Ground Sphere Mk1 Prototype Assembly Instructions¶

• Ground Sphere Mk1 Prototype Assembly Instructions
  • Introduction
  • The Antenna Base
  • The Ground Plane
  • The Antenna Body
  • RF Line
  • Phasing Loop
  • Preamp Output Line
  • Preamp Power Connector
  • Assembly
  • Fabricating and Installing the Aerials
  • Installation and Testing
  • Notes, Changes, and Errata

Introduction¶

The GroundSphere Mk1 prototype was designed to be a proof of concept device, and as such things which would normally be locked in place by design are instead designed to be moved. Building this device is not recommended unless the maker wishes to use it for parametric measurement and testing. Please refer to the end of this instruction sheet in the “Notes, Changes, and Errata” section for the lessons learned and the changes made in the Mk2 design.

The Antenna Base¶

The first object built was the antenna base. It is made from plumbing components available at any large hardware store. A 1 ½” pipe flange was screwed onto a 3” section of 1 ½” galvanized pipe. The diameter and length of the pipe was chosen to allow the preamp to fit inside the base to electrically shield the preamp and give the antenna a weighted base for stability.

Screwed onto the length of galvanized pipe is a female 1 ½ pipe thread to 1 ½” PVC adapter, then a 1 ½” to 1” bushing is inserted and secured in place with standard Schedule-40 PVC pipe glue. This allows the antenna body to press fit into the base, allowing easy assembly, disassembly, and tuning.

The Ground Plane¶

Using the dimensions found here, round pieces of conductive fabric and cotton muslin, 7” in diameter were cut out with a 1” hole in the center of the disks. These were quilted together using conductive thread. The quilting threads all end at one arbitrary location at the center opening of the ground plane so that this may be connected to the ground line. Attach a small gauge ring terminal to these threads with about 2” of slack between the top of the ground plane and the crimp.

On the muslin side, eight small pockets were sewn in which extend from the center of the ground plane disk directly to the outer edge at 45 degree intervals like spokes on a wheel. These ground plane strut sleeves are to hold the ground plane supports. The ground plane supports are 8 sections of ⅛” steel rod 3” long and bent at a 90 degree angle 2” from one end. The long side is fed into the eight ground plane strut sleeves on the bottom of the ground plane.

The Antenna Body¶

Cut 6” section of 1” PVC electrical conduit and deburr it so that it fits fully and snugly into the 1 ½” to 1” bushing. Temporarily place the ground plane on the antenna body so the support struts are flush against the 1 ½” to 1” bushing.

Take one Thomas & Betts L35-B2 ground lugs and position it so that the hole is toward the ground plane with the long portion in line with the antenna body. Adjust the distance from the top (screw side) of the ground lug to the top of the ground plane so that it matches the D plane figure in the spreadsheet above. Mark the position of the ground lug’s hole on the antenna body.

Draw a line around antenna body at the position of the ground lug hole we marked earlier. Along this line mark three additional positions 90 degrees apart starting at the ground lug hole. Drill a 3/16 inch hole in each position either with a drill press or carefully by hand. Drill another 3/16 hole ½” below one of the mounting holes you just drilled. This will be for the RF line (coax) to exit the antenna body and connect to the ground lugs.

Attach the ground lugs in all four positions with #10 nut and bolt hand tight. We will be loosening them again to make the electrical connections, so this is just to keep the bits together. Place the cap on top of the antenna body as well, but do not secure it with pipe cement or any other adhesive. You still need access to the inside of the antenna body.

RF Line¶

Cut an arbitrary length of coax cable long enough to go all the way around the antenna body, down the hole drilled for it (widen if necessary), and out the bottom of the antenna body with 3” to spare. Carefully strip back 2” of the outer cladding of the coaxial cable and gently pull it free without damaging the braided shield below. Gently push the braided shield back along the coax cable toward the insulation. This will gather and expand the braid like a chinese finger trap; allow it to expand.

Gently widen a gap in the side of the braid near the insulation and fish the center conductor out this hole with a fine screwdriver or dental pick, taking care not to unbraid the braided shield or breaking wires unnecessarily. Inevitably one or two may break, but this is okay. Gently pull the braid from the end in order to collapse it back down... help it a bit by smoothing it with your fingers. Once thin and smooth, you may wish to place the section of coax insulation you previously removed over the exposed coax braid. Without the center conductor in the braid, the insulation will slide over the braid easily protecting and insulating the ground from inadvertent contact.

Strip back about ¼” from the end of the center conductor and crimp a ring terminal on it. Use the smallest wire rating for this ring terminal that will accept #10 hardware (red jacket). You may wish to put a small loop in the end of the exposed wire before inserting it into the ring terminal in order to give the crimp more to hold on to. Crimp a second ring terminal to the ground side. For this you will need a larger lug (yellow jacket).

