The BIG and SMALL ANTENNA topic part 2

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.

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)

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.

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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.

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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.

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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.

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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.

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Its not surprising to me.

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!

I did mean to say dBm as in;

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

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

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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

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.

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!)

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It surprises me. Because we (foreigners to radio stuff) tend to think that distance is linear with size of the antenna. And I said size, not just radiation power or other tech stuff.

I would not expect X10 but X2 or X3 for 6db antenna. That’s why I was asking if it’s still possible 1-2km on city.

Anyway I also thought placement was important. But really I discovered is much more important. In fact my expectation was about 1km with antenna inside home, and about 1,5 or 2km in the outside. And my reading where 500m best case, in the window of the house (outside).

Now willing to put higher and out. To see who it improves. :wink:

But really antenna math is surprising.

I take the opportunity to ask: " 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) ." Why do you say 1000 times better? I know db is logaritmic scale, but how do you calculate it?

This is the process

  • Step 1 - Determine dB figure (71dB - 41dB) to get 30
  • Step 2 - Determine power by dividing dB figure by 10 (30/10 ) to get 3
  • Step 3 - Determine the multiplication using the power on 10 (10 to the power of 3) to get 1000

Example 1
OK lets say the dB figure is 3dB (which we know equates to doubling)

  • Step 1 3dB
  • Step 2 0.3
  • Step 3 10 to the power of 0.3 gives 1.9953 (which is usually taken as 2)

Example 2
OK lets say the dB figure is -3dB (which we know equates to halving)

  • Step 1 -3dB
  • Step 2 -0.3
  • Step 3 10 to the power of -0.3 gives 0.5012 (which is usually taken as 1/2)

what you have to do to understand logs is this

  • Be happy with the concept that any positive number can be expressed as a power of 10
    e.g.
    0.01 === -2
    0.1 === -1
    1 === 0
    3.16 === 0.5
    10 === 1
    31.6 === 1.5
    100 === 2
    1000 === 3

  • Be happy that once you have the “power” representation, then multiplication and division is really addition and subtraction of the powers
    e.g.
    10 (power of 0.5) x 10(power of 1) = 10(power of 1.5)

This is why dB is so useful - because it is the log representation of gain or loss, so whilst you are in this domain, it is easy to calculate the gain/loss because you use addition/subtraction.

If one talks about dB, then it is just a way of talking about “gain”
If one talks about dBi, dBd or dBm then it is no longer a “gain” figure as such because it is referencing “something”
If one talks about dBi, then it is not a gain as such, but a “perceived gain in a certain direction” referenced to the condition of an isotropic antenna . It is a slightly meaningless idea really, because no SI units are referenced. dBi references the “fluffy” notion of how the power shape of an EIRP situation changes i.e. it describes the Effective power gain in a particular direction which of course ignores the Effective power loss in other directions.
Because this “fluffy” nature of dBi (and dBd) can be misunderstood, a lot of people may think that dBi is associated with WATTS (a scalar SI unit), but is not, dBi is more about describing the “focussing shape” of an antenna, but it does this in the LOG domain

Now dBm also references something, but this time it is a SI unit (milliWatts),
CONSIDER THIS… dBm IS NOT A “GAIN” VALUE AT ALL, IT IS AN ACTUAL TANGIBLE DISCRETE VALUE OF POWER (described in LOG form).
e.g the power value of 25mW is expressed in the log domain as 14 dBm (The formula is 10 x log (25/1) ).
The advantage of doing this transformation of the power value from the world of “Watts” to “dBm”, is that power is now expressed in the LOG domain (and this means it can be easily added/subtracted with other “non-SI unit” LOG values e.g. dB /dBi / dBd, but you should not add/subtract a “SI unit” LOG value with another “SI unit” LOG value e.g. adding 2 dBm figures together - if you are doing this, then chances are, something has gone wrong when you transferred values into the log world)

N.B. The value in dBm describes a power in Watts by comparing what the gain would have to be compared to 1 milliWatt (which is an SI unit).

The reason why things still work if people conceptually misunderstand what is going on, is because the domain of logs just works anyway (addition/subtraction) - but there is the catch that if you use dBi, then the gain value is always the same as dBd + 2.15.

It is reasonable to assume (if the math is not known) to think that the more expensive an antenna, the more it can magically increase distance (a bit like thinking a more expensive sports car gives you more speed).
The truth is that there is no free lunch - if you want twice the distance with your existing antenna, you need 4 times the power (erp), and using an expensive appropriate antenna does not usually give the increase in distance as much as people may think because for Lorawan, an omni directional antenna in the horizontal plane is prefered (think of the torch example in the video) and people usually disregard the need for proper placement (because it is not told to them enough! :wink: )

There is a mathematical limit to how “good” an antenna can be - and it is not really linked to size of antenna - Whilst a “long” antenna may be needed to house the folded loop dipoles of a collinear antenna array , there is a law of diminishing returns here, and with impedance and loss considerations, there is a natural limit beyond which there is no benefit to having a longer antenna.

