CONSTRUCTION OF CYLINDRICAL FILTERS
SAM WETTERLIN
8/15/05
OVERVIEW
The objective is to construct a cavity filter of 2 or more stages operating on the same principles, and with comparable performance, to the plumbing pipe filter in the original Scotty’s Spectrum Analyzer, but using materials and techniques which hopefully make assembly easier. These filters are made of cylinders with a center resonator rod grounded at one end and open at the other, and are tuned by brass screws at one end. The cylinders are 1” in diameter; smaller diameters could possibly work but make it difficult to install coupling loops. The filters I have built by this method have included other experimental alterations, such a using brass or shorter resonator rods, and operate at higher frequency than Scotty’s original, so they are not directly comparable to his. However, I have every reason to believe they will deliver very comparable performance.
This paper does not duplicate the discussion of cavity filter principals or resistance issues which are covered in the separate paper “Construction of Box Filters”.
CONSTRUCTION
The photos on the next page are used as a guide to the construction process. The major difference between this process and construction of a plumbing pipe filter is that lighter weight material is used, which allows for less intense soldering procedures and easier alignment of parts.
Step 1—Construct the cylinders.
Use 28 ga. copper or 8-mil half-hard Alloy 260 brass (“Cartridge Brass”), Copper makes a better filter, but brass is much easier to solder, so if you don’t need the ultimate in bandwidth, use brass. Cut one piece for each cylinder of width=cylinder height (3.1” for Scotty’s, 2.6” for a filter in the 1.1-1.15 GHz range), and with a length of 3.375” for copper, 3.5” for brass (brass requires more overlap when you roll the cylinder. Wrap the sheet metal around a dowel or pipe of approx. ¾” diameter; when you have rolled it completely roll back and forth on the end of the metal to be sure it gets rounded. Remove the dowel/pipe. The sheet metal will spring open a bit.
Cut a 3.5” piece off a 1” wooden dowel. Wrap a piece of paper around the dowel and tape it with Scotch tape. This will be the form for the final cylinder; the paper provides a little extra diameter so that a wooden dowel without the paper will slide easily into the finished cylinder. Tin the edges of the sheet metal which will form the seam. The part that will form the inside of the cylinder should be tinned only at the very edge (1/8”); the other should be tinned about 3/8”. For copper, you should wipe the edges to be tinned with a flux pen before tinning.
Wrap the metal around the dowel/paper form, line up one end and solder it. Then solder the other end. Be sure the metal is wrapped reasonably tightly. Then solder the center of the seam and continue about halfway to each end. Finally, start at each end and continue soldering the rest of the seam. The solder should melt and flow along the seam as you slide the soldering iron along. All soldering can be done with a soldering iron. Using a glove on one hand to hold the tube together helps a lot.
You now have a lightweight pipe.
Step 2—Drill holes for coupling.
You need to drill the holes which will allow coupling between cylinders. How big depends on the coupling technique. If you want to duplicate Scotty’s technique, drill holes big enough for the teflon spacer. For my technique a hole of approx. 1/8” is good. In either case, locate the hole 0.625” from the end of the tube, about 90 degrees around from the seam. This dimension is shorter than Scotty’s—our tubes are a true 1” I.D., his is a little bigger. You need a pair of holes for each tube-tube junction. For the input/output coupling, you need only a very small hole for a wire to stick through, 180 degrees around from the interstage coupling hole.
When drilling, insert the 1” dowel inside the tube for backing. If for any reason you smush a tube during construction, put the dowel back in to reshape the tube. When you are done with all soldering, the tube will be very strong.
Step 3—Solder the tubes together.
For a 2-stage filter, this is simple. For more stages, just solder one tube on at a time. One advantage of the lighter weight tubes is that you can solder on one side of a tube without melting existing solder on the other.
Clamp two tubes together with their coupling holes aligned. You can probably clamp them together by putting a large paper clip in each end at the junction of the tubes; I used a small spring clip in that same location. Obviously, you don’t want to clamp them heavily from the side, since you will squash the tubes. Make sure the coupling holes are aligned by sticking a short piece of coax or tubing through them. With the clamps in place, lie the tubes flat on the bench.
Solder one end and then the other of the tubes. A soldering iron should work nicely. At this point you can take the clamps off. Then solder the middle of the tubes; but don’t get carried away and melt the ends. With copper, you probably need a butane torch for this. Even for brass, you may ultimately need a torch to reach down into the valley between the tubes. But with brass you can first spread solder around where you will need it and then use quick blasts of the torch to melt it into place. With copper you will need to use a flux pen.
There is actually no need to solder the tubes together continuously. The most important portion is the two ends and the area around the coupling holes, which may have a tiny amount of leakage if the solder does not surround the holes. Solder them on one side and then flip the tubes over to get at the other.
Figure 1 shows tubes after soldering is complete.
Step 3—Construct the end pieces.
You will need two end pieces. Copper flashing material (approx. 0.015”) works great; brass of similar thickness will work. If you go heavier than 0.020 you will have trouble soldering. Cut each piece about ¼” more than the dimensions of the end of the filter (which for a 2-cylinder filter is 1” x 2”).
Stand the soldered cylinders on one of the end pieces. One end of the cylinders may be flatter than the other, so use the flatter end for the bottom. Mark a “B” on the bottom of one of the cylinders, and mark a line on the bottom of the cylinder and onto the end piece so you can later know which part goes with which. Also mark a “B” on the bottom side of the end piece to mark it as the bottom, and a “T” on the other piece.
