Introduction/Scope of Project

In light of a new 900 MHz hobby-within-a-hobby, I needed a sharp band-pass RF filter.  The filter will be installed between an antenna on a radio tower and a small 900 MHz communications device.  The receiver in the device (shown below) would normally not be capable of operating properly in a high RF environment without such a filter.  The active device is a Heltec V3, a 150-milliwatt transceiver loaded with firmware such as Meshtastic or Meshcore.  It is one of many types of off-grid messaging devices that are quickly gaining popularity.  I will not elaborate further on the device or platform itself, as this is a hardware document concerning RF filtering.  Pulling up a search engine and looking for Meshtastic or Meshcore will be the easiest way to learn about the platforms.

The Celwave (RFS) 800 MHz duplexer

About 10 years ago, these were extremely plentiful in the surplus world, so much that many of them were simply scrapped for mixed metal.  They were primarily used for MDT (mobile data terminal) base stations, where the transmitter (usually a 100-watt Motorola Quantar) would continuously transmit data availability while the receiver listened to mobile field units.  The duplexer allowed the use of one antenna and transmission line.  Celwave (formerly Phelps Dodge), developed a genius one-box-fits-all solution without field techs needing to tune countless duplexers for every frequency imaginable.  Using sharp gang filters, pass bands of 18-MHz wide for both transmit and receive were achieved while attenuation in the reject band (a 45-MHz split) was sufficient to allow proper duplexing.  With an advertised loss of under 1 dB per side, many (and most) of the units sported much less loss than the specifications.  Typically, each port only showed 0.5 dB of insertion loss.  

The photo above shows the duplexer and the 19" rack panel it's attached to.  This two-rack-unit panel allowed the Quantar and duplexer both to fit in one of Motorola's standard cabinets with no room to spare.  With the disappearance of MDT's and most other site equipment sharing transmit combiners and receive multicouplers, these duplexers made their way out the door and unfortunately, the scrapyard.

Unit pre-check

If you are able to find one of these duplexers and you obviously have zero need for an 800 MHz public safety split and want to modify it for a 900-MHz filter (or filters--plural--maybe), then you will need to first do a pre-check to be certain the unit is operating properly and document pass responses BEFORE making any modifications.  At minimum, you will need a two-port tracking generator that is accurate and will measure insertion loss.  A one-port VNA will NOT work for this, although it is also nice to see the return loss or VSWR through each side of the duplexer to ensure that it matches the pass response with the two-port test.

In the screenshots above, the plots of the passband curve of the duplexer before any modifications were made.

Requirements - tools, experience and accessories

• Good understanding of RF, filter responses and test equipment.  As mentioned above, if you do not have a two-port pass response analyzer capable of going to 1 GHz, then stop here.  A one-port VNA will NOT work for this conversion.

• Good "shop etiquette" and experience in basic metalworking, cutting, filing, drilling, etc.

• Clean workspace, patience, and several hours of time to do things right

• Tools - drill, tubing cutter, pliers, rivet gun, screwdrivers, 7/64" Allen wrench

• Tools - 5/16" wrench, 1/8" drill bit, file, deburring tool, etc.

• Two gin-based cocktails (optional)

Reassembly

Install the six modified tuning rods back into the cavity with long-nose pliers.  With the filter in the position shown above, the SHORTER rod that was set aside in an earlier step should be installed in the position where the yellow box around the retaining screw is shown.  This is beside the "TX" decal next to the N-type chassis connector.  This is the NOTCH adjustment and will have very little effect on the pass band.  We will get to this later on the service monitor screenshot.

Tuning Rod Orientation - READ FIRST BEFORE ATTACHING END PLATES

It would be an extremely good idea to attempt tuning the filter BEFORE attaching the end plates with rivets.  The end plates should be loosely fashioned into the side of the filter as if you were preparing to rivet them in because capacitance to the end plates will affect tuning.  Use tape to hold the end plates securely while tuning the filter.  You may need to move/remove tuning rods (and therefore the coupling rod also) to make minor adjustments, such as a spacer/shim as I had to do on one of my coupling rods.  If you rivet the end plates into place and run out of adjustment room on one of the tuning piers, you will ultimately have wasted rivets and time while drilling the rivets out (again) to access the inside of the filter.  

