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I have say, 4 170 MHz signals coming in on 4 BNC cables. I would like to add these signals together. Is there a name for a piece of hardware that does this? Are there any special considerations I should make, is there a reason why I can't just use something like a BNC T-converter to add two lines into one?

Any help would be appreciated or even keywords I could google!

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You could use RF transformers - each primary connects to each input and the secondaries are all wired in series to produce A+B+C+D as an overall output. What do you next depends on you - you could terminate the combined secondary with 200 ohms and use this into whatever it needs to feed or, you could use a fifth transformer to convert the 4 series secondaries down to 50 ohm impedance.

Other problems (and only you know whether this is a real problem you might face) is that terminating the 4 series secondaries will produce cross-talk from A to B, C and D and any combination thereof so you might want to terminate each secondary in a 50 ohm but, if you draw any current from the "combined secondary transformer" this will also produce crosstalk.

This, of course may not be an issue to you. If it is then a small "high-impedance" buffer amp would "isolate" the secondaries reasonably well.

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It's called a power combiner. There are several different types, and the idea is to make sure that all of the connections are properly matched so that you don't get any issues with reflections under normal operation. For example, if you use a BNC tee, then both sources will see a 25 ohm load and a very significant portion of the signal will bounce off of the split and come back to the sources. A device called a wilkinson coupler uses two 1/4 wavelength pieces of 70.7 ohm coax and a 100 ohm resistor to carefully cancel these reflections over a narrow bandwidth. There are other types of couplers and power divider/combiners, but the wilkinson is pretty simple and easy to make.

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