If anything, putting a small dipole on a long stick would give most of the same antenna “distance increase” as a collinear (compared to previously having the antennae indoors) :wink:

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yes - its not suprising to think things that way.
However, the maths say that a 6dBi antenna will only give a 50% increase in distance when compared to a cheap dipole (e.g. from 300 m to 450m) if the antennae are used in the same place

Thus, on a 14 dBm limitation, you can’t have better than an antenna of 0 dBi if you’re transmitting power is already 14 dBm ?

In Eu I believe the 14db limit is set allowing for the use of a 2dbi ant (2.15dbi?), and if you have a feeder cable, pig-tails and connectors that induce some loss then you can compensate with better ant (1db of connector/adapter and (ultra)low loss, say 2-3m of RG58 or 5-10m of Aircell5/7 or LR-400, between GW connector and ant would allow for a 3db ant. Hopefully someone will shout if I’m wrong on this one… :slight_smile:

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Correct, but please see slight adjustments below - to try and make this tricky thing clear.

  • Thus, on a 14 dBm (ERP) limitation, you can’t have better than an antenna of 2.15 dBi if you’re transmitting power is already 14 dBm (ERP) (N.B. its often rounded down to 2dBi for easy calcs)

  • Thus, on a 14 dBm (ERP) limitation, you can’t have better than an antenna of 0 dBd if you’re transmitting power is already 14 dBm (ERP)

N.B. The ERP specification means that the power limit is expressed when using a dipole antenna.
N.B. A dipole antenna can be expressed as having 0 dBd gain or 2.15 dBi gain.


and as Jeff says, you can mitigate all this by adjusting for all the inevitable losses that occur (e.g. in the connectors and cable). Because of these losses, you can legally use an antenna of higher gain.
This is how you can calculate it.

e.g.
    output power limit        = + 14 dBm (erp)
    loss in SMA connector     = -  1 dB
    loss in cable             = -  2 dB  (for 20 metres @ loss of 1db per 10 metres )
    loss in antenna connector = -  1 dB 
    Antenna gain              = +  4 dBd (antenna is advertised as 6dBi) 

If you go through the calculation above, you see that the predicted power output from the antenna will still fit within the specified power limit because the losses match the antenna “gain”.
i.e. Even if an antenna with gain of 6dBi is used with RF power at the limit of 14dBm, the resultant field strength radiated by the antenna will still be within the limit.

Remember also, that the antenna is focussing the radiation and that the field strength is measured in the direction which gives the maximum strength.

For the calculation above, I used arbitrary dB figures for the connector and cable losses (e.g. 1dB per 10 metres). If you want more accurate “dB loss” values, you need to look up the spec sheets and check what the loss figures are for the frequency used e.g. 868Mhz
e.g. Have a look at this pdf link for a cable spec (check your downloads folder) which seems to suggest a dB loss value of about 1.5dB per 10m (i.e. 15dB per 100m)
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwi_8ZTP5P3cAhVOiRoKHfXKBJ0QFjAAegQIABAC&url=https%3A%2F%2Fwww.pasternack.com%2Fimages%2FProductPDF%2FRG400-U.pdf&usg=AOvVaw2F2LkOUIJW6NOrdeoaI20Y
Look for the “Electrical specifications by frequency”. What you see is that the cable loss value per metre is dependant on the frequency used - it increases with higher frequency.
Obviously for Europe and USA, we are looking for what the value would be at arround 900MHz).

If you have the correct specification sheet for the cable and the connectors you are using, and you know the antenna gain value (dBi or dBd), you should now be able to put it all together to calculate the dBm figures that would be output (by your antenna) at different lengths of cable.

Alternatively, you can use this on-line calculator
http://www.timesmicrowave.com/calculator/?productId=52#form

There are other considerations, but you can usually ignore them as long as you have a well matched antenna (50 ohm) and you are using a cable with the same “Characteristic Impedance” (50 ohm again),

The main thing to take away, is that antenna manufacturers always quote dBi figures - which is naughty really because power limits are almost always specified with ERP (and not EIRP).
What this means is that you have to “downgrade” the gain value to the correct figure by subtracting 2.15 dB from the dBi figure that the antenna manufacturer quotes.

Its no wonder that antenna manufacturers are quite happy for buyers to mistakingly believe that the more expensive the antennae, the more “magic smoke” the antenna has, and also the continued use of dBI to make their gain seem better (which just misleads the unwary :wink: )
The other takeaway is that distance gain is proportional to the square root of antenna gain

What this means is that it is realistically impossible to get an horizonally omnidirectional antenna with a reasonably focussed vertical radiation pattern (suitable for Lorawan) that achieves a distance gain greater than 4 times over a dipole positioned in the same place

Appologies for repeating the same thing

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