RF Hardware
The 868MHz antenna is a compact helical antenna developed by TI. By the datasheet's own admission, the efficiency is poor which ensures the range of the Internet Bridge device is less than it could be had a less space constrained design been used.
The differential RF output of the CC1101L, pins 12 and 13 feed a balun to provide a single ended feed to the antenna. The RF signal passes through an insert-to-break connector which is likely to be:
Finally the signal passes through a three element matching circuit and onto the antenna.
When connecting an analyser to the RF test connector it was observed no RF output. Upon further investigation it was found that the RF test connector is positioned in an unusual orientation; when the test probe is plugged into the RF test connector, the CC110L is disconnected and the test probe is connected to the antenna.
| Test probe disconnected | Test probe connected |
|---|---|
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Closely inspecting the RF test connector datasheet the orientation for signal in -> signal out can be observed by looking for the notch in the very centre part of the connector. The notch indicates the input side of the connector, but in the Tado design the notch is oriented towards the antenna.
One solution to improving the signal range might be to reverse the orientation of the test connector, and plug in an external dipole antenna. using one of the test probe cables mentioned above along with the appropriate SMA gender changer to an 868MHz antenna.
The test connector was removed and a replacement fitted (Digikey part 490-4981-1-ND / Murata MM8130-2600RA2) in the correct orientation, note the notch in the centre is now to the left side towards the CC110L:
A test connector with 868MHz dipole antenna was connected and it was found the range of the device improved by a factor of about 50%!