Slide the ground plane over the bottom of the antenna body with the conductive fabric side up and the struts on the bottom. Thread the unstripped side of the coax through the lower hole in the antenna body from the outside to the inside, leaving ½” or so and the stripped and crimped ends hanging out the hole. Mount the 1 ½ to 1” bushing on the antenna body and the 1 ½” pipe to 1 ½” PVC adapter to the bushing. You can use PVC cement to secure these parts if you wish.

From the coax hanging out the bottom of the 1 ½” pipe to 1 ½” PVC adapter, strip ½” of insulation from the coax and unbraid the shield. Gather the shield on either side of the center conductor and twist to form two ground leads. Strip the center conductor back about ¼ to 3/16”, ensuring that the center conductor has enough insulation to prevent the ground wires from making contact with some margin. Solder the center conductor to the input side of the preamp and the ground shields to the ground shield on either side of the input line on the preamp’s PC board.

Phasing Loop¶

Cut a section of 90ohm cable to a little over 2 ¾” in length. The math comes to 2.77456 or about 2 199/256”, but don’t sweat the accuracy... just measure 2 ¾” precisely and add the width of an Xacto knife blade. The more accurately you can make this cut, the higher gain the antenna will have at the desired frequency.

Strip the cable ¾” back on both ends and push the insulation back and fish the center conductor out as you did before on the RF line. Strip back about ¼” from the end of the center conductor on each end and crimp ring terminals on the ends. Use the smallest wire rating for this ring terminal that will accept #10 hardware (red jacket). You may wish to put a small loop in the end of the exposed wire before inserting it into the ring terminal in order to give the crimp more to hold on to. Crimp larger ring terminals (yellow jacket) to the ground braids.

Preamp Output Line¶

Take the cable terminated with the MCX connector and on the unterminated side strip ½” of insulation from the coax and unbraid the shield. Gather the shield on either side of the center conductor and twist to form two ground leads. Strip the center conductor back about ¼ to 3/16”, ensuring that the center conductor has enough insulation to prevent the ground wires from making contact with some margin. Solder the center conductor to the output side of the preamp and the ground shields to the ground shield on either side of the output line on the preamp’s PC board.

Preamp Power Connector¶

Using a pre-wired barrel connector, solder the red lead to the input side of the preamp’s voltage regulator. This can be best accomplished by soldering it to the pad labeled L5 on the side closest to IC1, the voltage regulator. Solder the black lead to the ground shield at a convenient point around the outside of the preamp PCB.

Should you not be using a pre-wired barrel connector, ensure the center conductor of the connector connects to the wire connected to the positive input of the voltage regulator (the pad of L5 as indicated above), and the outer conductor goes to the ground shield of the preamp PCB.

This connection scheme will allow any standard power supply with a center positive barrel connector to be used, as the regulator can handle any voltage from 6V to 18VDC. In a pinch, the power supply can be connected directly to the preamp PCB as indicated in the preceding paragraphs. The expected current draw of the preamp is around 35mA, so a good value for a fuse would be 1/4A, though most “wall wart” power supplies rated at 100mA will fail before then, so there is little point in putting a fuse inline, but plenty of reasons to double check your work.

Assembly¶

Remove the hardware connecting the four Thomas and Betts ground lugs to the antenna body. As you look at the antenna body with the top up and the hole where the wire exits facing you, arrange the RF line’s coax so that the ground shield goes to the right and the center conductor to the left. These are somewhat arbitrary, but it is necessary to know where things are in order to connect the phasing loop correctly.

Connect the ground of the RF line, the ground of the ground plane (the conductive thread pigtail), and the ground of one of the ends of the phasing loop together by stacking the ring terminals on the #10 screw. Ensure the RF line connector angles toward the RF line hole, the ground plane ground goes straight down, and the phasing loop’s angles away from the RF line’s hole.

Place the ground lug on the screw last with the screw terminal at the top, then insert the screw into the hole in the antenna body on the right side as viewed with the RF line hole facing you. Tighten the screw securely with a nut on the inside of the antenna body making sure the ground lug remains in line with the axis of the antenna body and the ring terminals do not rotate causing strain on the wire crimp connections.

Do the same with the center conductor on both the RF line and the phasing loop (the center conductor from the same side as the ground used in the previous paragraphs) bolting them and another ground lug to the hole on the left when viewed with the RF line hole facing you. What this should look like is the RF line straddles the antenna body on one side, while the phasing loop straddles the antenna body on the other.

Take the phasing loop and gently turn it to the right. Heating it a little with a heat gun or in the sunshine may help it the phasing loop proves to be stiff. Connect the ground wire on the phasing loop to the lug directly above the RF line hole and the center conductor to the remaining hole 180 degrees opposite of the RF line’s exit hole. Tighten as before, making sure the ground lug remains in line with the axis of the antenna body and the ring terminals do not rotate causing strain on the wire crimp connections.

