The BIG and SMALL ANTENNA topic part 2


(Bart S.) #69

Hi everyone,

I’m currently using my TTN gateway inside attached to my window, however it seems the isolation works really well… Which makes the signal really ineffective.

I’ve been walking around the neighbourhood with a sodaq one to send out packets with GPS data in them to map it to TTN Mapper and I’ve found out my range is only about 125m before packages are starting to be missed.
To make it even more embarrassing, someone else was actually able to receive those packets my gateway missed almost 5km away. Absolute defeat!

What options would I have for running a gateway outside? I’m alright with getting an antenna running on the roof if that would mean beating my competitor :face_with_monocle:

Any ideas? Tips? Let me know


#70

option 1
antenna outdoors… keep the (quality) cable short and use a good antenna

option 2
more complcated, make your TTN gateway waterproof and power it via POE… (only for experienced) and put it on the roof


(Bart S.) #71

I feel like option one is not going to work out as the shortest cable I would be able to realize from my room to the antenna would be about 2.5 meters.

Powering it via POE is no problem as I actually have a POE switch, does the TTN gateway have POE equipment on board? What options do I have for waterproofing the gateway?

What antenna would you recommend to reach distances of multiple kilometres?

Thanks in advance :smiley:


#72

2.5 mtrs is no problem imho and I don’t know your budget for an antenna… you could also make it yourself
I suggest you use search and read a bit on this board… lot of answers can be found :wink:


(Gaguilar) #73

Hi @CurlyWurly,

Thank you for the response. If you take a look to the pictures, I have some of those antennas, I brought 4. But then, the don’t fit specs. In fact the measurements are just so different that I don’t know how to fix them.

What I found is that everyone is getting 15km while I fight to get 500m. Something that bugs me.

About the connectors. You are soooooo right. I made the mistake because didn’t figured out what’s the RP stuff in the SMA connector. So I plugged it because it fitted, and only 64m range. Then I discovered that one connector was RP and the other SMA alone. So no connection on antenna itself. When I fixed it I got 500m.

But hey… How people is getting 10-15km on cities? How is that possible?

I don’t see it possible with current hardware. I replaced some BSfrance arduino modules with original ones (first 4 modules bought on china) and the range is exactly the same. The gateway seems to be ok. But I will also replace it.

Any help will be appreciated. And I think antenna is the major difference.


#74

The picture here is the insides of a cheap antenna I bought (should be like the ebay link given in post 59). The pic also shows the extra bit I soldered onto the tip.

The antenna was advertised as 868MHz, but when I checked with my N1201SA antenna analyser, it resonated more at 915MHz. I added the extra length, and it brought down the resonant frequency to 868Mhz (happy days!). Whilst I can’t give exact values for performance, I can say the RSSI did improve, so for sure, the antenna is still radiating OK etc.

Be aware that for resonance, lengths that are measured can be shorter than that predicted by the simple formula “C =freq x wavelength” This is because this simple formula does not include some other calculation factors that are required to be included in the formula. These extra parts can be too compicated and variable to include for this sort of antenna - Trial and error in cutting the length seems to adjust fine!).

In the end, the presence of the sheath reduces the overall reactive part of the impedance. In other words, you must put the plastic sheath back on. If you don’t the impedance is out. antenna_868_with%20extra_bit

Have a look at the following link (post 432) for more info about what I did

In the end, I decided that I wasn’t going to get a significant performance boost using a different antenna. However, I did find that if this antenna had been placed high up (with direct line of sight) in free space (and not inside a building on the ground floor), the performace boost was about 1000 times better (i.e. from -71 db to -41 db) .

When you are looking at antenna options, you won’t get one (outside of yagi) that is practically (and within reasonable cost/size considerations) more than 4 times better than a standard correctly resonant dipole (such as this cheap antenna when tweaked) . If you do, the polar pattern becomes compromised e.g. like a directional yagi.

This is the point - 1000 X performance increase is a lot more than 4 - and you will not get an antenna that gives the sort of boost that placement does !!

