The article IoT – exploration of LoRa – part 3 showed some components of a simple LoRa system.
This article reports measurements made on two antennas used in the prototype system.
Above is a view of the prototype system.
nanovna
Measurements were made with a nanoVNA-H v3.3 modified to fix a power supply decoupling design defect, ttrftech firmware 0.6.0, and nanovnasharp MOD3 PC client.
The scan spans the transition from third to fifth harmonic mode, and since it appears little is done in the firmware to deal with the risk of glitches, we might expect some. We might also expect worse measurement noise above 900 MHz because of the fifth harmonic mode.
RF Supplier GM2-1669-S01SP4-035
The RF Supplier GM2-1669-S01SP4-035 is the rubber ducky antenna on the LoRa – Wifi gateway module.
Lots of antennas in this form are sold out of China, and many if not most are just frauds by the Chinese cheats. This one is from RF Supplier who has a better reputation than nameless eBay or Aliexpress sellers.
Above is the |s11| plot (-ReturnLoss, -RL) which looks fairly good in the sense of wanting to see medium to high RL around 920MHz.
There is considerably more measurement jitter above 900MHz as expected, and there is a glitch at 900MHz.
Above is the same data expressed as VSWR.
Again There is considerably more measurement jitter above 900MHz as expected, and there is a glitch at 900MHz.
Notwithstanding the shortcomings of the nanovna, the antenna appears to be acceptable for the Lora 920MHz application.
Molex-105262
The Molex-105262 is a broadbanded dipole antenna with integral feed line and U.FL connector. It has a self adhesive back and is designed to be applied to an plastic insulated equipment enclosure.
For the purpose of these measurements, the antenna was secured to a ABS box to get a fair idea of the actual tuning of the system.
Above is the |s11| plot (-ReturnLoss, -RL) which looks fairly good in the sense of wanting to see medium to high RL around 920MHz.
There is considerably more measurement jitter above 900MHz as expected, and there is a glitch at 900MHz.
Above is the same data expressed as VSWR.
Again There is considerably more measurement jitter above 900MHz as expected, and there is a glitch at 900MHz.
Notwithstanding the shortcomings of the nanovna, the antenna appears to be acceptable for the Lora 920MHz application.
Frustrations
These measurements were made with the nanoVNA attached to a PC, and the thing kept resetting as the USB cable wiggled a little. Substituting a known good USB cable did not fix the problem.
It appears that when the nanoVNA loses the USB connection, even for the briefest period, it resets. It may be that the USB-C socket on the nanoVNA is actually faulty, and I see reports of others complaining the the sockets or the cables are fautly, but I am of the view that it may be that the nanoVNA is not tolerant of very small transients that are normal with wiggled USB connectors… IOW another design defect.
This constant resetting wasted so much time that I would reach for anything else rather than endure this.
Update 05/12/2020: The problem described here got so bad that the nanoVNA was destined for the bin in no time. I did source a replacement USB socket and it was replaced, ,nanoVNA-H – continuing USB-C woes #2, it no longe has the USB jiggle issue. Chinese quality… bit of an oxymoron really.