Jun 17, 2020

6 min read

Testing and Reviewing LoRa Antennas

Part 2: Detailed antenna measurements

This is a continuation of the last article about measuring LoRa antennas. In this section I go through the impedance measurements (VSWR or Return Loss) for each antenna in gory detail.

There is one more antenna (called Yagi):

The beam covers the ISM band (900MHz) as well as some higher frequency bands. The label says 698–960MHz & 1710–2700Mhz.

FrSky 900MHz Moxon Zipp 9

This is one of the more interesting antennas. It comes with radiation pattern documentation (+1). Not documented, it ships with an SMA-RP connector so I had to add a barrel SMA-RP -> SMA adapter (-2). $25.99 at Amazon.

Looking at the traces on the back of the antenna it appears to be a dipole with an additional dipole beam that I assume adds some directionality/gain.

The return loss chart is very good. The 900Mhz dip is deep (so good match) and the rest of the range is appropriately unmatched, other than a strange dip at about 750MHz.

The manufacturer provides this very-nice claimed radiation pattern picture which shows the greatest gain upwards and broadside. I’ve never seen that but it makes sense.

FrSky 900MHz Moxon Zipp 9 Radiation Pattern

FrSky 900MHz Super 8 Antenna

The Super8 antenna looks like a folded dipole and acts much like it. This was $15.98 at Amazon. Like its brother above it uses an SMA-RP so I needed a barrel adapter to SMA.

The impedance plot is good. Just like the Zipp9 it has a strange match area around 730MHz. Maybe these are artifacts of the short coax or the barrel adapter.

There is a less-exciting radiation pattern provided by FrSky:

This is extremely uniform except along the X axis (to the left and right). Spherical (technically cylindrical) while retaining a good impedance match.

Linx 916-HW

This is a conventional center fed dipole. This antenna is 5" end to end. This was $9.50 qty 1 at DigiKey.

The impedance plot is classically pretty.

There’s a small dip near Wi-Fi (maybe caused by my active Wi-Fi field) and otherwise it’s a narrow spike in the frequency band.

Linx 916RH

This is a quarter-wave helical whip that desires a ground plane. It’s about 2" tall. The full-range impedance chart uses a ground plane. This is $8.40 at DigiKey.

The impedance chart is very pretty. There’s a second moderate dip around 2GHz and that’s it. For a small quarter wave, the frequency range is fairly wide which suits the ISM band well.

Molex Flexible 102562

This is a flexible small antenna at about 3" by 1/3". It’s supposed to be attached by the single-sided tape on it to a plastic wall of the unit. My test unit did exactly that since any other testing wasn’t as optimistic. $3.74 at DigiKey.

The pattern visible on the front of the antenna shows the trace.

The display here is very good. Nice low return loss from 860 to 928 and then out-of-band elsewhere. It’s a bit lower in frequency than I’d like but usable — later note: the U.Fl barrel connector seems to be introducing un-fixable errors in the measurements so don’t take this as gospel. For <$4 this is a steal in terms of impedance matching. The antenna ends in a U.Fl connector and so this test requires an SMA-U.Fl barrel connector of unknown effect.

Linx Monopole LTE

The LTE band is near the ISM band and this worked really well in my last tests so I added it despite it not being a ‘real 900MHz’ antenna. I’m also not 100% sure this is the Linx Monopole because it came as part of a Samsung kit — but it sure looks like that antenna. It’s about 2.75" in height and has a hinge.

The impedance chart is pretty, but as expected the frequency range is lower than a typical 915MHz ISM antenna and it includes sub-bands for cell. I’m glad to be redoing these tests.

I don’t show those results here but without a ground plane this is a bad antenna since the impedance shift is substantial.

870–960 MHz Whip

This is a quarter-wave whip, so it requires a ground plane.

The impedance chart is again very pretty, although tuned slightly low in frequency for my purposes with best match at 874MHz.

Siretta Echo1A

The Echo1A is a pcb antenna. $6.21 qty 1 at DigiKey. Their doc calls it Quad band GSM and 3G as well as ISM: 868 and 915MHz.

As you can see in the image, it kind of supports ISM bands but matches far better with the GSM/3G bands. I’ve used it when I need something to provide a load for U.Fl but. Their documentation shows a bigger dip in the ISM range (although it’s not clear) so it may have been testing error.

Phonetone 7/9dbi Outdoor Directional Yagi

This is a rather large beam antenna. The doc claims less gain at ISM bands but it was worth taking a look at. It was $17.99 at Amazon.

The impedance chart is fairly spectacular given the specification. Not great if you want to ignore some of the frequency bands but I’ll bet you could pull a few beams off to improve that : )

It shows a near 1.0 VSWR starting at 700MHz rising to 2 at 940MHz. I’m looking forward to measuring the gain of this beast.