Testing & Reviewing LoRa® Antennas

Part 1: Overview and Summary Information

Mark Zachmann
Jun 13, 2020 · 5 min read

Selecting a LoRa antenna is tough. Measuring an antenna well is a hard task and almost no one does it. Many antennas available via the hobby channels report gain or vswr, maybe at a frequency, that they clearly often pretend to measure.

So, since I have some test equipment and I’ve spent quite a while picking LoRa antennas… I decided to review a bunch of them.

LoRa Antennas for Testing

I have a product that requires uniform high-quality LoRa communication and deciding on how to send that data has taken a very long time. Start by analyzing the various available antennas.

Earlier Article

What make a good antenna

Impedance Match

If an antenna is not 50 ohms at the frequency you’re using, then power is wasted (sometimes as heat) and component stress may increase. If an antenna is 50 ohms at frequencies you aren’t using then nearby radios could interfere with your reception or the radio could send noise on the wrong frequency (which the FCC really doesn’t like). We want the antenna to be a perfect match in our band and an open circuit outside our band.

Measurement of a reasonable antenna

The antenna impedance is measured at each frequency and we calculate how much of the radio power is being transferred and how well guarded we are from interference/interfering.

Radiation Pattern

A high gain directional antenna radiation pattern

Good commercial antenna manufacturers provide radiation patterns for their antennas. Most do not. Take a look at the very directional pattern shown above to see how important this is.

For my applications, I have two preferred patterns.

There’s a device that gets carried around with an antenna. It needs a uniform XY pattern so no matter how it’s pointed it still gets a good signal.

There’s a base station that communicates with the mobile. The base station is far enough away that a directional antenna can be pointed at the mobile area for additional gain over an omnidirectional pattern.

Nearby Objects and Ground Planes

Whip without and with ground plane

In the picture above are two ways to measure this whip antenna. The long coax and the raised platform give some distance from any metal surfaces so the left-hand image is ‘clean’. The right hand image includes a piece of aluminum foil that provides a ground plane (if the foil is a bit smoother).

The reason for including a ‘ground plane’ is that vertical whip antennas are designed to work best with a reflective floor (ground plane) to bounce off. Without that ground plane the design frequency and impedance all wander off.

Even non-metal things that are nearby matter. The Molex flexible dipole is supposed to be stuck on a plastic wall of the chassis and it was measurably better when that was the case.

In my case, there’s a nice plot twist at the end of this article.

Who is being tested

Impedance Analysis

Then, I retested in a narrower range to just look at performance around the 915MHz spot I’m using. The full US ISM band is 902–928 MHz. The below charts were imported into Excel so I could compare the various antennas at one time.

Return Loss Charts for all Antennas

I ran two narrow tests for the antennas that ‘require’ a ground plane. I tested without a ground plane and with a ground plane, so those tests are separated.

The Return Loss chart above shows the how much power each antenna reflects back to the source (figuratively). The return loss is the amount of power wasted — the bigger the number the greater the waste. A returns loss of -10dB (0.1) means that 10% of the power is wasted, -3dB (0.5) means 50% of the power, and so on (power x in dB = 10log10(x)). When you see a huge dip in the charts above that’s a good thing.

Wasting half the power loses 30% of the range.

Amateur radio operators usually use a measurement called VSWR which is based on return loss but is best at 1 and gets progressively worse as the number increases. VSWR is a good indicator of the additional stress on components caused by bad impedance matching. Here’s a comparison of both types of charts.

Return Loss compared to VSWR charts for the same data

See Part 2 of this review — details for each individual antenna.

Trademark: LoRa is a registered trademark of Semtech.

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