Dolphin UHF Duplexers Build



Now that Dolphin duplexers have made there way on to the Amateur market we have had the opportunity to try convert them for 1.6 MHz split use. After a few modifications we hare happy to say that the modifications shown below have given us a good cheap set of duplexers. There was no silver plating in the cavities or any associated parts on the sets that we had so you might consider having them silver plated.

Our sets that we acquired had the centre tuning stub control mechanism and 19 inch rack fittings still attached which held various control circuits and circulator still attached. Stripping down the unit until you have the main thick walled copper box with the 2 stepper motors still attached is all e need at this stage. Keep the Black Circulator / Isolator as we will also need this in this design.

The stepper motor wires can be chopped off as we will be tuning the tuning stub via the slotted adjuster a the top of the motor. We did not strip down the stepper motor as it was so small and helped with the strength keeping the centre tuning tube in place and something to connected the screening cans that we added later.


Remove the centre "N" type socket arrangement and attached coils. These will not be needed. You will need to remove the top plate that is held with 16 small star headed screws. File flat any wire that you may have sticking up from the plate (Copper side)

Remove the 4 star retaining screws holding the stepper motors to the main body. Remove both tuning mechanism. These will slide out very easy. If need you can move the position of the copper tube by slackening the small Alan key at the bottom of the tube and sliding the tube that will make the tube go deeper or shallower. You will not know if the needs adjusting until the time of the tuning up and you are unable to get the peaks in the correct place. We did not need to make any adjustment here.


We need to make the rotatable coupling mechanism. This is made by using a "N" type socket draw a circle on the back of the socket with the line just touching the inside of the 4 original holes on the new socket. Filing away the 4 fixing holes and making a circular collar as shown in the picture above. The loop is made from 2mm silver plated wire. Sizes for the loop is shown in the diagram below and soldered to the socket.


Replace the plate and secure with a few screws to hold it in place. As seen in the above picture using 2 of the original screw holes as a guide drill out to 15mm to accommodate a modified "N" type socket that will house the rotatable coupling loops. I used 3 self tapping screws to hold the sockets in place. Do not have the screws to close to the socket that it doesn't rotate but close enough that the screw head holds down the socket when tightened. Use your modified socket as a guide. See above pictures as a guide. Remember not to over tighten the screws as it is copper that you are screwing into


2 x Loops made from 2mm silver plated wire required



The screening cans measure 50 x 50 x 25.

A 7mm hole in the side is needed for a trimming tool to pass through so as the tuning capacitor can be adjusted as shown below also cut out 2 slots as shown 6mm wide and 5mm from the edge of the can to enable the coax runs to pass through.

Prepare your coax


I used Thermax RGU 142 Coax cable (50 Ohm) for making up the interconnections  between the cavities and the circulator / Isolator. It has not got the best loss figures but it is silicon based and will not melt when soldering the coax especially to the tin screening cans. I'm sure that you will use what you have. Just keep in mind about the heat and melting the coax. The variable capacitor is 1p


Looking at the pictures the TX section is on the left. Remove the fixed loop and rotate it 180 degrees you may need to file a little to make it fit without fouling the case when re fitted. I marked the earth end of the loop on the socket so I could see which was the earth end. I found rotating this loop gave me the notch in the correct place when tuning. and again with the RX side rotate the loop 270 degrees. See the above photo and note dots on the sockets denoting the earthy end.

The fixed loops I did not change as they worked very well so I used the original loops here

A good vacuum cleanout will be needed here to remove and filings that have fallen into the cavities. 


The Circulator / Isolator used is the one that we removed earlier its made by AMP in Ireland and covers a frequency 410 - 430 MHz. I have used them up to 434 MHz without not much problems. You may want to replace with one of your own that you have if so you will need to make the coax harness to fit your Circulator / Isolator connections.

On the original Circulator / Isolator the connections made were   1 = TX  "N". --  2 = Antenna "N". --  3 = RX "SMA". This Circulator was not adjustable


I used 1 right angle "N" Coax plug and 1 right angle SMA coax plug that connects to the Circulator / Isolator as seen above


The stepper motor casing makes a great place to cable tie the screening cans to. I used Velcro to hold the Circulator / Isolator in place.

Use metal tape (Found this in the local DIY shop It is sticky on one side) to cover any open holes. I also used this to cover the slots in the screening cans.


RX . Pass = 434.900 MHz - Notch = 433.300 MHz       TX . Pass = 433.300 MHz - Notch = 434.900 MHz

RX 53dB 2dB loss & TX 60dB 2db loss including the Circulator / Isolator

Tune one side at a time. Set the tuning capacitors to nearly minimum capacitance. Connect the RX side of the set to a spectrum analyzer with sweep generator set for the RX frequency. With the RX connector to the input to the analyzer and the circulator connector to the output of the analyzer. Rotate the screw driver on the top of the stepper motor pass peak waveform is achieved in the display as in the above picture on the frequency that you are using. Adjust the loop until you have the best notch you can get 1.6 MHz away on the TX frequency. Rotating the loops you will achieve either a peak or a notch on this side you need the notch. It will take you a little time when tuning and rotating the loops so be patient. Your notch will be at the frequency of the repeater TX frequency. The 1p tuning capacitors will fine tune the notches after the coupling loops have been rotated to get the best notch.

Do the same with the TX side except the peaks and notches are the other way round this time and rotating the input and output connections to the spectrum analyzer

The Circulator / Isolator is not tuneable that came with the original setup

HP 8444a tracking generator and 8554L Spectrum Analyzer also a Marconi 2955a Test Set Used during the tune up.

Ian G8PWE & Peter EI4JR

Webmaster Peter EI4JR  (C) 2000 - 2014