Be sure the cylinders are approximately centered on the end piece. Use a pointed object to trace the cylinders onto the endpiece at one end. Remove the cylinders. Clip the two corners of the end piece about 1/8”-3/16” from the marking for the cylinders. You now want to make a tab which will fold up and hold the edge of the cylinder. See Figure 3. For each tab, make two cuts about 1/8” apart into the corner of the endpiece ending just short of the marked lines for the cylinder edge. Fold up the resulting tab a little more than 90 degrees, so it bends inward a tiny bit.
Now place the cylinders back in place, lined up with the tabs you just made, and mark the cylinder at the other end. Create two more tabs on that end. When you are done you should be able to place the cylinders between the tabs and the tabs will hold them in position. A little bit of play is no problem, but if you have too much just bend one or more tabs inward to take up the play. Be sure that there is a little bit of end piece exposed for soldering all around the cylinder. If not, bend some tabs.
Do the same thing for the top end piece.
Step 4—Drill holes in the endpieces.
This is the clever part. Getting properly centered holes is the hard part with the plumbing pipe filter, but is quite easy here. First you need to drill a center hole in the 1” dowel. To locate its center, use a strip of brass or other material that is approx. ½” wide—it need not be exact. Place the strip against the end of the dowel and draw a line. Rotate the dowel 120 degrees and draw another line, then draw a third. If the strip was exactly ½” wide, the three lines intersect at the center of the dowel. If not, there will be a small triangle on the dowel. It is easy to eyeball the center of the triangle, which will be the center of the dowel. You can somewhat see the lines on the dowel in Figure 1.
Now drill a hole in the center of the dowel, into which you will stick a finishing nail with its head cut off—so get a finishing nail and figure out what size hole you need for a snug fit, and how deep it should be so the finishing nail sticks out 1/8-1/4”. Insert the nail with its point sticking out, as in Figure 1.
Now the nail may not be in quite straight, and you may not have drilled exactly on center. But who cares? With the cylinders in place on the bottom piece, stick the nail end of the dowel into one cylinder. With a small amount of pressure, rotate the dowel in a circle. Do the same on the other cylinder. Remove the cylinders. The bottom piece should now have a circle scribed around the center of the holes to be drilled. If the nail was perfectly straight and perfectly on center, there will only be a tiny dot; the greater the errors the greater the diameter of the circles. But they will be small enough that it is easy to eyeball their centers. Mark the center with a punch. Drill a small pilot hole and then a 7/32” hole (which will later be reamed up to ¼”).
Do the same with the top end piece, being sure that you also use the top end of the cylinder. You can drill the same size holes in the top piece, though they only need to be 3/16”
In the bottom piece, you will also have to drill two holes for the coax input and output, which will enter the bottom and run up the side 0.625” and go out the side for soldering. This is the reverse of Scotty’s original, but the idea is the same. You should be able to locate the holes on the bottom piece by reference to the scribed marks for the cylinder’s edge (allow a tiny bit for the width of the walls). Drill holes sized for your coax. See the completed endpieces in Figure 1.
Step 5—Install the interstage coupling.
Refer to Figure 2. The interstage coupling loops are soldered in a notch at the bottom of the common cylinder wall, run up 0.062” from the wall to the hole you drilled previously, and come back down on the other side. This is the part I dislike, and I have no great way to do it. Figure 2 shows wire soldered to 0.062 PCB used as the spacer. I wasn’t really happy with this technique and suggest you follow Scotty’s use of teflon spacers, though it is not necessary to have a spacer in the coupling hole if you leave the insulation on the wire. My only suggestion is that you notch the end of the cylinders for the two leads to be hooked over, and then soldered. Be sure you mechanically hook the leads to the notch before soldering so they won’t come loose if the solder is melted when the bottom piece is soldered on. But if you prefer, you can let the leads stick out holes in the bottom per Scotty’s method.
Step 6—Solder on the bottom.
Tin the cylinders around both the top and bottom edges using a soldering iron. If you use brass, tinning is not necessary. With copper, be sure to prep with the flux pen. Place the cylinders in place on the bottom piece on a heat resistance surface. Be sure there are no bumps from the tinning that lift the tubes off the bottom. Solder the cylinders at several points to hold it in place. You may need to apply pressure to get the cylinders flush with the bottom. Excessive gaps allow solder to run to the inside. Then solder the cylinders all the way around. To solder the area around the junction of the cylinders, you may need a butane torch, because it is not easily accessible. You can get some solder in position with the soldering iron and then finish with the torch. As soon as the solder melts into place, remove the torch and allow the solder to cool somewhat; it is easy with the torch to melt a broad area of solder if you don’t pay attention.
Step 7—Install the resonator rods.
The resonator “rods” are made of ¼” tubing. The holes drilled in the bottom in step 4 were only 7/32. Use a reamer to gently expand the bottom holes so you have a snug fit for the rods. Cut the rods to a length which allows about ¼” to project out the end when they come to within 0.4 inches of the other end. Make two cuts in one end of each rod to create a tab which you can bend up. Bend it so that when you insert the rod through the bottom piece all the way to the tab, the rod will stick into the cavity the required length (going to 0.4” from the top end).
Tin the bottom piece on the outside around the area of each hole (including the coax holes). Use flux pen for copper.
Insert one of the rods in position. To hold the far end in place, you need a piece of tubing or wooden dowel 3/16” in diameter which will fit inside the rod and extend out through the hole in the top piece. The top piece will have to be taped into position for this. When you cut the rod, you may have compressed the cut end (as tubing cutters do), so you may have to ream/flare it with a file handle or similar object to open it up to 3/16”.
You should also tape the supporting tubing or dowel in place so it doesn’t fall out when you put the cylinders upside down, which is useful for soldering. Or you can solder the thing on its side.