The BIG and SMALL ANTENNA topic part 1

which do you recommend ?

  • stupid question… off course the ones you sell in the webshop :wink:

did you ever tested this type of outdoor node antenna ?

http://www.ebay.com/itm/868MHz-communication-antenna-RG58-50cm-inner-hole-waterproof-IP67-RP-SMA-male-/263022485657

By far the best antenna I’ve used (and probably the ‘general best’ 868MHz antenna available) is the Procom CXL 900-3LW-NB/868MHz, but at €165 it’s not particular realistic for the average tinkerer.

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Unfortunately I’ve never tested those kind of outdoor antennas, usually we try to get the most performance out of small and cheap antennas! I am a big fan of the molex antennas, but you need to mount them to plastic in order to tune them properly. I’m still in the process of finding good ‘whip’ antennas that have a SMA connector!

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yes I’ve bought also several different ‘china’ types :innocent:

We did an 868 MHz antenna test recently:

https://www.coredump.ch/2017/04/13/lorawan-868mhz-antenna-test-part-1/
https://www.coredump.ch/2017/04/30/lorawan-868mhz-antenna-test-part-2/

Interesting how far some antennas diverge from their specification.


I have a question related to dipole antennas: A single element dipole cannot have more than 2.15 dBi gain, correct? So all those dipole 5dBi gain whip antennas available on Ali Express are by definition impossible to have that much gain, correct?

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like this one, less then one euro in quantities and shipped with all kind of lora products

Whip antennas are by definition monopoles, but when placed on a conducting surface act (somewhat) like dipoles. Some whips have grounding radials in the base of the antenna, but that does not make it a real dipole though. Most whip antennas increase gain by adding resonators or loading coils, but I would take those gain figures with a big grain of salt.

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A new and very interesting outdoor antenna option: Sirio GP 868 C (dedicated for the European 868 ISM-band) is now available for ordering: http://www.k-po.com/SIRIO-GP-868-C.2.html

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Did someone do actual VSWR & gain test for those 868MHz fiberglass antennas on Aliexpress?



I’ve seen that @GryKyo and @Epyon use similar antennas but I find the technical characteristics announced by the seller to be, ahem, extremely dubious. Still, even if you ‘only’ get 6dBi out of their 10dBi antenna, it’d be cheaper than the Procom.

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received this one last friday … no time to play yet :sunglasses:

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I did some quick testing of the popular Sunhans 13dBi antenna and the A&C 14dBi compared to a regular ground plane antenna (2.19 dBi) by analysing the RSSI of a fixed node around 500m LOS away. While both antenna’s functioned better than the ground plane, they certainly don’t reach the advertised gains. I would guess something between 3-6dBi at most.

While I’m at it, I should add my contribution:

A few months back, we bought cheap antennas on eBay, advertised as “7dBi 868MHz antennas”.

They were dirt cheap, so while we knew the 7dBi figure was utter nonsense we decided see if they couldn’t replace our plastic indoor antennas.

Well, no. We had them tested in a RF chamber at Orange Labs, and they expert opinion was that they were “unusable”. Something about a 15dB loss…

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Hi, is anyone using this collinear coaxial antenne solution for lorawan?

http://jeroen.steeman.org/Antenna/collinear-coax
http://www.rason.org/Projects/collant/collant.htm

The solution seems very popular on 1090MHz ADS-B;

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Mostly, antenna gain is misunderstood.

For example, if you have a mobile antenna you’d better off with a 0dBi antenna rather than a 10dBi antenna. The more gain the antenna has, the more energy will be bundled in one direction. Think of putting a reflector behind a light bulb. All the light hitting the reflector will be reflected, no light will go through.

For a mobile antenna you don’t want ‘black spots’. Effectively, you want your sensitivity/gain be the same in each direction (0dBi). This is called isotropic. Any misalignment in the pattern will yield gain in some direction. A dipole is not fully isotropic (radiating less in line of the dipole and radiating more perpendicular to the dipole), think of a radiation pattern like a squeezed ball.

Stationary (gateway) antennas will have more likely gain in horizontal direction and less gain vertical direction. This can be achieved by stacking dipoles for example.

An antenna can not output more energy than goes in. On the other hand, an antenna can ‘loose’ energy. When there’s a mismatch in the antenna (system), part of the energy get’s reflected and will not reach it’s destination. This is measured in VSWR (or in S parameters). A VSWR of 1 is most ideal: no energy is reflected.

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And when you see that thin coax cable (mostly RG174), you can easily count 1 dB loss for each meter (@ 868MHz) .

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It is not necessarily correct to say a pure isotropic antenna would be better for a mobile node, for two reasons:
1/ 1km away from your antenna, the ‘narrow’ cone of your radiation pattern will be hundred of meters wide, so you wouldn’t have any issues hitting a gateway
2/ Closer to your antenna, there would be radio blind spots. Most of them are above or bellow it, so in most circumstances they matter precisely squat. It does matter, however, when you are close enough to a gateway you somehow ended in its shadow area. Thankfully, it’s not the end of the world, because the indirect path capabilities of LoRa are pretty impressive, and you would probably get a decent signal reflected from somewhere.

Another note: isotropic antennas and 1:1 VSWR are impossible in real life, they’re just used as handy points of references.

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This is the best document I found so far for understanding antennas and their specifications:

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Gig,

  1. These lobes (or cone as you describe) will remain the same angle, regardless of distance. In other words, if your lobe is 36˚ wide and your antenna is in any random direction, you’ll have 10% chance this lobe hits your gateway.
  2. These antenna blind spots are due to a non-isotropic radiation pattern. That’s why an isotropic pattern is best if your antenna is mobile in any direction.

And agree on your notes, of course ‘ideal components’ doesn’t exist in real life :wink:

I think you misunderstood what I meant.
Let’s assume there’s 36° between the -3dB axis on each side of your lobe, on the vertical plane.
At a distance d, the vertical distance h between those two -3dB axis is still 36°, so h = d*tan(36/2).

Unless I’ve screwed up my calculations as I often do, at 1m, that’s 30cm high. At 100m, it’s 32m, and at 1km it’s 325m. So, unless your target is more than 325m above or below you, at 1km you are ALWAYS in the -3dB lobe.
Conversely, if your objective is to cover a wide area, using an antenna with a lesser gain (relatively to an isotropic antenna) will make you radiate a lot of energy toward the sky and the ground.

To make an analogy with light: many streetlights are big, round globes illuminating equivalently all around them. You could see them as low gain antennas. They also illuminate straight up, which is completely useless, because it’s the ground to want to see. Now, there are also streetlights that are pointed toward the ground - and with the same energy, they will provide more useful illumination than the omnidirectional streetlights.

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