Coaxial Cavity Filter for
Modularized Spectrum Analyzer

This Page was Started Dec. 14, 2004 Go B
Updated Dec. 28, 2012 

     This page will act as a design guide for building a Coaxial Cavity Filter for the Modularized Spectrum Analyzer.  This 1st I.F. filter is composed of four individual coaxial cavities, cascaded together using "hairpin" coupling.  Traditionally, coaxial cavity filters are built using rectangular shields with aperture coupling.  This design is a change from the "norm" and makes for a very simple construction.
    A second filter, on this page, is a two cavity version.  It is used as a tuning filter following a Step Recovery Diode Multiplier Module.  The dimensions are identical for both filters, the difference being the number of cavities.  Assembly is the same for both.  Both will tune from about 900 MHz to 1050 MHz with a bandwidth of approximately 2 MHz for the 4 cavity filter and about 4 Mhz for the 2 cavity filter.

    The main purpose of this filter is to attenuate the second LO (LO 2 = 1 st I.F. + Final Xtal Filter) and the image frequency, which is 2 times the final xtal frequency below the commanded center frequency (image = 1 st I.F. + 2x Final Xtal Filter).  Expect an insertion loss of 5 to 8 dB for the 4 cavity filter and 2 to 4 dB for the 2 cavity filter.  Most of the pictures were taken when the 2 cavity filter was constructed.
    A secondary goal of the filter is to attenuate the response at the pass band of 10.7 MHz., the frequency of the Final I.F.  Best attenuation here is wanted to decrease intermodulation products from MSA input signals that are 10.7 MHz apart. A good "rule of thumb" is about -50 dBc.
    For more good information on the construction of a single Coaxial Cavity Filter, I suggest visiting this link:  A good build and description by Mike Suhar, W8RKO.

Coaxial Cavity Dimensions (inches):
 1013 MHz Cavity Filter, 2 MHz BW
    I admit that construction of this filter looks daunting, but it is really easy to build.  I used RG-141 hard pipe for input and output, and its center conductor for interstage coupling with teflon dielectric for spacers.  It was "fairly" close to 50 ohms and had a loss of about 7 dB.  Silver plating would improve insertion loss, but probably not more than 1 dB.  However, loss is not a concern here.  The MSA is not designed for low loss as a goal.
    I bought a 24"  length of 1 inch copper tubing at Home Depot and used their pipe cutter to make 7 pieces, each 3.1 inches long.  You will notice the above dimensions show that the pipe is not exactly 1 inch diameter, even though, that is what it is called.  The inside diameter is really not that critical.  The only critical lengths are the center resonators.  These coaxial resonators have a characteristic impedance of 85 ohms.
    Use quarter inch copper tubing for the resonators and cut each to 3 inches in length.  A portion of the stub will protrude through the bottom plate, which makes soldering easier.  Any excess can be cut off when the filter is completed.
    Use .062 brass for the bottom and top plates and thread the top plate for the 4-40 tuning screws.  Thicker material would be sturdier, but not necessary.  Drill the bottom plate as per the above dimensions.  A drill press would be nice tool to have.  Wish I had one.
Each cavity wall is drilled to accept RG-141 hard pipe.  A 9/64 drill bit is perfect.
parts for cavity filter  center and end plates  dualdrilled
Cut and polished                 Drilled                                 Pieces fit
These photos were taken during the building of a dual cavity filter.  The 4 cavity filter is cut and drilled the same way.
    Once the holes are drilled, clean and polish the inside and outside of the pipes as best you can. I used emory cloth wrapped around a piece of foam rubber, stuck on a long drill bit to polish the inside of the pipe.  The cavities are now ready to be soldered together.
    Clamp the 4 pipes side by side, with a wooden dowel (or a very long drill bit) through all 8 coupling holes for alignment.  The following picture shows the clamping for the two cavity filter.  For 4 cavities, lay them flat and use a larger clamp.  Use a propane torch and solder the pipes together, top to bottom using regular 60/40 rosin core solder. Let cool and remove dowel (or what's left of it).
dualdrillbitdual soldered cavities
Clamped and ready to solder. Cavities soldered and sitting on bottom plate

    Make the input and output probes. I used SMA connectors on RG-141, but you can connect the input and output of the filter directly to the modules if you wish. Either way, use RG-141 as the drawing shows. Strip off the outer conductor and dielectric leaving about two inches of raw center conductor. Bend the center conductor 90 degrees as shown, wedge the RG-141 into the input and output holes.
    Use the center conductor and teflon dielectric from some RG-141 for the hairpin couplers.  Take about a foot of RG-141 and remove the internal center conductor and dielectric. Cut the center conductor into 3 ea, 4 inch pieces. Cut 19 spacers from the dielectric, each one about 3/32 inches long. Install the spacers and install the probes into the cavity as shown. Leave extra length on the probes to stick out the bottom plate when installed.  I suggest you stagger the extra length on all the probes for easier insertion through the bottom plate holes.  Install the bottom plate and cut off the excess probe wires.  Bend the leads flush to the bottom plate.
make probes  coupling probes  dual bottom 2
Input and output probes         Interstage installed            Bottom plate positioned

