The BIG and SMALL ANTENNA topic part 2


(Heath Raftery) #149

14dBi? Phwoah. Probably a great option if you can also add an additional low gain antenna within the building for reception below the high gain antenna.


(Mrpackethead) #150

Im not sure if i belive that it really is 14dbi


(Ud Lo Ra) #151

Thanks - this is the serious version of my incidental finding :slight_smile:


(Tony Smith) #152

@dicktonyboy, early last year you were modelling antennas. I’d appreciate your advice if think the modelling software would be suitable for a Meandering Monopole, something like this one http://www.rroij.com/articles-images/IJAREEIE-1596-g002.html


(Kylix) #153

Interesting antenna (maybe for a node): https://www.amazon.com/RFSPACE-Ultra-Wide-Band-Antenna/dp/B07NPRC1SR/


#154

interesting yes, and it probably works on a node, but with an VSWR >1.8 @868 there are better (and cheaper ) choices imho.


(LoRaTracker) #155

On the large size, maybe useful on the front end of a spectrum analyser.


(Tony Smith) #156

I would have thought an antenna on both a node and a gateways needs to be somewhat frequency selective to filter out unwanted signal and improve SNR. The antenna is the first filter in the receiver chain. A wideband antenna will pass un-necessary RF to the receiver thereby reducing SNR and performance. Seems OK for a spectrum analyser or a broad band receiver but can’t see a benefit for Lora based systems.


(Jofleck) #157

Hi I’ve built a outdoor gateway using a IC880a and a raspy zero W. Im totally satisfied with the range but I didn’t use the whole potential because currently the Antenna of the gateway is located inside of the case.

Maybe someone made already good experiences with cheap outdoor antennas?


#158

I am working on a future LoRa/LoRaWAN YouTube tutorial (Mobilefish) about Antenna’s. In the EU antennas are used with a freq. of 868 MHz. In the US an antenna freq. of 915 MHz is used. But which antenna freqs are used in other regions? I have searched the web but did not find good answers.
I have included a table with the min and max freq as can be found in the TTN freq plan and also included the average value (fmin+fmax)/2).

My question is:
If you use one of the other TTN freq plans what antenna freq are you using? The last column in the table is MY ASSUMPTION it may be completely wrong.antenna_freq


(Tony Smith) #159

You should only need 3 different antennas, one for bands in 400Mhz range, another for the 86xMHz bands and a third for bands in the 900MHz range. Have a look at the “The big Antenna S11 catalog” where you can see actual antenna performance displayed on a VNA. You will observe they work across a range of frequencies not just a very narrow band.


#160

Thanks @TonySmith! S11 yet another antenna parameter to study :grinning: and it makes sense.

Is the following statement correct?
Devices (end nodes) using US902, AU915, AS920, AS923 and KR920 freq plan can use an off-the-shelf rubber duck antenna 915MHz but make sure the antenna S11 < -10dB and VSWR < 2 at the required freq.
Devices using EU863 and IN865 freq plan can use an off-the-shelf rubber duck antenna 868MHz but make sure the antenna S11 < -10dB and VSWR < 2 at the required freq.

But for CN470 I am not sure which off-the-shelf rubber duck to use (433 MHz??).


(Tony Smith) #161

@robertlie, you could make that recommendation. We all have our own “rules of thumb” and I prefer VSWR to be < 1.5.

A far more important message is “BEWARE many antennae do not work as advertised” For example I have a 915MHz antenna which is tuned to 868MHz. It sort of works in the 915MHz band but has severely limited the performance of my nodes. Knowing a lot of antennae are not as advertised I purchased from a reputable supplier and still got caught.
Therefore to have some idea of what you are buying, information sites like “The big Antenna S11 catalog ” are an important reference. Otherwise check with the supplier for the antenna’s data sheets / performance graphs.


#162

@TonySmith, I will mention the message “BEWARE many antennae do not work as advertised” in my tutorials, because it is absolutely true. I bought a N1201SA Vector Impedance Analyzer and tested it on TTN gateway antenna (Kickstarter) and to my surprise it was a 915 MHz antenna instead of 868MHz and to make sure that I did not make any mistakes I opened the antenna and take some measurements to confirm it.