Anyway, it sounds like your gateway antenna is a bit iffy or its placement is wrong (near metal etc?)

When I checked the antenna of my TTN gateway, I found it was exactly the same as these cheap antenna. I made the change as above it definately improved the situation.
If its good enough for TTN gateways, its good enough for me (possibly the wrong assumption :wink: )

Also, be aware that even though you can stick a gateway against window glass, some modern glass has metallic coatings which severely attenutate the signals.

Theres no getting around it - place high up and free for max performance. There are other reasons why it should be even higher, mainly to help the resulting polar pattern NOT being skewed up away from a downwards direction (where your devices probably are!).
Also bear in mind that you do not need a ground plane with these cheap antennae

I know it doesn’t answer the question of “which antenna is better”, but I hope this info assists you with another direction for improvements - In that you should check “placement” first if you want long distance. If the antenna is for the gateway, then outside is better, so you should look for an outside antenna which is placed somewhere appropriate.
If it is for a node, then consider cheap antennae (if you can get ones with the correct lengths!), or a straight up wire at the correct length.

N.B.It does look that the ones you have pictured are not resonant for 868MHz
Hope it helps :slight_smile:


(Bart S.) #75

My budget would be between 100 and 250 euros (excluding some costs for cables etc.), if a slightly more expensive antenna would give a way bigger range I’m even willing to go for that. It just feels silly to run a gateway that barely gets further then your street right now

I prefer buying a pre-made antenna because I don’t have any proper equipment to verify the build and neither am I very skilled with producing things.


#76

Buy a Cheap antenna such as the one I have linked to (make sure it is correct connector - can’t remember if male/female).
It’s possible your antenna is faulty? - its cheap, and you can do a trial of “adjusting” it if you ever want to (or perhaps get someone else to do it for you)


(Gaguilar) #77

Yep! I saw your adjustments days before. And I followed them.

What you say about placement looks right. Since I don’t see any difference between setting a pigtail antenna and the antenna in my previous post. So as you say placement is all.

Currently my tests where done on the wild (inside building or on the window as you say) because no radio stores near me. Spain is a shame on tech shops, when I went to buy items they ask me. “What’s this?” “What are you talking about?” “This is impossible to work”, and they even don’t have material tunned to 868. So it’s tricky. No good cables, connectors, antennas and so on.

I will build the one that’s posted here and place it in the roof. Let’s see how it improves.

Thank you a lot for your assistance! Hope I get you in range.


(Jac Kersing) #78

You are reading the forum selectively, there are people getting 15k, however that is (mostly) line of sight. More people have complained about the lacking distance and have been told antenna placement is a key factor, In an urban environment with the antenna inside a building < 1km is realistic. With the antenna outside < 3km is realistic. With the antenna outside on a high building things are different, but still a limited range for nodes at street level and inside buildings.

Get a GP tuned to 868MHz and place it at a good location (as high as possible, nothing shielding it in any direction and no metal constructs parallel to the antenna). Even the most expensive antenna can not make up for bad placement.


(LoRaTracker) #79

Its always worth remembering that if you are already transmitting at the legal limt (25mW\14dBm ?) then you can use an antenna with a bit of gain to compensate for cable and connector losses etc.

However if your already transmiting the limit then adding a 10dBm gain antenna ‘for long range’ can take your actual transmitted power up to 24dBm, which is too much.


#80

Also bear in mind that 6dBi gain is effectively a doubling of the distance when compared to an isotropic radiator (if within the squashed polar pattern).

Lets say you’re only getting 300 metres at the moment with a reasonably performing cheap chinese dipole antenna (as above) which has been badly placed indoors.
You might think that your new “6dBi” antenna would double the distance to 600 metres if you put the antenna in the same place.
Nope - Whilst Field strength (E) equates to 7 * Root(erp) / distance, and the square root of 6 db is 3db (that is why it only equates to a doubling of distance rather than x4) the cheap Chinese dipole already has about 2 dBi gain anyway, so when you do the math, there is really a 4dBi gain (6-2), which equates to a “square root” distance gain of 2dB (4/2), which is roughly about a 50% increase (e.g. 300->450 metres ).

