How to best connect 868MHz RF to slot antenna in aluminum enclosure ?

[opprud]

Hi all,

I Am currently working an a design, featuring an RFM22B 868MHz module, transmitting at upto 20dBm.
The PCB i am designing fits into two slots in a aluminum profile, and the in bottom i will mill a slot antenna at 164,3mm. (See attached photo)
Tests indicate a feedpoint at 1/20 wavelength, on each side of the slot.

My problem is, how to establish a good RF connection from the PCB to the Aluminium surface, preferably something spring-like or pointy mountable on the pcb, so the pcb can be pushed into the slot, fixed, and has a good rf connection to the slot

Any ideas, or experience with "RF-to-aluminum-enclosure-connections" would be highly appreciated

Best Regards
Morten Opprud

 

[jiripolivka]

Hi all,

I Am currently working an a design, featuring an RFM22B 868MHz module, transmitting at upto 20dBm.
The PCB i am designing fits into two slots in a aluminum profile, and the in bottom i will mill a slot antenna at 164,3mm. (See attached photo)
View attachment 119024Tests indicate a feedpoint at 1/20 wavelength, on each side of the slot.

My problem is, how to establish a good RF connection from the PCB to the Aluminium surface, preferably something spring-like or pointy mountable on the pcb, so the pcb can be pushed into the slot, fixed, and has a good rf connection to the slot

Any ideas, or experience with "RF-to-aluminum-enclosure-connections" would be highly appreciated

Best Regards
Morten Opprud

In a classical design one needs coax connectors on the PCB and an antenna, to define impedances and matching.
If you desire to somehow "directly" couple RF output power to a slot antenna made in the metal enclosure, you must experiment. Maybe the rigid ground connection is not needed but some loop or capacitive coupling to your slot may work.
Most design rules call for the 50-Ohm interface. Other methods must be tried and tested.

 

[FvM]

Spring contacts can be an option, but anodized aluminium is not conductive and bare aluminium not corrosion resistant. There may be a problem.

A capacitive or inductive coupler could work, but would be preferably designed with an EM solver. The slot antenna idea looks elegant, but most likely involves considerable design effort.

Good luck!

 

[opprud]

In a classical design one needs coax connectors on the PCB and an antenna, to define impedances and matching.
If you desire to somehow "directly" couple RF output power to a slot antenna made in the metal enclosure, you must experiment. Maybe the rigid ground connection is not needed but some loop or capacitive coupling to your slot may work.
Most design rules call for the 50-Ohm interface. Other methods must be tried and tested.

Correct, that the connection is "direct" PCB to aluminium. Its only 4.6mm, from RF output on the PCB, to the antenna slot, so an antenna connector and cable probably won't fit.

Basically what i am trying to find is something like a PCB "spring", perhaps a shield finger, like these: https://www.ttiinc.com/object/Harwin-EZ-Shield-Fingers
I might try yo investigate their RF properties, if nothing more obvious comes up.


best regards
Morten

 

[FvM]

Surface mounted spring contacts should work well, if you can provide a suitable finish of the aluminium case. The small inductive series impedance can be compensated by the antenna matching network.