Carsten Groen's (OZ9AAR) personal place

Low(er) power 90 degree hybrid couplers for 23 and 70cm (and 2M)

I needed some low power splitters (90 degree hybrid combiners) for both 23cm and 70cm. Previously I made two high powered 90 degree hybrids for these bands (combiner for 70cm and combiner for 23cm). These combiners are fitted with DIN 7/16 connectors and can both handle substantial power levels. Usually these are used for combining two PA modules. In that case, a low power splitter is needed to split the input power to the PA modules.

This triggered the design and construction of the two hybrid couplers shown here.

(As a note, there is even a hybrid for 2M band also, seems to fit the 70cm PCB as well!)

All the design files (STEP files for the aluminum box and lid as well as the Gerber files for the PCB (0.8mm FR4, ENIG surface, JLC7628 substrate from JLCPCB) are available for download here:

1. File for aluminium box, STEP file (6061 aluminium)
2. Description of threads for aluminium box, PNG file
3. File for lid for aluminium box, STEP file (6061 aluminium)
4. Gerber files for 70cm PCB, ZIP file (JLCPCB, JLC7628 substrate, 0.8mm thick, ENIG surface)
5. Gerber files for 23cm PCB, ZIP file (JLCPCB, JLC7628 substrate, 0.8mm thick, ENIG surface)  

I can produce a limited number of the combiners for anyone interested (or you can have your own made from the data linked above), 

Assembled and tested combiners is sold for €170,- each plus shipping (same price for 70cm and 23cm version, these are WITHOUT trimmers inside. You can add these yourself if needed).

The design

The two couplers uses the same aluminium housing and SMA connectors. It is only the PCB board that are different for the two bands. The outside dimensions of both PCBs are the same, holes for screws etc. are also the same.

The aluminium box has 4 mounting holes on the back for M3 screws (fastening to a chassis etc).

The 70cm coupler uses a SMD mounted hybrid from TTM Technologies: XC0450L-03S

The coupler can be purchased from Mouser: 620-XC0450L-03S and can handle 200W of RF power.

The 23cm coupler is also an SMD mounted device, this one is available from RF Microwave in Italy: D-1-1.5 and covers 1 to 1.5 GHz. It will handle max 50W of power (I did not find further datasheets for the hybrid, only the info from RF Microwave on the website for the hybrid).

Both designs uses four SMA female connectors (with 4mm long PTFE center), manufacturer code

SMA14-2H-04E from QAXIAL also available from RF Microwave. 

The PCB board, connectors and the lid are all fastened with M2.5 4mm long screws (24 screws needed in total).

Both designs can also, if needed, be fitted with two Sky Trimmers if small adjustments of the phase is needed (3 to 10 pF or less for the 23cm. You could also try and mount small "flaps" to either of the outputs to change the phase a bit).

Assembly

The SMD hybrids are soldered to the 0.8mm FR4 PCB using solder paste and a hotplate. This is an very easy and safe method (you could also use hot air). The 23cm hybrid needs four small pieces of wire to bridge the connections from the top of the hybrid down to the PCB, keep these as short as possible!

Test and measurements

I connected both hybrids to my Oscilloscope (2 GHz BW), P2 (-90 deg) to CH1 and P3 (0 deg) to CH2.

Cables and connectors were not calibrated, so this is just a relative test to show the phase difference.

At 432 MHz, -10 dBm was fed into P1, P4 was terminated in 50 Ohm, yellow (CH1) trace is P2 and red trace (CH2) is P3.

Same measurements for the 23cm version.

At 1296 MHz, -8 dBm was fed into P1, P4 was terminated in 50 Ohm, yellow (CH1) trace is P2 and red trace (CH2) is P3.

Below is a test setup with the 23 cm version and one of the QRO 90° hybrids. The setup shows excellent phase and amplitude match, the insertion loss is also very low, in practice it is dictated by the loss of the QRP splitter shown on this page, the QRO 90° hybrid adds practically nothing with regards to insertion loss!

Adjustable phase

I made the design so it is possible to add two small trimmer capacitors. Adding a bit of capacitance to the outputs (P2 and P3) is a simple (and somewhat crude) was of altering the phase of the signals. Normally you would change the  length of the coax cables going from P2 and P3 to the input of the PA modules in order to get minimum measured power on the isolation port of the (QRO) combiner at the output of the PA modules.

On the 70cm version, I have tried adding two 1.8 to 10 pF Sky trimmers, and on the 23cm version two 0.7 to 5 pF Sky trimmers to see the effect.

A side effect of adding these trimmers is that the return loss on P2 and P3 will be heavily influenced (which is normally not too critical).

On the 23cm version, the return loss on P2/P3 will go from around -23 dB to -13 dB at minimum capacity (0.7 pF).

On the 70cm version, the return loss on P2/P3 will go from around -25 dB to -18 dB at minimum capacity (1.8 pF).

Using the trimmers, it is possible to change the phase up to 40 degrees on the 70cm version, and up to 50 degrees on the 23cm version. You might want to try and find even smaller trimmers as 40 or 50 degrees is a lot, and this is not needed. You can also experiment soldering small strips of copper/foil to the P2/P3 PCB traces, this will also change the phase (but can quickly become tedious).

Calculate phase difference based on output power and power on isolation port

I made a small Python script that can calculate the phase difference on the two ports of a 90° splitter/combiner setup. You need to enter the power measured on the output port and the power measured on the isolation port. You also give it the frequency (in MHz) and the velocity factor of the coax cables used.

The program will then tell you the phase difference between the two signals that corresponds to the power measured on the isolation port as well as the length of cable this corresponds to depending on the entered velocity factor.

REMEMBER! There are many things that can influence this, the program is just meant as a simple tool that can visualize the phase difference!

As an example, if there is not a perfect load on the output port of a combiner, typically 50% of the reflected power will be present on the isolation port (with the last 50% divided with 25% to each of the two input ports), this will "mask" the result, giving you the impression that there is a phase error between the two PA modules, when in reality some/all of the power measured on the isolation port is due to mismatch on the output port!

As a fun exercise, I asked Grok (the AI on the X platform) about the isolation port and mismatch:

(read with caution, but I believe the text to be pretty correct)

Grok on isolation port power in a Hybrid combiner