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My goal is to connect mobile devices using a coax cable with a SDR board to be independent from using the air interface and antennas. I don't know how to connect my devices with the SDRs since I'm an absolute beginner.

Instead of using:

antenna <-> air <-> antenna 

it should be:

antenna <-> antenna

Just bridging the signal without using emitting the signal should be possible and I found references that this can be done.

BladeRF SMA RX1 <- coax <- \
BladeRF SMA RX2 <- coax <-- \
                            <--> coax <-> TRX SMA mobile phone
BladeRF SMA TX1 -> coax --> /
BladeRF SMA TX1 -> coax -> /
BladeRF SMA RX1 <- coax <- \
BladeRF SMA RX2 <- coax <-- \
                             <--> coax (MIMO?) <-> SMA netgear
                             <--> coax (MIMO?) <-> SMA netgear
BladeRF SMA TX1 -> coax --> /
BladeRF SMA TX1 -> coax -> /

I'm using a BladeRF Micro A9 with 2 RX ports and 2 TX ports (SMA ports)

As devices I have:

  • a mobile phone with a single internal antenna (I assume) which I want to unplug and connect over the onboard Ipex U.FL MHF2 connector to an SMA adapter.
  • a mobile LTE router Netgear AirCard 785. This router has two antenna ports as TS9 ports. I assuem for LTE MIMO? I have 2 adapter to SMA here.

I have purchased also some 30 db and 10 db attenuators to get not issues with the power input.

Question A: How can I connect the 2 port from the mobile router with the BladeRF. Can the coax cables be simply joined? Which cable from the mobile router goes into which port from the SDR?

Question B: Can I use an Y cable to split the coax cable from the single mobile device antenna and connect it to the RX 1 and TX 1 of the BladeRF SDR?

Question C: Are the output ports of the mobile router for RX and TX? Or just for RX?

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  • $\begingroup$ Can you edit your question and provide more info, such as a drawing and specs on these devices? $\endgroup$ Feb 6, 2023 at 3:31
  • $\begingroup$ @ha3flt Air interface is the transmission between a cell tower and the mobile device. Instead of using antenna <-> air <-> antenna it should be antenna <-> antenna. $\endgroup$
    – k_o_
    Feb 6, 2023 at 18:37
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    $\begingroup$ @MikeWaters: I added some schemes. $\endgroup$
    – k_o_
    Feb 6, 2023 at 18:49
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    $\begingroup$ Especially with the latest edits (thank you — and welcome to the site BTW) the OP's goals/questions seem relatively clear, basically how to hook up a system of RF devices to a shared coax cable. So as to let the devices communicate and monitor as they would in air — but without radiating. $\endgroup$ Feb 6, 2023 at 18:54
  • $\begingroup$ This reminds me of en.wikipedia.org/wiki/10BASE2 where many Ethernet transceivers could all be tee-ed into a single run of coax. In that case the tees had to be direct and the coax had to be terminated at both ends. I don't know enough to say whether that could apply to any set of 50Ω/shared-Z₀ RF devices or if the transceiver equipment was specially designed to support the impedance matching involved . $\endgroup$ Feb 6, 2023 at 19:04

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You can connect the device to the SDR with a cable.

The choice of ports is up to you, depending on your application. Your devices are capable of (at least) 2x2 MIMO so to fully evaluate them you probably need to connect each port to a separate transceiver. The bladeRF claims to do 2x2. If you're looking for the bare minimum you can probably just connect both ports together, with an RF splitter so that they are somewhat isolated, or even try just terminating one port and ignoring it.

You might need more than 30 dB of isolation. The phone will transmit at ~ 1 W, 30 dBm. Your receiver will probably expect signals in the range of -40 dBm to maybe -20 dBm, unless it has an attenuator that can be switched in. Also the other way - the phone will expect signals over the air in the range -90 to -50 dBm. If you transmit at (say) 0 dBm it might not work with just 30 dB attenuation.


If your SDR has a dedicated Tx and Rx port with no T/R switch, you need a wilkinson splitter at the SDR side too. Wilkinson splitters have reasonable isolation between their ports, so very little power comes back to the device. Something like this:

connections
The modem won't have a problem with the -20 dB coupling, because a) it has T/R switch(es) so it doesn't transmit and receive at the same time, and b) it will commonly be used with two antennas that couple strongly anyway.


Testing of mobile devices is usually done in a benchtop test chamber. This shields the device from the networks outside and allows your test equipment to transmit to it in real mobile bands, without requiring a license.

There are many manufacturers, the first one I could find is this one from R&S:
enter image description here
but there will be others that are less expensive, or you can make one yourself. It's basically just a welded metal box with a special RF gasket, a means of clamping the lid down, and a simple antenna inside. Some RF absorber will help but isn't essential for just testing basic communications. Things like transparent windows, or data ports, make it more expensive.

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  • $\begingroup$ Thanks for the answer. Regarding the termination: is this needed? I read that unused ports on newer devices have some built-in protection? Can I connect RX also with the TRX port of the phone? Since the mobile phone is only using one antenna, it seems to use it for TR and RX. Also, when using an attenuator, does this attenuator work in both directions? I'm using mauritz.de/en/attenuators/sma-attenuator-6db-1w-dc-6ghz $\endgroup$
    – k_o_
    Feb 6, 2023 at 20:23
  • $\begingroup$ It's possible they won't mind being left open. It's also possible that if you don't make a connection, the on-board antenna will remain connected, this may pollute your tests. $\endgroup$
    – tomnexus
    Feb 7, 2023 at 16:40
  • $\begingroup$ Nice answer. What tool was used to produce the image? Is it a text based tool like plantuml? $\endgroup$
    – k_o_
    Feb 7, 2023 at 22:31
  • $\begingroup$ Google docs / drawings :) PlantUML looks interesting. $\endgroup$
    – tomnexus
    Feb 8, 2023 at 21:24
  • $\begingroup$ I have practically verified it and it works. As values for the BladeRF I have used tx_gain = 40 and rx_gain = 20. $\endgroup$
    – k_o_
    Feb 15, 2023 at 0:06

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