I have a NanoVNA V2 Plus (50kHz - 3GHz) and decided to test my LoRa antennas:
- an unknown antenna delivered with a LoRaWAN gateway (RAK831)
- a 868Mhz antenna from Linx Technologies (ANT-868-OC-LG-RPS)
As the range was poor between my gateway and my devices using the first antenna (1), I thought that maybe this was an antenna for US LoRa frequencies (915MHz) so I bought the second antenna (2) that works in the 868Mhz band. The datasheet of (2) contains the following figures:
However, when I measured the antenna response with the NanoVNA I got very different results:
In my results, the VSWR is ~2.7 at 868Mhz and never go under 2.5. It also seems from the smith chart that the antenna has too much inductance. As a comparison, the response from the unknown antenna (1) looks much better:
I configured the sweep from 840Mhz to 940Mhz with 0.25Mhz steps. I calibrated the VNA with the included calibration kit. The antenna is connected directly to the NanoVNA SMA connector. Since the antennas use SMA-RP, I used an adapter and compensated for it in the calibration by adding 57ps in the port length extension configuration. This is the first time I use a VNA and mostly followed the Andreas Spiess NanoVNA V2 tutorial.
Could such a large discrepancy between the datasheet and my result come from the lack of precision of my cheap VNA? The fact that the SWR is much better for the unknown antenna (1) makes me think that the Linx antenna (2) is crap. However, it does not match the analysis from the datasheet.
I am misusing the VNA? What could explain this behavior?
Edit: I made some new measurement, this time with a 30cm SS405 cable between the antenna and the NanoVNA.
unknown antenna (1) without touching the coax (antenna end): unknown antenna (1) touching the coax (antenna end): Linx antenna (1) without touching the coax (antenna end): Linx antenna (1) touching the coax (antenna end):