Our first guest post! (thanks to Nathan @plainsimple) and a follow up to the first antenna shootout we did a few months back. Enjoy!
Quick disclaimer – I am not a RF expert rather I am a technician with some knowledge in test equipment so take this commentary as my impressions.
The setup for this testing is pretty rudimentary in that it is conducted indoors with the RigExpert being supported in the upright position attempting to create an impression of free standing in air. **Spoiler alert** I deployed the RF Shop 6dBi with a Helium hotspot and got some readings that represent an actual installation.
Let’s start with the newest kid on the block the recently released RF Shop 6dBi antenna. According to the good folks at RF Shop this antenna has been tuned for AU915 band ( 915MHz to 928MHz). Out of the box and directly connected to the RigExpert I got a SWR reading of 1.16 at the 915MHz with a band of +/- 25MHz.
Remember this number as it is useful to compare to my latest hotspot deployment. For the purposes of LoRa this is pretty good, anything below 1. 5 SWR is what you are needing for your setup. So let’s take a quick look at the second pic of the SWR range curve.
Here you can see as the frequency moves towards the 928MHz it is getting close to 1 before springing upwards. So this accords with the RF Shop information that states that the centre frequency of this Antenna is not 915MHz but closer to 928MHz. Does not having the centre frequency at 915MHz matter? In my opinion – No! All that matters for Helium/LoRa is for the anything around 915MHz to have a SWR less than 1.5. Now for those in the 868MHz range this antenna also performs exceedingly well. There is another SWR curve dip at this frequency….but I forgot to take a pic of this. A follow-up test once I order another antenna will come and to confirm from my memory that at this frequency this measured around the 1.18MHz as well.
Next is the McGill 3dBi. Ok taking a look at the SWR measurement it gives a solid and beautiful 1.08 and the SWR graph shows a dip in the 912MHz to 913MHz.
So all up an excellent reading.
Finally, I have an Elsema 4dBi. I have this antenna deployed on a hotspot and it is performing ok. It scores a 1.37 SWR and the range graph shows that it’s centre frequency is probably towards the 900MHz area and not 915MHz.
In comparison this antenna is performant but not as efficient as the McGill or RF Shop antennas.
Testing with brackets
For kicks I thought I would quickly run the same tests with the antenna bracket from the McGills to see if the aspect of the ground plane was a factor. I found a difference but not being a RF specialist I am cautious about what it is meaning. However it definitely influences how I will approach my hotspot setups.
So let’s take a look at the results starting with the RF Shop 6dBi. Here you can see the SWR went down to 1.06 from 1.18. This says to me that this is an optimised setup.
Next the McGill 3dBi….and the SWR reading is 1.32 which is substantially up from the SWR of 1.08.
I do not know what is happening here. Any RF Engineer care to advise? Note in practice I have been running this setup with one of my Hotspots for a month now and it is a solid performer. This lends weight to the assumption that provided your antenna SWR is less than 1.5, then the performance of the antenna and its setup has less of an impact on the performance overall.
Let’s not leave out the Elsema 4dBi. The SWR comes in right on 1.5.
Note though this antenna has a different bracket for its ground plane and the fit on this for test purposes was not great. I would probably discount this reading because of the poor setup.
Testing flat window pass-through cables
Changing tack a little, raw antenna SWR numbers are good for generic antenna selection but in reality they are just one part of the antenna install. The other parts are the Hotspot (RF filter and concentrator and internals which is manufacturer design parts) and the coax cables that connect the hotspot to the antenna. Conveniently I received a couple of window pass through coax cables I was in need of as I was given permission to install an external antenna provided I didn’t ‘hack’ through the walls. I had ordered two different lengths – 240mm and 460mm. Let’s hook these up to the RF Shop 6dBi antenna (the one I was about to deploy) and see how much difference these things make.
My going in impression would be that longer cable would have a bigger influence on the overall SWR than the smaller one. Note the test setup is not optimal here as I had them laid out on the floor. Ok the the results are in reading SWR 1.27 and SWR 1.32. But the slightly poorer performer was the shorter cable (240mm).
Influences here are obviously the length, the connectors including how tightly I reconnected the cables to the RigExpert etc. A SWR difference of 0.05 I would say is within the noise. Finally take a look at that graph curve and its dip towards the 928MHz – exactly what RF Shop said it would do!
The installation – tested
Alright so the final part to this test is to see what the SWR would be for my hotspot install – a real world installation. The installation consists of a Heltec Hotspot, the 460mm window pass through cable, about 6m of CLF195 coax, a RP-SMA male solder/crimp connector, a N type female connector, a 320mm antenna eaves mast, the McGill antenna attachment, the RF Shop 6dBi antenna and some self-amalgamating tape. For completeness I have the Heltec hooked up to the Gl.inet myfi 4g modem.
For this setup I was expecting a SWR closer to 1.4 and was indeed hoping for less than the upper limit goal of 1.5. Surprisingly I got a reading of the SWR at 1.18.
I’ll take that!
There are many things here that may not be adding up particularly if you just read the raw numbers from the tests I completed. Honestly some of it I have no idea. I will surmise that testing horizontally is not optimal, testing not in ‘free air’ and needing to mix and match the right components make a heck of a difference.
Overall though this exercise and my short 3 months of having a hotspot of different makes deployed with different setups lends me to conclude for the Antenna components just keep your SWR under 1.5 and all will be well.