Details of the construction and needed materials:
Fine, dry abrasive sponges for cleaning of copper surfaces before and after finishing. Clean inner and outer surfaces of the pipe and the copperclad with dry, clean, abrasive sponges, before and after working on it. If possible, work with cloves to prevent swet coming on the copper surfaces. Espessially the copper surface of the copperclad oxidises very rapidly. After building the cavity, the still remaining oxide and sulphite on the internal surfaces, will mostly be removed by the use of CRC 2-26. But still, avoid unneccasery polution of the surfaces.
Spray-can 300mL CRC 2-26 electro (or 3-36 or 5-56, CRC industries Europe - B-9240 Zele - Belgium). This is very nessasery to prevent the filter from aging.
300mL spray-can blank paint for spraying outer surfaces right after finishing each individual cavity, and the top-plate after final adjustment.
Tin solder (Good electronic quality! You need a lot).
5 mtrs of PTFE coax abt. 3mm dia for internal wiring. Remove the outer insulation, as the cable will be soldered directly to the surface of the cavity-walls and topplate. This ensures optimum grounding, and shortest connections possible. No plugs are used for internal wiring. Resulting in cheeper, easier and more relyable construction. It makes very accurate effective cablelenghts possible, for better performance.
A little blowtorch for soldering and helping to heat-up copperpipe etc.
A good 50W solder iron with adjustable temperature and a short, wide tip plus a long, medium wide tip.
Two pieces of wood abt. 75x75mm and 50mm long for making two calipers (supporting the central conductor and cavity-walls during positioning and soldering). See "caliper.gif". Use this caliper for keeping the correct dimensions and positions of the cavity walls and central conductor. This ensures easy construction, and strain-free hanging of the pipe, when the completed filter is in upright position.
Silvered or tinned (earth-) copperwire of 2.5mm dia for making the coupling-links "C" (see "top.gif") and notch-L (see "ln.gif").
Copperclad: Cut all pieces of copperclad as exactly as possible at right angles, preferably on a so called "guillotine cissor". Try to find one at a metal-sheet-workshop, printed circuit-board workshop or at a technical school. Plan the order of cutting! Cut coppercladpieces with equal width or lenght directly after each other, with the same adjustment of the cutting-dimension. This to be sure, that "same" dimensions are really the same for different pieces. It prevents a lot of trouble and unnessacery work afterwards.
Topplate "A" HAS to be sturdy, as the mechanical and electrical stability of the construction FULLY depends on it. Use doublesided copperclad abt. 2.5mm thickness for topplate "A".
See "top.gif". If only thin copperclad is avalable, i suggest to glue 2 pieces =single= sided, 1.5mm thick copperclad together. Make the non-copper surfaces raw with abrasive sponge or sandpaper. Then glue with f.i. 10 seconds glue or two component glue while pressing with a weight. The copper surfaces must face outwards.
Double sided copperclad abt. 1.5mm thick for all other plates.See "platen.gif".
Beware! The INNER surface of the side-lid "M" may NOT make contact with any internal part of the cavity! So file the INNER-edges at 45 degr. angle, to create insulation. The inner surface of the sidelid is, by means of soldered-through rivets or wire-pieces, only connected to the outer surface of the lid (see "cavlit.gif").
Do NOT use to much solder when soldering the outside of the lids into the cavity. This to prevent intermittend short-circuiting (cracking noise).
The toplid "L" may not touch the sidewalls either. It only is connected to "mass" through the metal standoffs.
Copper waterpipe abt. 22mm dia. for the central conductor "B", plunger "E" and the copper connectingstrips for Cn. See "ond1.gif", "top.gif", "mkeplun.gif" and "cn.gif".
Be sure to have clean (new) pipe. Cut the ends of the pipes with a pipe-cutter.
By cutting the central conductor with a pipe-cutter, the edge of the pipe is automatically somewhat sharp-edged towards the centre of the pipe. See "DETAIL" in "top.gif".