I was also not able to get a good exclusive photo of the small copper strips soldered to two of the six tuning rods.  You can see this on the rod in the photo below.  These copper strips are capacitance adjustments and they MUST be mounted next to and facing the input/output coupling rods attached to the N-type chassis connectors.  This gives maximum coupling and therefore less loss.  More loss does not necessarily mean a sharper filter response in this case.  Although this isn't critical, it does lower the insertion loss substantially if the copper strips on two of these tuning rods are located next to and rotated to face the N-flange connector coupling rods.

What happens if you cut a tuning rod too short?

If, while tuning the filter pass-band you find that you are running out of capacitance on a section (turning the screw clockwise until it bottoms out) then the rod is too short.  A quick easy fix is to use a few small washers as a spacer where the rod is mounted into the cavity.  This will effectively increase the rod length, therefore, increasing capacitance to the tuning slug and lowering the frequency for that section.

Tuning

With the tracking generator configured as before, attach the cables appropriately and begin tuning the filter.  Adjust each screw alternatively.  This will take time and patience, as each section of the filter affects another.  Try to adjust the pass-band to appear like the photo below.

If desired (and you should), check the VSWR through the filter into a known, dependable 50 ohm load.  I get 1.3:1 throughout the entire passband which is excellent.  I get extremely irritated at people who say "what difference does it make.  It's just 150 milliwatts!"  The effect that a 2:1 VSWR has on 150 milliwatts is the exact same ratio as a 2:1 VSWR has on a megawatt transmitter.  Efficiency is best when impedances are matched, which is exactly what VSWR and return loss show.  The amount of reflected power, in watts, will certainly be less with a one-watt transmitter versus a kilowatt transmitter--but the apparent and net effects in percentages of lost efficiency will be the same!  

Update 3/6/2026 - How did it work?

I mounted my RF device in an aluminum enclosure with a 3D printed tray which was custom built by K4ROM.  This mounted nicely to the filter itself, making a 19" rack-mountable all-in-one device.

The Meshcore repeater blends in very well with other commercial equipment at the site.

The antenna array

 The antenna is mounted at the 500-foot level on a TV broadcast tower.  The DB408 UHF antennas (and probably the DB404 and DB420 versions) seem to work fairly well on 910 MHz.  It is nearly impossible to come up with a conclusion to why, but logically a half-wave on 450 MHz is a full-wave on 900 MHz.  Does this give reason to why it works fairly well?  There's no way to know unless range tests were done because the pattern is very unpredictable outside of the antenna's normal frequency range.

The filter in this article does an excellent job at keeping unwanted signals out of the Meshcore device.  Right beside the antenna being used for Mesh is a powerful 10 dB gain 851 MHz antenna attached to a multi-channel trunked system.  It has no effect on the Mesh device.  Furthermore, the 20KW television station (561 MHz) along with a NOAA WX transmitter and two 100KW FM broadcast stations right across the street, the Mesh device performs flawlessly with noise floor of -118 dB according to the device itself.  The device shows a -120 dB noise floor with no antenna attached.  That is pretty outstanding.

Conclusion

I am very satisfied with the performance of the filter.  I plan on converting the other half of the filter (the smaller portion) in the near future

Update 3/12/2026

The smaller of the two filters has been modified and tuned.  The modification required cutting about 5 mm off each of the six tuning rods.  However, this meant that the plungers (small brass threaded inserts) were too short to do any tuning.  I picked up six #8-32 brass machine screws (stainless will also work) along with some #8 washers and nuts.  The screws are 2" long, which is enough to tune each section effectively.

The small filter actually surpsied me in overall performance.  The rejection in the lower portion of the spectrum isn't quite as good as the larger filter, but the insertion loss is about the same.  Perhaps the reason for the larger filter's size is due to power handling capability (it was formerly the TX part of the duplexer).  I am extremely satisfied in the results and will be modifying more of these in the future as long as Meshcore/Meshtastic continues to be a useful thing

N4DBM