(3dB)  R.F splitters  and combiners   for H.F.

I needed to split an H.F signal from one antenna between two H.F receivers, this can be done two ways 

1. by making a resistive splitter using 3 resistors in a star formation  with the centre of the coax  ,the resistors are chosen to perform an impedance match at all 3  ports, there is loss to this configuration but there is next to no  isolation between the split signals , The circuit is simple and performs an adequate function in several scenarios but  it has the undesirable property  of what happens to one of the split ports will have a large effect on the other port .

2. A more complex but much improved  Rf splitter can be made with  judicious connections of ferrite cored wound transformers , the connections cunningly arranged by impedance matching to achieve the best match at the input and output and still maintain the sharing of the output signals into the wanted impedance be it usually 75  or 50 ohms. but of course the circuit can be designed for other impedances

      rf splitters  

call in3odt                                              

DH1tw

I did the usual trolling of the web ,searching for 3dB splitters  or combiners  . There were lots of commercial varieties for sale but the home brew  recipes were comparatively  thin on the ground . I did find some excellent  information  and circuits  from the following web sites of avid constructers like W8JI,  IN3OTD, W6PQL and DH1TW  to name a few  ,Im sure if i kept looking I would find more  but the circuits  looked much the same in most places I looked I did have some small binocular cores  and some small 20mm dia ferrites  toroids  so I decide to have a crack at two types of construction.   The  dual Binocular splitter with auto transformer  and transmission line construction  and the twin toroid version made with two transmission line transformers.  I was somewhat surprised at the few turns required on ferrite material  and the resulting operating  bandwidth of the devices .

           two rings                                 

The first device was made in a small diecast box to achieve good  rf screening , I used 3 so239 sockets for connections as I was going to use the splitter at HF  namely the  80 m band .The transformers were wound with small diameter  insulated  twisted wire transmission line of about 8 turns per inch, The input ferrite was wound with 8 Turns of trifilar wound wire  and wired such that the output of this transformer was close to 25 ohms  output impedance . 
The splitting ferrite transformer was wound with 8Turns of bifilar form wire  and connected to feed both output ports with equal signal  ,  see the wiring diagram  : note the inclusion of a reactance cancelling capacitor to improve the output  port to port isolation .

                             rings with wire  

two rings with pin wire

All was fitted into the box and soldered together   a little bit of a squeeze  but i wasnt concerned to much due to the self isolation properties of ferrite toroids .

 n  60 pf cap

                                Finished 3db splitter combiner   note 6-60 PF  trimmer  to adjust for maximum  parallel port isolation   we had a top for it as well

2011 02 13  11;58

                        "output" port isolation versed frequency ,all ports terminated in 50 ohms ,performance not as good as the binocular ferrite splitter

12:20

                           I measured to VSWR  as well to see how it was   and was pleasantly surprised    all ports terminated on 50 Ohms
                           the vswr of the "split ports was much the same

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