sure the notch is at the Pin 1 side.
· Install variable resistor VR1 on the bottom side of the board.. All other parts go on the top of the board.
· Install several parts, check to insure the correct values were installed and then solder. Be sure not to cover the pads where the battery, meter, switch and RF connectors will solder later.
· Solder the battery wires as shown in Figure 4.
· Set the board in place so the power switch pins and the BNC pins are ready to be soldered.
· Solder the three switch terminals. DO NOT solder the BNC center conductors or ground wires.
· Remove the switch nut and remove the board .
· Cut a 3 inch black and green wire and remove 1/4 inch insulation from both ends.
· Cut a 4 inch red and yellow solid wire. Remove 1/4 inch insulation from both ends.
· On the circuit board, connect one end of the 3 inch solid green wire at Audio + pad and solder.
· Connect one end of a 3 inch solid black wire at Audio - terminal and solder.
· Connect one end of a 3 inch solid red wire at the Meter + pad and solder.
· Connect one end of a 3 inch solid yellow wire at the Meter - pad and solder.
· Install the IC’s in their sockets.
· Preset VR1 to the center of it’s adjustment range
· Connect a 9 volt Battery.
· Toggle the power switch to “ON”.
· Using a Volt/ohm meter, verify that 9 volts DC is present at IC1 pin 1 and IC2 pin 16. Note the meter reading.
· Measure IC1 pin 3 and verify the voltage is 3 to 6 volts VDC.
· Verify the voltage at the collector of Q1 is 3 to 6 volts VDC.
· Verify the voltage at IC 2 pin 5 is 3 to 6 volts VDC. Turn the power off.
· Mount the pc board on the front panel and verify the BNC connector pins and the BNC ground wires are through their holes.
· Verify the audio green and black wires are coming out the bottom between the BNC connectors.
· Tighten the switch hardware. DO NOT over tighten.
· Connect and solder the solid green wire from Audio + to the terminal with the green wire on the 3.5mm Audio In connector.
· Connect and solder the black wire from 3.5mm Audio - to the terminal with the black wire on the 3.5mm Audio In connector.
· Solder the BNC and ground pins. The board will have a tilt in the box due to the difference in the heights of the BNC and switch pins. If you wish, you can solder pins in the BNC connectord to make the board mount flat.
· Solder the solid red wire to the Meter’s + terminal.
· Solder the solid Yellow wire to the meter’s - terminal.
· Install a 3.5mm plug on each end of a 3 feet piece of RG-174 cable.
· Install the 9 Volt Battery and bottom cover.
If an oscilloscope is not available, set VR1 to a point where the voltage of the collector of Q1 is .15 volts higher than one half the battery voltage. This will be very close and can be verified later. If you have an oscilloscope, measure the waveform at the Antenna In BNC and adjust VR1 for a 50% duty cycle.
If the Battery voltage is 9.2 volts, set VR1 to a point where the DC voltage of Q1 collector is 4.75VDC
(9.2VDC/2 = 4.6volts +0.15 = 4.75VDC.)
You will need a friend to help with the checkout.
· Using a VHF receiver like a 2 meter HT, tune it to simplex frequency.
· Connect the Antenna Direction Finder II to the Antenna In terminal.
· Connect the 3 ft BNC cable between the Antenna Out and the Antenna connector of the HT.
· Turn the Direction Finder and HT power on.
· Set the audio level to a normal listening level. Leave the 3.5MM connector off the HT so you can hear the audio.
· With your helper standing 15 to 20 feet away, have him transmit a carrier on your receive frequency while you rotate the antenna. Listen for a 1000 to 2000 Hz Tone to get very weak. this should be at a point then the antenna elements are equal distances from the transmit antenna.
· Plug the audio connector into the HT and repeat the test but instead of hearing the tone null, watch the meter for a “Zero” indication when the elements are the same distance from the transmit antenna.
Note: You will need to determine the correct phase of the radio you are using so that you will be able to tell the direction of the signal. The phase can be different for different radios so you must remember to check this if you change radios.
· Face your friend with the antenna in front of you.
· Have your friend emit a low power carrier and rotate the antenna for a zero reading.
· Once centered, move the antenna to the right. If the meter needle moves to the left, you have the correct phase and you should make sure the right side elements have Red Beads. If not, change them. If the meter deflects to the right, turn the antenna 180 degrees and repeat the test. It should now deflect correctly.
With the PCARS Direction Finder II set up correctly, you will be able to determine the true direction from which a carrier is coming. Remember to insure the red beads are on the right. (RED ON RIGHT)
Always turn the antenna in the direction the meter is pointing and you will know that when the meter reads zero (centered), the transmitter is in front of you. If the meter goes the same direction, it’s behind you . Turn around and repeat the test.
An earphone or headphones can be inserted in the Audio Out jack on the unit so you can monitor the tone while the meter will continue to function.
That’s all there is to it. It’s now up to you to learn how to use it and understand what the meter and audio is telling you.
One last thing worth noting, This unit works on a Doppler principle and depends on good quality signals. At times you may get reading that seem off if you know where the signal is coming from. Move over a few feet and check again. If the reading changes, there is a good chance you are seeing multipath. This means that you are receiving the signal from more than one path. It can be from reflection off buildings, a plane or some other metallic object.
Even the best units can have trouble with multipath. It does happen.
Thanks to George K4PX, Jim KC4HW , Dan WA4RGK, Kiper KE4ZMZ and a special thanks to John Houser KE4MMI for helping with this Club Project
U1 LM-555CN Timer
U2 CD4066 CMOS Switch
D1,2 1N914 Diode
Q1. PN2222 Transistor
L1 0.47uH Inductor
R8,9. 220 ohm 1/2 watt 5% resistor
M1. Meter 250uA -0- 250uA
R3. 2.2K ohm 1/4 watt 5% Resistor
R4,5. 10K ohm 1/4 watt 5% Resistor
R2 22K ohm 1/4 watt 5% Resistor
R1. 47K ohm 1/4 watt 5% Resistor
VR1. 10K ohm Variable Resistor (VR1)
C6. 100pF Capacitor
C1,2 0.01 uF Capacitor
C5. 100uF/16V Capacitor
C3,4. 4.7uF Capacitor, NP.
2 EA. 3.5MM Phone Jack
2 EA. 3.5MM Phone Plugs
8 IN. Phone Wire (Red, Yel, Grn, Blk)
1 EA. BNC male connector (RG-58)
3 FT RG-174 coax
11 FT. 12 ft. RG-58A cable
2 EA. BNC jack, chassis
1 EA. BNC to BNC 3 Ft. cable
1 EA. 8 Pin IC socket
1 EA. 14 Pin IC socket
1 EA. Switch SPST
1EA. 10” black #16 wire
1EA. 8” white #16 wire
1 EA. 1/2”X1/2”x1/2” PVC Tee
1 EA. 60” PVC 1/2” Schd 40 pipe
2 EA. Antenna mounting board
2 EA. 1/2” couplers (modified)
4 EA. SS antenna element
2 EA. #8 3/4” SS screw
4 EA. #10 flat washer
4 EA. Plastic bead (2RED, 2 GRN)
1 EA. Project box
1 EA. 9 Volt battery holder
1 EA. 11/2” X 1/2” Heatshrink
2 EA. 1 1/2” X 5/16” Heatshrink