Fabricating and Installing the Aerials¶

In the spreadsheet mentioned at the beginning of this build, the aerials are defined as loops 13.71 inches long. Cut two pieces of romex house wiring, at least one insulated, to a length of 14 ¼”. Strip off ¼” of insulation on both ends. Position a pair of pliers to grip on the bare wire directly against the insulation, and bend the end 90 degrees. Do the same for the other end, taking care to bent them in the same direction. To be clear: If the wire is placed on the ground with one stripped end pointing up, the other would be pointing upward also. Repeat this process with the second aerial.

Next, take the aerial between thumb and forefinger clamping with a bit of pressure and pull it through with your other hand in order to curve the wire, As you do so, ensure that the stripped ends end up on the outside of the curve. To be clear: when the wire comes full circle, so that the ends touched, the stripped ends point away from the center of the circle. Repeat this process with the second aerial taking care with both to form a smooth arc.

Partially unscrew the set screw from the ground lugs on the side of the antenna body (not the ground lug mounting screws). Insert the loops into opposite ground lugs and tighten, taking care not to torque the lugs off true. A large amount of torque is not required. If the lugs were numbered 1 thru 4, one loop would connect 1 and 3, while the other would connect 2 and 4. Once tightened, arrange the loops to be circular and when viewed from the top, cross at the center of the antenna body and at right angles.

Installation and Testing¶

At this point, secure the ground plane struts with a zip tie or two making sure the ends are in contact with the bushing under them and are in line with the antenna body. Arrange the struts to make the ground plane as flat as possible.

Thread the preamp output line and preamp through the steel pipe and pipe flange base. Without torquing the preamp, thread the base on the antenna body. At this point you may wish to cover the preamp with electrical tape or heat shrink tubing to insulate it, but any part of the preamp PCB that can touch the sides of the pipe should be a ground anyway. Gently push the preamp assembly into the base of the pipe. You should probably place some rubber or felt feet on the base of the antenna assembly (the pipe flange) to keep from scratching paint or furniture.

Plug the amplifier into a 12VDC source. Current draw should measure about 35mA +/-5mA. If not, inspect the unit for damage or poor solder connections. Connect the MCX connector to the SDR dongle and tune the SDR software to 900 MHz. Using a cordless phone or other 900 MHz source verify that the antenna is receiving and amplifying signals. Further testing of the preamp is out of the scope of the average hobbyist, but you can play with the length of the aerials, phasing loop, and distance between the aerials and the ground plane by simple adjustments and replacement of parts to tune the antenna and align your GroundSphere satellite ground station to peak performance.

Notes, Changes, and Errata¶

During and after the assembly of the Mk1 GroundSphere prototype, many lessons were learned which translated to a lot of design changes.

1. We discovered that the steel sleeve over the preamp and the base does little to keep the noise out, particularly when it is not grounded itself. Instead it makes a conduit for static discharge to the ground of the preamp which could damage the preamp.
2. The 1 ½” and 1” size of the antenna body and base are excessive for the 915MHz version. It would be better to mount the preamp horizontally in a base and reduce the size of the antenna body to ¾” or even ½”.
3. As the ground plane is quilted, it pillows the fabric. This reduces the radius of the ground plane and circumference by a set amount per stitch, but since the circumference is about 6 times the length of the radius, the shrinkage is 6 times greater in the radius than in the circumference. This causes the ground plane to become floppy even when stretched taught by the struts. A better solution is to secure the conductive fabric with a minimum number of stitches.
4. The struts do little to keep the ground plane flat, primarily because of the issue noted above, and that the struts cover very little of the ground plane’s surface. A better solution is to use a plastic mesh to support the ground plane over the whole surface.
5. If a plastic mesh is used to support the ground plane, it could be affixed using a minimum of stitches and use an adhesive to secure the conductive fabric to the surface of the mesh, further, the mesh could be silk screened with a logo or a decal applied with little to no performance loss. We should experiment with this concept.
6. The RF line and phasing loop mounted to the outside was convenient for testing, but made the Mk1 prototype more fragile than it should have been. Bringing the RF line up from the bottom and the phasing loop in from the top on the inside would be much more rugged. The only outside connection should be the pigtail from the ground plane. To fit in a ¾” or ½” antenna body, the phasing loop would have to be tightly coiled and covered in heat shrink to keep it’s shape during assembly and parts storage.
7. Use pre-terminated lines for the RF line (preamp side) and the preamp output rather than soldering them to the preamp.
8. Build the preamp with SMA connectors for the input and output side. These can ve machine soldered, while the coax cannot.
9. The preamp can be powered with a power inserter, but this would add cost to the completed ground station and may induce noise. Experimentation would help drive decision making.
10. A jig can be made to precisely cut and strip the phasing loop once the ideal measurement is identified.
11. With smaller diameter components the assembly can be cleaner, cheaper, and have better repeatability.
12. With a plastic mesh backed ground plane, the completed ground plane can be sandwiched between two plumbing components and secured there with PVC cement