    Now, we are ready to solder the bottom plate to the cavities.  Reclamp the cavities together so they won't fall apart during soldering.  Push the 1/4 inch stubs into the bottom plate leaving a total length of 2.72 inches above the bottom plate (for tuning to 1013.3 Mhz).  This length is not critical but try to make each stub the same length and make sure they are shorter than the natural resonant length of 2.81 inches (1013.3 MHz).  Make sure the stubs are centered in each cavity.  If, after soldering, the stubs are not centered, simply insert a drill bit into the stub and use it to bend the stubs to a centered position.
    I suggest using a vice to hold the bottom plate parallel to ground, leaving enough working space below the assembly to sweat solder the stubs and probes to the bottom plate.
    Use a propane torch and start by soldering the bottom plate to the cavities. Next, solder the probe wires and stubs to the bottom plate.  Try to keep the torch from directly hitting the input/output connectors. There will be more than enough indirect heat for the RG-141 to sweat solder onto the cavities.  Let cool and inspect for a good sweated joint between each cavity pipe and the bottom plate.  The extra stub length below the bottom plate can be cut off or left intact.  It makes no electrical difference.
 base soldered
Bottom plate soldering complete

    I left the top plate for last.  Verify the cavity resonators (stubs) are centered in each cavity.  We are now ready to solder the top plate to the filter assembly.  Clamp the top plate to the filter assembly and solder it to the tops of each cavity.  Without a top plate and tuning screws, this filter will resonate at approximately 1060 MHz. A top plate without tuning screws will lower the resonant frequency less than 5 MHz.
This set of pictures are the completed 2 bank cavity filter:
dual finished 1  dual finished 2  dual finished 3
   Bottom                            Side and bottom                Side and top
    Install the 4-40 tuning screws using a lock washer and nut. As a starting point before tuning, allow the tuning screws to extend into the cavity about 0.25 inches.  Blob some RTV or some other "pucky" over the stub holes on the bottom plate. This will help prevent the internals from tarnishing and corroding, as these are the only holes exposing the cavities to outside air. You could even stuff a small package of silica gell "absorbant" into the stubs before "pucking".

    The following pictures are from W4ZCB.  His 4 bank filter is a derivation of my specified filter.  Notice that he is using solid resonators rather than hollow 1/4 inch pipe.  He threaded the rods and bottom plate.  He does not need tuning screws for the top plate.  Tuning is accomplished by lengthening or shortening the resonators from the bottom.  The nominal length of each stub resonator will be approximately 2.81 inches at 1013.3 MHz.  He has also silver plated the assembly.  How's that for class!  The final assembly (right) is shown without the top cover plate installed.

Tuning the Filter
    If you are building this filter for the MSA, you may tune the filter during the initial Set Up and Calibration of the MSA.  No special test equipment is required. If not, you will need an appropriate frequency source and detector for tuning. A VNA is a logical choice.
    Alignment results of Coaxial Cavity Filters are mainly dependent on the physical dimensions of the hairpin couplers. 
You will notice, in the drawing, that a teflon spacer separates the vertical portion of the hairpin from the wall of the cavity. You would assume the distance from the hairpin to the wall is fixed.  However, the length of hairpin conductor between the spacers will allow the it to be bent a minor amount. If it is bent closer to the wall, the Filter will exhibit higher insertion loss, with narrower bandwidth. If it is bent farther from the wall, the Filter will exhibit lower insertion loss, with wider bandwidth.  Of course, the top plate cannot be soldered in place if you "tweek" the hairpins. You can temporarily affix the top plate by using rubber bands to hold it in place during alignment.
The following is a representation of the minimum and maximum characteristics of a typical 4 bank, coaxial cavity filter:
    One Filter I built has a 3 dB bandwidth of 2.2 MHz, with an insertion loss of 7.1 dB. The rejection at 1034.7 MHz is -112 dBc.

    The following sweep is that of the Coaxial Cavity Filter built by Jim McLucas for the Verification MSA.
The Magnitude trace shows the response of the Cavity Filter when installed in its final configuration in the MSA. The frequencies displayed are offset below the actual frequencies of the Cavity Filter. That is, the center of 0M corresponds to 1013.3 MHz, the -20M corresponds to 993.3 MHz, and 20M is 1033.3 MHz. The ultimate rejection is much better than the graph indication of -88 dBc. The real ultimate rejection cannot be seen due to the noise floor of the MSA (-110 dBm) masking it. The true response of the filter is -108 dBc. The attenuation at 1034.3 MHz is not shown but would be a bit higher than the 20M point. Obviously, it would be as good or below what is indicated at 20M.  The graph also shows that the response at the 10.7 MHz bandwidth (near the -100 scale line) is about -78 dBc. This is 28 dB better than the "rule of thumb" design goal of -50 dBc.