VHF - UHF DIPLEXER  Project

by Rob Stokes ZL1RJS

This is my version of the VHF / UHF diplexer.
Please note, Please read my Disclaimer and you understand it before continuing.

Photos

Photo 1. Shows the completed assembled version.
Photo 2. Shows the Top layer of the PCB.
Photo 3. Shows the underside of the PCB.
Photo 4. Shows the coils with front and side view.
Photo 5. Shows the VNA diagram.
Photo 6. Shows the VNA diagram.
Photo 7. Shows the VNA diagram.
Photo 8. Shows the VNA diagram.
Photo 9. Shows the Circuit Diagram.

Parts List

2x 18pf Capacitors
2x 4pf Capacitors
2x 8pf Capactors
1x 4.5t 5mm Coil (L2)(Made from 1mm Enameled Copper Winding Wire)
2x 3.5t 5mm Coil (L1, L3)(Made from 1mm Enameled Copper Wire)
2x 1.5t 3mm Coil (L4, L5)(Made from 1mm Enameled Copper Wire)
1x Diplexer PCB
1x 2cm x 3.4cm Blank Double or Single sided PCB (For screening)
1x 5cm x 5cm x3cm Aluminium Enclosure Diecast Box
1x UHF SO239 Female Jack 2 Hole Socket
1x BNC to BNC RG58 lead 60cm (Cut in half) to fit Tait Radios
Or
60cm of RG58 cable. (See optional parts list)
4x 10mm Stand-off M3
4x M3 Nuts (To hold the PCB board)
4x M3 6mm Countersunk bolts
2x 1/4" x 1/2" Rubber grommets
2x Nuts and Bolts for the UHF SO239 socket
1x Lug terminal (To earth the socket to the PCB)
1x 4cm wire (For connecting the shields/earths of the RG58 together)

Optional Parts List

If you are not using the BNC cables, then make you own leads to suit your radio, using your choice N type or PL259 Plugs.
1x 5mm Bolt (To make the coils)
1x 3mm Bolt (To make the coils)

Let's Begin the Project

Making of the coils

There are two sizes of the coils that we need to make.
One with a 5mm air gap and the other with a 3mm air gap.
See photo 4. The top 3 coils were wound on the 5mm bolt and the bottom 2 coils were wound on the 3mm bolt.
To make the coils, use a 1mm Enameled Copper Wire.
First, make the coil for L2. start winding the wire tightly around a 5mm bolt.
Leave a start tail around 10 - 15mm and turn 4.5turns around the bolt and stop.
End with a 10 - 15mm tail and cut the wire.
See photo 1. L2 is the middle coil left of picture.
In photo 4. it shows all the coils are made. Make sure to leave enough tail wire to make the connection to the board. After the coil is wound, using a craft knife or a small file, scrape away the enameled coating off each end of the coils to expose the copper wire. This will allow the copper wire to be soldered to.
It may be easier to scrape the coating off while it is still on the bolt. Also to tin each of the ends of the coil.
To get the coil now made off the bolt, unscrew the bolt from the coil.
Once you have made 1 coil the rest will be a lot easier to do. You need to make 2 more of these but now with 3.5 turns, still using the 5mm bolt.

Next do the same for the next coils, except wind the coil around the 3mm bolt, for this coil you only need 1.5turns.
Follow the same instructions as above to make last coil.

These coils are not through hole on the board, so you will need to bend each of the legs of the coil at right angles so they can be fitted to the board.
Once all the coils are made and shaped, trim any excess off and fit each coil onto the board. Tin one side of the coils pad circuit board first with solder, fit the coil onto the board. You may need to use tweezers to hold the coil as it will get hot. Once the coil is in place, solder the other side of the coil to the pad of the circuit board.
Using tweezers or equivalent should make it easier when fitting these coils to the PCB.

Capacitors

Fit the capacitor's to the PCB. There are 6 capacitors in total to fit.
Sort the 3 values together, 2x18pf (C2, C3), 2x8pf (C4, C6) and 2x4pf (C1, C5). fit the same value capacitors on to the PCB as not to mix them up.

Screen divider

I added a blank copper clad board onto the PCB as a screen divider to keep the RF away from the UHF/VHF signals.

Drilling the holes in the case

Drill a 14mm hole for the SO239 socket on the side of the case. Drill 2 x 10mm holes to fit the gromments on the opposite side. The gromments I used were 1/4" x 1/2".
Open the enclosure there is 33.2mm between the hole pillars. Measure and mark the center 16.6mm. Measure down 10.5mm on the 16.2 line and mark. Drill the 14mm hole. I used a stepper drill to drill 14mm. After drilling the hole, continue with the 16mm very slightly to countersink as the SO239 socket has an edge slightly bigger than 14mm until it is flush with the case. On the opposite side, mark the center again of 16.6mm. Next measure 8.3mm from the center and draw a line. Do the same the other side from the center. Measure 8.3mm down and mark. Drill 2 x 10mm holes. Fit the grommets.

Fitting the PCB to the Case

cut the corners off the PCB to make it fit into the case. Use a marker pen to mark the holes to drill. Drill 4 x 3mm holes. Countersink the holes using a 9mm drill until flush to the case.
Fit the 10mm spacers and loosly tighten. Fit the PCB and then tighten the hex bolts. Fit the nuts to hold down the PCB board.

BNC Lead

Cut the BNC lead in half. Trim back the coax, when cutting and stripping the ends, make sure you do not pull the center cable otherwise you may pull the certer pin out of the BNC socket. Fit the grommets to the dicast box and push the coax through each grommet to join onto the PCB. Once they are joined, fit the 4cm wire to join the two braids together and also solder to the top of the screen divider.

Tuning the Diplexer

You MUST terminate the other side of the diplexer first with a 50 ohm dummy load when checking the measurments with a VNA or Spectrum Analiser.
If you are measuring the VHF side of the duplexer then you put the 50 ohm dummy load on the UHF lead, and vice versa when measuring the UHF side, you put the dummy load on the VHF lead.
Failure to use a dummy load will give false reading.
When doing any measurements, it must be in it's case.
See photos 5, 6, 7 and 8. The blue line shows the waveforms cutoffs.
When I build this proto-type diplexer, I found the waveforms were not quite right and further tests, I had to earth both sides of the cables where the VHF and UHF cables come into the case. I used a small 4cm piece of wire to join both shields together and also soldered to the top of the wire to the screen divider where it crossed over.
This was done to reduce the waveform for the cut-off on the VHF side. On the VNA it changed from -33.64dB to -62.32dB by adding this wire. The UHF side had no problem.

Final Testing

Once fully assembled you should end up like the picture of photo 1.

Credits

A special thanks to Keith ZL1BQE for showing me tips on how to design, size the printed circuit board and tips on making this project possible.
I hope you enjoy and have fun building this project.

73 de Rob