# Combining/splitting three different RF signals

Not being so fluent in RF, I decided to ask here if my current approach seems doable or how one would try to achieve something like this:

I'm doing research and deciding on an approach for my proto-board, to understand if and how combining signals from three different antennas into single coax and then splitting(filtering) them at the receiving end for different radio modules would even work (and for transmitting as well, so bidirectional).

More precisely - let's say I have antennas for 1) GPS, 2) 2G/3G and 3) Bluetooth.

I think the frequency bands for them are rather separated, based on the antennas I have:

• Cellular: 800 - 960MHz / 1700 - 2170MHz
• GPS: 1560 - 1590 MHz (1575MHz center frequency).
• Bluetooth: 2400 - 2500MHz

so to me it seems it would be possible to join these into single coax, without greatly disturbing the signals (they are not in the same bands thus interference should be minimal - especially with filtering).

This where my discussion on different approaches begin, so I currently plan to test two things:

a) Bluntly join antenna traces together into single coax after some impedance transforming and filtering.

b) Use a passive combiner/splitter to join the antenna traces together - in hopes for better isolation when signal is transmitted the other way back.

For the receiving end I would mirror these blocks, before any ANT input on the chips.

simulate this circuit – Schematic created using CircuitLab

SO basically:

• 1) Does just joining antenna lines like that make sense or there's smth completely wrong with this idea? (based on the proposed protocols)
• 2) Would using combiner/splitter make this approach more feasible ?
• 3) I'm completely off and I should try smth else

In any way I believe I need filtering for each of those bands for sure - in order to get better SNR on my receiving end.

• GSM filtering is probably more complicated since there's two bands to deal with. That's why it's red on the schematic.
• you need to do b). Your variant a) doesn't work (unless your "blunt" way of join coaxes amounts to building a wideband impedance matched combiner... in which case a) becomes b) ;) ) – Marcus Müller Jun 25 at 15:04
• anyway, what is your goal here? What's the purpose of all this? – Marcus Müller Jun 25 at 15:05
• @MarcusMüller Idea is to join three antennas into one coax. Have that cover some distance and then divide it again on a separate board (idea is not to have three separate wires going full length to my radio receiver board). – crypton Jun 26 at 8:34
• but for what purpose? – Marcus Müller Jun 26 at 10:30
• erm, umff, that IS the purpose :). I guess to elaborate, maybe 'to achieve easier assembly with a single cable at the expense of more complex hardware... mostly to understand if and how this could be possible to do - so educational purpose as well'. – crypton Jun 26 at 10:37

As shown in the diagram, a triplexer is a bidirectional device comprising three filters that separate (or combine) the signals from the input (or output) port into band-limited signals at the three output (or input) ports. A properly designed triplexer will present 50$$\Omega$$ at all four ports across the unit's usable frequency range.