The pa0nhc 2mtrs repeater duplexfilter.
General info, construction details, needed materials.
In literature, i found at first little information about: Wich mechanical and electrical constructiondetials, have what influence, on the performance and stability of cavityfilters.
Older ARRL-publications gave incomplete and confusing information.
At last, i have got a copy of a pencil-handwritten article from G8AMG. This article described systematically again some to be solved problems, when connecting a repeater-receiver and -transmitter to one antenna. It was enlighting. It also discribes some filter-constructions, and important mechanical and electrical construction-details. From him i learned the importance of a clean transmittersignal.
Some important points:
KISS = Keep It Stupid Simple!
Mechanics: Solid copper is very expensive. This material alone can cost hundreds of guilders. It is difficult to get it in shape and soldered/welded together. The same is valid for aluminium. It is only a bit cheaper.
HF is only running at the surface (skin) of the metal. So why not use more easy to shape , cheaper glass-fiber copperclad?
Make the filter as stiff as possible. This enhances mechanical long-term stability. In this construction all eigth cavities are mechanically one-piece. No separate frame is then needed.
Especially the "top"plate, with the central conductor an coupling-links in it, should be sturdy. This plate must be abt. 3mm thick.
How important is the couplingfactor of the coupling-links and how to adjust them? I did a lot of experimenting. Correct amount of coupling prooved to be crucial for good filter-performance of this type of bandpass- and notch-filter. This was mentioned nowere in the literature i read. Also not how and when to determan and adjust it.
Adjustable coupling is difficult to construct and adjust. Turnable plates, secure in position when locking them, secure in contact, are difficult to make. They only need te be adjustment ONCE, during the construction of each individual cavity. So why bothering with elaborate constructions?
My couplings do not rotate. I change the effective suface (length), by pulling them out of or pushing them in to the cavity. Then locking them by soldering the "cold" end directly to the cavity. Secure contact and position is garanteed.
How long can cavities be for good performance? Shortened cavities with capacitive top-loading have often lower Q and stability. So use full 1/4 wave or 1/2 wave cavities, with low top-capacitance. There must be enough room between the bottom-side of the central conductor and the cavity-bottom. This ensures low temperature-coefficient of the cavity.
What size must a cavity have? In the article of G8AMG was stated, that when the cavity is seen as a coaxial cable, the characteristic impedance of that "cable" must be 78 ohms for optimum Q of the cavity.
Wider cavities have bigger central conductors. They are more stiff and have lower losses (they say). Current-density is lower. So for high transmitter-power-levels they may be better, as temperature-effects are smaller.
I wanted to use readely avalable materials. 22mm copper waterpipe can be bought everywhere, and is not to expensive.
Copperpipe of abt 80mm dia. for the outer cylinder is very expensive and difficult to obtain. So i make cavities square. With an inner conductor size of 22mm a square cavity must have a width of 74 mm. Not to big, so cheaper.
How needed is silver- or goldplating? Silverplating adds little to lower insertion loss of the cavities, i think. It merely acts as a anti-oxidant. So why expensive and difficult silverplating? Take measures to prevent oxidation of the copper-surfaces. Cheaper and effective.
Tuning-mechanism. Mostly there is some possibility of finetuning in cavities. This often complicates the construction of the tuning-mechanism.
I use course-tuning of a simple, but stable construction. After all, course-tuning is only done once, during construction of the filter. It then can be forgotten, if stable.
As it is difficult to install and adjust a filter, when (fine-)tuning is in the bottom of each cavity, capacitive finetuning is done here with a bolt on the side of the filter, near the top of the central conductor.
Cables, are they critical? The lengths of the interconnecting cables depends on the actual type of notch-cavities. In this design (ARRL) lengths must be totally an 1/4 electrical wavelength. They are not to critical.
