Updated GRB septum measurements and construction details
Since the only square tubing I could find has an inside width / height of 4.5" (114 mm), I made some adjustments to the septum. The actual septum measurements are in the diagram below.
I ordered the square tube, septum plate, and back plate from
onlinemetals.com A list of what I ordered is below. For a small fee they will cut their metal to the requested size, rounded to the nearest 1/4". When I received my order, I was pleased to see that the 4.5" x 11.5" plate fit perfectly inside the square tube.
I used a hand held circular saw with a Oshlun SBNF-070054 blade to cut the septum to the correct shape. After cutting I used a metal file to remove the burrs. The septum was mounted inside the square tube using
MG Chemicals Silver Epoxy Adhesive, which is electrically conductive (and very expensive!).
The probes are made of 1/4" aluminum rod, which has a bevel I made by using a drill and file. Before beveling I drilled a small hole in the center of each rod for the copper wire, which was removed from some RG6 cable. After the beveling was done, I used "Aluminum Solder Wire 96.5 Sn/3.5 AG .062 Flux Core" to solder the copper wire into the holes I drilled in the aluminum rods.
For probe mounts I used female 'F' connectors. I drilled holes in the square tubing about 34.6 mm from the back which were slightly larger than the connectors. Drilling through the square tube was not easy! Its an extremely strong alloy, probably intended to be a structural support beam. Two nuts on each 'F' connector hold them in place.
Finally I inserted the probes into the 'F' connectors and taped the back plate onto the septum. So after all this, you may be asking why I used tape for the back plate? Because I wanted to be able to easily access the probes in case I needed to adjust them.
The final septum weighs in at 12.5 pounds! I wish I could have used a thinner / lighter square tube, but it was the only tube I could find.
Now, how to mount the big heavy thing to my dish struts? I made a 5 inch (inside dimension) square out of 2x4 wood. To get the mount to fit tightly I nailed the 2x4's together using the metal tube as a 'form'. Then I drilled holes in the corners of the wood square and bolted it to the struts using big wood screws. The septum slides into place, and the distance from the center of the dish is adjustable. My best results were obtained with the dish focal point slightly inside the mouth of the tube.
I built this last December. Initially I thought it didn't work very well. But recently I decided to give it another try. I attached a new Nooelec SawBird+ directly to the 'F' mounts using an 'F' to SMA adapter. Its powered using a home made bias tee / DC blocker that I made by gutting the insides of a TV splitter box. A 47 picofarad capacitor serves to block any DC from the DVB card on one side, and the other side has 5 volts from the computer's USB port to power the Sawbird+.
This septum performs very well for GRB. It's performance is on par or slightly better than the RF HamDesigns septum, and provides a strong enough signal to pickup GOES-EAST (75W) from California at 9.7 to 10.4 dB. (It varies).
I have no idea if this design is optimal. I may try variations on the probe length. Comments and questions are welcome!