Testing LoRa Antennas at 915MHz

I’ve been working on LoRa systems and really wanted to find a ‘best of breed’ antenna to use. So, I found an inexpensive but accurate antenna tester and rounded up a bunch of antennas.

Antennas with the Analyzer

This started as Testing Antennas using the LimeSDR Mini, but what I found was that, at least for now, I’m unable to get consistent results from it — and it’s very slow. So, instead I’ve switched to an N1201SA.

Over time, if I get more antennas, I’ll update this article to include them.

Antennas

  • Short stub: I have a bunch of these, not necessarily all the same (but they all look the same). The ones I purchased deliberately are here. They are advertised as GSM on 868/900/915 (?).
  • Folding stub antenna for 868–915 RFID. This came from Microcontrollershop.
  • Skinny foldable antenna. I have no idea where this came from.
  • Wire. A simple 3.1" wire is a quarter-wave at 915MHz.
  • Tall (dipole?) antenna rated at 915MHz. This was purchased here.
  • Molex internal ISM (868/9156MHz) antenna 105262.

Impedance Testing

To test the antennas, I measured impedance and SWR at 902, 915, and 928. The actual ISM band range is 902–928 and my devices wander around the band so I tested the entire band.

Although the AAI unit has a calibration mechanism I wasn’t able to get good results from it without a calibration kit (on order), so I used the short connection. This suffers from the fact that there is a partial metal base (ground plane) which changes results somewhat.

I did a second test with a really crude ground plane, so I’ll come back and update this when the calibration set comes in.

Short Stub

The stub antenna had a reasonably steep SWR curve with a minimum SWR of 1.0 at 890MHz rising to 1.2 at 902Mhz and 1.6 at 920MHz. At 915Mhz the impedance was 56+22j.

With a ground plane this had a minimum SWR of 2.0 at 870MHz rising to 3.7 at 920Mhz.

Folding Stub

The folded stub antenna (when straight up) had a moderately flat SWR curve with a minimum of 1.4 at 990MHz and rising to 2.0 at 900MHz. Impedance of 66–33j at 915Mhz.

With a ground plane this had a minimum SWR of 1.1 at 890MHz rising to 1.4 at 920MHz.

Skinny Foldable Antenna

The skinny antenna (when straight up) had a moderately flat SWR curve with a minimum SWR of 1.4 at 900Mhz through 920MHz (!) with an impedance of 71–1.5j at 915MHz.

With a ground plane this had an SWR minimum of 2.0 at 850MHz rising to 2.4 at 920Mhz.

Long 915MHz Dipole Antenna

This antenna never worked right. It has a minimum of 1.0 SWR at 450MHz and increasing to 2.0 at 700Mhz then going totally out of range. When bent it was just as bad. I don’t think it’s a 915MHz antenna.

With a ground plane this still sucked.

Summary

This test had a small partial ground plane. As such the best antenna (totally surprising me) was the skinny foldable antenna. The foldable RFID antenna was second, followed closely by the stub.

With a ground plane the foldable stub was by far the best antenna.

Transmit and Receive Testing

For the first test, I’ve set up two transceivers using LoRa protocol. One is based on the InAir9B module while the other I’m going to swap modules around.

They were placed on tripods about 20 feet from each other and then, on the second unit, I swapped out antennas while watching the RSSI (signal strength) values. Crude but useful.

There’s no reason to test the garbage antennas, so I’m just looking at the three best-of-breed. The foldable antenna and skinny antenna are both fully unfolded.

Station with InAir9B Semtech-based module

Antenna   |    Result
----------------------
Stub | -57
Skinny | -54
Foldable | -56

Station with RFM95W module

Antenna   |   Result
--------------------------
Stub | -53
Skinny | -52
Foldable | -53

Summary

Not surprisingly, the antennas perform almost identically. I’m not at all sure why the RFM95 seems 3dB better than the InAir9B. Either it matches the antenna better, transfers power better, or I’ve simply not done the placement of the transceivers well.

I’ll redo these tests far more carefully when the calibration kit arrives in a week or two.