Last week I have evaluated the RAK 6,1 dBi beta antenna. I evaluated mechanical, S11 (reflection), antenna gain, and radiation pattern. These are my observations and conclusion.
The antenna is delivered with 2 clamps for installation. while I initially did not know how they should be used I eventually found a hint in the document “RAK_860-930MHz_5.8dBi_Fiberglass_Antenna_Specification.pdf” that was published earlier in this thread.
The disadvantage of this construction is that the stiffness with which the antenna is held up depends on the type of L-shaped rod. When the antenna is under windy conditions the vibrations of the antenna might result in metal fatigue. See Figure 1.
The advantage of this construction is that it is possible to install the antenna against a wall or mast. Disadvantage is that this will influence the radiation pattern and result in directivity away from the structure against which the antenna is installed.
Conclusion: The delivered clamps are usable but are not the best solution available. It is adviced to use clamps that allow installation on top of a pole.
Figure 1: Viberation that might result in metal fatigue
S11 measurements (return loss)
The antenna is specified to work from 860 to 930 MHz while delivering VSWR better than 1:2.0. This VSWR is equivalent to a return loss of -9,5 dB. The S11 parameters ware measured using a minivna tiny of mRs miniRadioSolutions. On the band edges of the antenna and on 896 and 915 MHz as these frequencies are of primary interest. The result is in the following table.
The figures below are the source of the previous table.
Conclusion: While professional antennas of Kathrein are specified at an VSWR of better than 1:1.5 (equals -13,9 dB return Loss) the RAK antenna performs better than this target value and can compete with the “professionals”.
When analysing the full band the RAK antenna does not meet the “professional” target but they are close.
Figure 2: S-11 measurement (Returnloss) of Rak gen.2 antenna over specified band
Figure 3: S-11 measurement (Returnloss) of Rak gen.2 antenna specific for 868 MHz
Figure 4: Smith-chart of Rak gen.2 antenna over specified band
While I have no anechoic chamber nor a Open Air Test site (OAT) available I used the gateways of TTN-Apeldoorn and a reference antenna to compare the RAK beta antenna to. This results in a method know as the substitution method to determine the gain.
Gateways included in the test.
For data collection I used TTN Mapper app and a dedicated node.
I two runs of approximately 15 minutes I collected around 100 measuring points over 6 gateway’s. The first run the RSSI was measured using the RAK beta antenna. The second run was measuring using the reference antenna.
RAK antenna measuring
Test equipment, tripod with node
While studying the measurements it was found that one gateway had obstructed line of sight. This results in an anomaly and this data was discarded.
Both averages are subtracted and the result is the gain difference between the antennas. While the reference antenna is 0 dBd the gain is in dBd.
-103,8 – 107,5 = 3,7 dBd gain.
While the reference is 0 dBd this is 2,15 dBi.
2,15 + 3,7 = 5,9 dBi.
Conclusion: The gain specified by RAK is measured with this measurement.
The radiation pattern is required to plan radio coverage using simulation tools like Radio Mobile. Therefore the radiation pattern is required in both H- and V-plane in a 1 degree resolution.
RAK only delivers data form a TRP measurement (3D) from which the required patterns can be extracted. The TRP pattern is at 30 degree intervals.
Conclusion: While competing manufacturers like Sirio provide the required radiation patterns RAK does not.
The new 6 dBi RAK antenna really delivers what is being promised. While the antenna mount is not a rigid solution it might work in many situations. It would be a welcome improvement when the clamps ware replaced by more mature ones that allow installation direct on the top of a pole. Th eproposed ones are shown below:
As I do simulate my gateways before installing I find the poor resolution of the radiation pattern a disadvantage. When RAK issued high resolution radiation patterns the can compete with the professionals.
Compared to many other antennas that have chinees origin the new RAK antenna has serious advantages. Serious value for money at low price.