Yes, you should always check the data sheets, but how do you check the antenna itself if you have no special equipment such as the N1201SA? I can only think of 2 basic tests:

  1. If it is a simple sleeve dipole (rubber duck, no coils inside), open it and do some measurements. You can find the difference between 868 and 915.
  2. Measure the RSSI if gateway and end device is 10m apart with direct line-of-sight. If the RSSI is > X than the antenna is wrong.

Of course both tests are NOT great but what else can you do? A software developer like me, who just want to learn LoRa/LoRaWAN technology and build software applications, is not going to spend money on special measuring equipments.


(LoRaTracker) #163

Comparing antennas good v bad is easy, if you have a couple of Arduinos and LoRa modules, see here;

Testing antennas is easy


#164

@LoRaTracker, great read! I was planning to do almost the same. At my location there are several gateways at different distances and directions. I was planning to build several simple antenna’s (1/2 wave, 1/4 wave with radials, coil antenna …) cheap rubberduck and these antennas will be mounted on the same test setup on the same end device. Measuring the RSSI’s at different gateways on TTN console and demonstrate how the antennas compares to each other.


(Tony Smith) #165

@robertlie, in addition to the post by @LoRaTracker, some points you need to consider when setting up a test site with a remote gateway.

As you would understand consistency in the test site is important. Since the wavelength of the radio signal for the 868/915 is in the order of 300mm (1/4 wave in the order of 80mm) then moving the test site even a few millimetres from one test to the next can have an effect.
Nearby objects including the person conducting the test will have an effect. They need to be at least 3 wavelengths away from the antenna and transmitter and 10 wavelengths would be better. eg don;t hold the transmitter during a test.

In free space the signal attenuates by the distance squared (d^^2) but if located indoors (eg an office with cubicles, furniture, wall and ceilings) it can climb to the 4th power of distance (d^^4) . So be outdoors.

You need to be careful of multi-path. Not only does the signal arrive at the receiver by the direct line of site, but also a number of indirect paths bouncing off nearby structures. Therefore its important to locate the test site not only in a direct line of sight of the gateway but also pick a location where multi-path is minimised. How do you quantify this? Well, in your video blog of Fresnel Zones if you have a reflected signal from a object/building located just outside the first Fresnel Zone then this signal subtracts from the direct line of sight signal. Compare this to a multi-path signals created inside the first Fresnel Zone, this one would add to the direct line of sight signal. FYI, the definition of the First Fresnel Zone is the cross-over from adding to subtracting.

@LoRaTracker set up a test site with both the transmitter and receiver clear of any obstructions, which you can’t do when using a Gateway as the receiver. So the message is, pick a test site to minimise the effects and be sure the transmit location is at the same location each time (ie within millimetres).


(Remko) #166

Antenna measurements that can be reproduced and deliver either a gain or radiation pattern are labour intensive to setup. A general known principle is the substitution measurement method that can be used in either an anechoic chamber or on an Open Air Test site (OAT).
ETSI is describing the method as well as other standarisation institutes:


I think this method is out of scope of what we do here. For the purpose of indication of a gain while comparing antennas we can use simplified methods like described before.


#167

@TonySmith, thanks I will take your points into consideration.


(Radius) #168

Hi guys,
I wish to build gateway in Czech Republic, I will use unused RP1 (SW from this repo https://github.com/ttn-zh/ic880a-gateway/wiki).

And which antenna should I buy?
this


or this

or another one?

Regarding on limitations, max transmit power is 25mW + 0.1% “on air time” in Czech, right?


Site 3, label h, h1 or h2 probably.

Antennas are high gain, is possible to limit from software power (and probably “on air time”)? I have found that on new ttn router is it possible (but no information on limiting on air time), but this repo has some kind of older (generic) router? Is another supported pack of software for RPi1?

In Czech is very bad (mostly no) coverage, I wish to get most coverage what could it be.

Thanks Radius


Antenna_gain in dBi or dBd?