Topband: RX Splitter for 160/80m

[Tom Rauch]

A "magic T" splitter or combiner is very easy to build, and that's all 
Minicircuits or a CATV manufacturer will sell you. I often just use a 
chunk of old PC board as a "chassis".

1.) Wind a bifilar winding on a ferrite core, 73 to 77 mix will work 
fine. If the core thickness is about 3/8 inch, you'll need about five 
passes of twisted pair wire (any gauge you can work with).

2.) Connect the start of one winding to the finish of the other. That 
point becomes the antenna input.

3.) The remaining wires become outputs to the receivers. Connect 
a resistor that is twice the impedance of the receivers across from 
wire to wire. A 100 ohm resistor works for 50 ohm receivers, or 150 
ohm for 75 ohm receivers.

4.) Feed the center-tap with a 2:1 impedance transformer (a turns 
ratio of 5 to 7 will work fine, and that can be an auto-transformer 
wound on the same style of core used to make the splitting 
transformer). The full winding is the input, the tap point (5 turns off 
ground) is the output that drives the center tap of the splitting 
transformer.

A word of caution, all splitters are sensitive to output port 
termination. If one port is misterminated, the output voltage (and 
loss through the splitter) will still vary quite a large amount.

The splitter prevents a short or open from totally killing the signal, 
but it does not guarantee equal power to each port unless 
impedances are matched. 

Turn this around, and it becomes a combiner. About 3dB loss 
when matched and splitting, almost a perfect sum of the inputs 
when combining.


73, Tom W8JI
w8ji@contesting.com



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