The length of the cables, which connect the outputs of the Tx-filterhalf and the Rx-filterhalve to the antennacable, must be EXACTLY 1/2 electrical wave and 1/4 electrical wave. This is critical. The performance of the completed filter depends on it.
So why use plugs for connecting cavities to the antenna and eachother? They cost money, construction-effort, and couse possebely bad contacts when oxidating.
I soldered 50 ohms PTFE cable directly and shortly at the required points. It is easy to accurately determen the effective cable length. Braid can be soldered directly to mass, without melting the cable. But remember that PTFE has a different shortening factor than normally used cable. Double sheelded PTFE cable should be used.
What do these 1/4 and 1/2 wave output cables do exactly?
I concluded the following:
At the transmitter-notch-frequency, the L-notch-cavities in the receiver-chain, have a LOW impedance with a high VSWR to 50 ohms.
The 1/4 wave line between the last L-notch-cavity and the antennaconnector, transforms this to a very high impedance.
This 1/4 wave cable in combination with the cavity-properties, acts like an open switch for the transmittersignal in the direction of the receiver. Thereby disconnecting the receiver at the transmitter-carrier-frequency from the antenna-connector.
At the receiver-notch-frequency, the C-notch-cavities in the transmitter-chain, have a HIGH impedance with a high VSWR to 50 ohms.
The 1/2 wave line between the last C-notch-cavity and the antenna-connector, does not transform this.
This 1/2 wave cable in combination with the cavity-properties, act again like a open switch for the receiversignal in the direction of the transmitter. Thereby disconnecting the transmitter at the receiver-frequency from the antenna-connector.
Received signals only can go to the receiver. Transmitternoise at the receiver-frequency is isolated from the antennaconnector.
Interconnecting cables between cavities have to be exactly 1/4 electrical wave long, including plugs and connecting wire-pieces. They transform the very low notch-frequency-impedance of the L-notch-cavities to very high.
Again this cables act like open switches at the notch-frequency. The same applies for the C-notch-cavities. but form high- to low impedance.
As the VSWR of the cavities at the pass-frequencies is low, the 50 ohms cables have no influence then. They then just act as a interconnection.
Recepticals. Use only plugs with PTFE insulation. At first i used cheap BNC. But after some time one made trouble.
N-connectors are more sturdy, water-tight and can withstand some strain. So use N-type.
Selectivity. Notch-cavities have little selectivity beside the pass- and notchfrequencies. Some types give no more then 6 dB. So adding a extra bandpass (notchless-) cavity, is very usefull to suppress wideband transmitter noise, and strong signals from transmitters in the neighbourhood. Preventing interference and desensibilisation of the receiver.
Transmitter-sideband-noise can be mixed with transmitter-harmonics, generating a noise-signal on the repeater-input-frequency.
This can happen in the transmitter, circulator, bad contacts in the antenna, bad contacts in surrounding metal structures, and in the receiver.
So make the transmittersignal clean. Make the receiver-oscillators noisefree. Best use well designed cristal-oscillators.
Use the right amound of drive in every stage of the transmitter and receiver.
Make receiver-selectivity good.
Add transmitter- and receiver-selectivity by adding filterselectivity.
Is a circulator needed? Only use a circulator as a compensation for a duplexfilter with insufficient suppression of the transmitter-carrier.
When used, first separately adjust the circulator for BEST VSWR at all ports on the repeater-frequency. Then the port-separation it also optimal.
The antenna-VSWR must also be PERFECT. All reflected transmitter-power returning from the antenna is directed by the circulator to the receiver!
Using a circulator with 30dB port-isolation, and an antenna with a reflection damping of only 10 dB, results in only abt 9 dB isolation between TX- and RX-antenna connectors, and not 30 dB.
Remember: circulators can make intermod! So do not use a circulator. Make your duplexfilter good.
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 double screened 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.
IMPORTANT! 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 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 +=+=+.
It 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 male receptacles N-type. Use best quality! They must have PTFE insulaton, to allow soldering 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".