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We have an RF chain where a full-duplex radio transceiver with Tx and Rx port operating on different frequencies. These are connected to a single antenna (that can Tx and Rx) via a diplexer, so that antenna can be connected to a single common port on the diplexer while transmitting and receiving.

However, we have switched the design to have 2 antennas that each can also do Tx/Rx. With this change, the common port from diplexer needs to be connected to both antennas via a splitter/combiner. The concern is that the splitter/combiners we have bought are apparently not for full-duplex (according to the vendors, but have not verified experimentally myself yet).

enter image description here

Is it actually possible for the above schematic to work in full-duplex?? Or is there any splitters/combiners available that actually do allow for full-duplex?

thanks

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  • $\begingroup$ Why did you switch to two antennas? How different are the two frequencies? $\endgroup$
    – tomnexus
    Oct 6, 2023 at 1:36
  • $\begingroup$ We added an antenna in the opposite direction, in order to get more coverage. Without giving the exact frequencies, the ranges for Tx is 2200 to 2290 MHz and Rx is 2025-2110 MHz $\endgroup$
    – ITried
    Oct 6, 2023 at 7:23
  • $\begingroup$ Sorry I didn't see the orange thing said Diplexer. So you've already taken care of Tx / Rx isolation. $\endgroup$
    – tomnexus
    Oct 6, 2023 at 23:00

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Couplers or splitters are passive, bi-directional components.
As long as the power is within limits, they don't mind if they are transmitting or receiving. The loss from ports C to 1 is the same as from 1 to C (just in the combining direction, the signal is absorbed internally, not split). Here's a low power splitter with some specifications.
So you can use a splitter as in your diagram.

Mini-circuits also has a nice application note about splitters.

Be careful when using two antennas to improve coverage. In the region where both antennas can be seen, they will be an interference pattern of stronger and weaker signal, where the signals from the two antennas constructively and destructively interfere with each other.

Finally, if you have a high power transmitter (over 10 or 50 Watts) and a sensitive receiver, you could run into problems with Passive Intermodulation, where third-order intermods of the transmitted signals land within the receiver band. Antennas, cables, diplexers and splitters for cellular phone base stations all have tight PIM specifications, and have to use special materials and cleaning.

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  • $\begingroup$ Thanks for the response. I did test this exact setup experimentally end-to-end, and it seems to work fine with full-duplex as u had mentioned. Regarding the interference when 2 antennas are visible, this is certainly a risk that we are tracking. To replicate this, I simply made the coax cable to one of the antennas longer than the other by 1 ft. Since the frequency is in the 2GHz range, that extra length should be enough to phase shift it to a meaningful amount. The result was that communication was virtually uninmpacted to the naked eye. Do u think that was a valid test? $\endgroup$
    – ITried
    Oct 12, 2023 at 22:55
  • $\begingroup$ No that doesn't change anything really. A half wave (40 mm) of coax would turn the nulls into peaks and vice versa. Another half wave undoes the effect. There's no way to eliminate the effect, except to prevent overlap of the sectors (leaving a dead zone), or use different channels in different sectors. $\endgroup$
    – tomnexus
    Oct 13, 2023 at 6:48

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