However, if you put that new “6dBi” antenna on the roof…Well, all of a sudden it becomes the best performing thing since sliced bread. :wink: Hence the reason why a lot of people rave about their new outside antenna, but the real reason why it is so good, is because it is better placed.


#81

Its suprising really, that even exceeding the limit as you say, when you do the math, a “10dbi” gain antenna does not magnify the distance by X10, its more likely to be 4 times less than that - at x2.5 when compared to a cheap dipole !!!

(10 - 2) / 2 === 4db == X 2.5

So if someone was getting 300m on a cheap chinese dipole, the “10dBi” antenna would only gain a distance to 750m (i.e. not what many may think they should be getting i.e. 3KM === 300m X 10 )

I think this is one of the reasons why so many get dissapointed with their new antenna, and hence the constant “same question” posts on this thread about their new expensive antenna not performing as they should!
Only reason for posting this is that it seems no-one has posted previously about this basic maths effect before.


(LoRaTracker) #82

Its not surprising to me.


#83

Can you recommend any “10dBi” antennas that can be used?
(its dBi by the way not dBm - bit surprised you got that wrong :wink: )

…only joking Stuart!


(LoRaTracker) #84

I did mean to say dBm as in;

“then adding an antenna with a power gain of 10dBm”


#85

Yes an interesting point (it was clear what you meant really :wink: ),
However, its worth exploring the definition of antenna gain though.

Antenna Gain: A relative measure of an antennas ability to direct or concentrate radio frequency energy in a particular direction or pattern. Typically measured in dBi or dBd.
http://www.antenna-theory.com/basics/gain.php


(LoRaTracker) #86

Sure, but when your trying to explain that adding an antenna with gain affects the actual radiated power, then using dBi or dBd add an unecessary step of confusion.

What I originally said;

“However if your already transmiting the limit then adding a 10dBm gain antenna ‘for long range’ can take your actual transmitted power up to 24dBm, which is too much”.

And reworded;

“However if your already transmiting the limit then adding a 10dBi antenna ‘for long range’ can take your actual transmitted power of 14dBm up to 22dBm, which is too much”

And no doubt if I had posted that, a lot of people would wonder why 14 + 10 = 22


(LoRaTracker) #87

It is of course, although most of the time spent in here on discussion and build of gain antennas is perhaps of academic interest only as such antennas are often illegal to use.


#88

I sort of get what you are saying, perhaps lets say it in the following way?
This is an area that confuses many and it seems to be confusing you too!

“The difference between EIRP and ERP is that ERP compares the actual antenna to a half-wave dipole antenna, while EIRP compares it to a theoretical isotropic antenna”.

so if the max power for 868Mhz is 25mW erp, then this is expressed in dBm as 14 dBm erp (10 x log (25/1) ).

Right…

  • There is no such thing as a passive 10dBm gain antenna - its a bit like saying you have 15 apples when there are really 15 oranges - Whilst the qty is correct. the UOM is wrong) . If it really was a 10dBm antenna, then an ampifier would have to be involved because you really are talking about POWER
  • if you use a 10dBd gain antenna, then the equation is corrrect = 14 + 10 = 24 dBm
  • If you use a 10dBi gain antenna then the equation is 14 + 10 - 2.15 = 21.85 dBm

If you use the term “dBm antenna”, you confuse those who know what dBm should mean, Using the term “dBm” means it would be reasonable to think that you must be talking about an active antenna system (which uses an powered amplifier to amplify the signal) rather than a passive one that merely shapes the total power accordingly (and doesn’t really alter the total value of the power being radiated out of the antenna)

This video nicely explains the concept of antenna gain. Note, even though he seems to say so, there is NO overall power gain as such - i.e. there is no energy being magic’d out of thin air, its just focussing - this is why it is so important to make sure that when discussing this, the correct terms are used for antenna gain (the dB figure is NOT based on power) otherwise it gives the impression that extra energy is appearing from somewhere (and it is not!)