+=+=+ This sharp edge at the lower end of the central conductor is very NEEDED , in order to get a good contact-pressure between the edge of the central conductor and the plunger +=+=+.
This prevents bad contact and electical instability of the filter.
Saw the slits in the central conductor with a metal-saw. Best use (if possible) a thick saw, so the slits will become wider than 1 mm.
Remove the rim at the top-end of the central conductor completely. It must be possible, to insert the plunger downwards at the top of the central conductor.
Saw the 10mm wide connecting-strips for Cn from a piece of 22mm copper pipe, and flatten them. Clean the surfaces. Do not use 2.5 mm wire for them. Wide copperstrips have lower impedance and result in deeper notches! The difference can be 6dB per cavity.
The plunger "E":
is made from the same 22mm copperpipe as the central conductor (see "mkeplung.gif"). Saw/file the bottom-end of the plunger at 30 degr; and roundoff the outer edge (see "plunjer.gif"). This makes it easier to get the plunger into the central conductor. The tuning-rod will be soldered at the top-end of the plunger. Saw and file the slot in the plunger abt. 8mm wide.
Finishing the plunger:
STEP1: (see "mkeplun.gif") Flatten the plunger a bit by pressing. Then turn 90 degr. and (STEP2) press again. The plunger will fit with some force into the central conductor, and gets a rounded (a bit triangle-like) shape. It will touch the inner surface of the central conductor only at 3 straigth lines, beeing mechanically stable in position and making good electrical contact. This ensures stable operation.
If nessacery widen or close the slot until is is possible to insert the plunger in the central conductor with some force. When moving the plunger up or down in the central conductor, you MUST have to use some force to let it slide! Lever up the rod with a long-nose plier. Tap the rod down. The ends of the fingers at the end of the central conductor are making good contact to the plunger becouse of the use of the pipe-cutting tool, resulting in a sharp edge on the inside-end of the contactfingers on the central conductor.
Steel or brass rod abt. 6mm dia. The coures-tuning rod "F" has to be soldered with a blowtorch or a big soldering-iron into the top of the plunger "E" (see "plunjer.gif"). The total length of plunger+rod should be abt. 53 cm. See to it, that after thet plunger is inserted into the central conductor, the rod does not touch the inner surface of the central conductor, or Ln or Cn. It must be possible, to access the top of the rod for tuning.
Thermal glue pistol + glue
for fixing the course-tuning rod.
After final adjustment of the filter, fix the position of the tuningrod with a thermal glue pistol (see "cn.gif"). Pre-heath the rod and copper pipe with a soldering-iron, and immediately apply the glue. The glue will melt to the metal surfaces, and make good mechanical contact. This ensures a secure, strain-free locking of the end of the tuning-stem after final adjustment of the filter (see "cn.gif").
After heating-up the tuning-rod, the course-tuning can easely be re-adjusted.
(Stainless-) steel bolts M6 + nuts.
Brass rod abt. 20mm dia. for the finetuning bushing "D". Ask if your supplier can saw the pieces at the specified length, and at right angles. Drill a hole of 5mm. Tap M6 in it (see "bus.gif"). Mount the M6 stainless steel bolt with a nut in it. The bushing will be soldered later on the outside of the side-lid of each cavity.
A small amount of slow hardening two-component glue. The "cold" side of the tuning-link is soldered to "mass". After final adjustment of in- and output coupling of each cavity, the "hot" side of the coupling-links is fixed to the topplate "A" with 2 component glue.
Three N-type male receptacles. Use best quality! They must have PTFE insulaton, to allow soldering the flanges and nuts to the copperclad filtersurfaces.
Metal stand-offs 45mm long (4 per cavity) with M3 holes in the ends. The top-lids "L" may NOT make contact with the side-walls, only with the stand-offs! File all edges of toplids "L" at 45 degr. Solder the standoffs and the screws after assembling, at the topplate "A" to "mass".