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I'm looking a bluetooth modules for a project i'm working on. I want to have a small form factor version of the board be usable with a trace or chip antenna, and have the option of connecting a external antenna for better performance if need be.

However, when looking at bluetooth modules, i've noticed that the ones that include both options have a selection via a 0R link rather than simply having both connected (see pictures). enter image description here enter image description here

This is at odds with things like Nordic's nRF52 dev kit which has both a trace and IPEX/u.FL connector available at all times (see picture).

enter image description here

What is the reason for this? I can only assume that this is due to performance considerations but i'm not sure. Ideally, like the dev kit, I want both connections to avoid soldering to make my selection.

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  • \$\begingroup\$ The part in your first example looks more like a capacitor than a jumper. The RF output will often have a DC component. The capacitor removes that DC component and prevents DC short circuits if you manage to short circuit your antenna. The Nordic board also has such a capacitor - but I don't see where it has a built in antenna. All I see on the Nordic board is the connector for the external antenna. \$\endgroup\$ – JRE Jul 15 at 11:07
  • \$\begingroup\$ Ah. Wait. I see the antenna on the Nordic board now. Looks like a simple dipole rather than the F antenna on the other boards. \$\endgroup\$ – JRE Jul 15 at 11:09
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I took a look around the Nordic documentation.

You don't need a jumper on the Nordic board because the RF connector it uses has a switch.

The connector is of SWF type (Murata part no. MM8130–2600) with an internal switch. By default, when there is no cable attached, the RF signal is routed to the onboard PCB trace antenna.

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    \$\begingroup\$ I would also add SWF connectors are meant for probing, and are not compatible with u.FL cable assemblies. They serve other purpose than solder-selectable u.FL / PCB antenna. \$\endgroup\$ – Nipo Jul 15 at 17:22
  • \$\begingroup\$ Good spotting! That makes sense with my assumptions about the implications of connecting two antenna at once. It simply disconnects the trace antenna when connecting an external antenna. \$\endgroup\$ – Joe Jul 15 at 23:30
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JRE's answer has already identified that the connector on the Nordic board is a switch type, so none of these examples have both antenna options connected at once.

As to the other part of your question - why can't both be connected at once - the answer is impedance matching.

The integrated antenna will be designed to present a particular characteristic impedance (usually 50 ohms) to the transmitter at the operating frequency. This ensures that the power from the transmitter is transferred efficiently to the antenna.

The connector will be designed to present that same impedance, but will only do so when a cable and suitable antenna are connected.

If the empty connector and the integrated antenna are both connected to the transmitter at once, or if an external antenna is connected at the same time as the integrated one, then the impedances would not match.

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