PtP C5x with different sizes of modular antenna (horn and dish) in each side

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#1

Hi Everyone,
I am planning a PtP link between 2 buildings that are 3km far away, with quite the same elevation and good line of sight.

I have already done the Mimosa Design Tool to choose the better modular antenna. It says that the better results are with N5-X20 and N5-X25 and it is supposed to be no better behaviour from X20 to X25. So if i am a good boy, I should choose X20 or X25.

BUT…
In one of the sides of the link (the side where the internet is coming from…) I have a “aesthetic” limitation, so i cannot use “dish-like” antennas.
In the other side (the side where they are waiting for the internet), I have no limitation at all.

SO… i am planning to use X12 or X16 in one side and X25 in the other side
What do you think about this scenario? I know it is not the better scenario, but it is what i have…

It would be better to use X16 in both sides because is much better to be the same than more dB in one side?
Maybe X12 in one side and X25 in the other side be enough? This would be better for me for avoiding esthetical problems with the property…

Thank you all for your advice

#2

Looks like you have a couple questions I will answer them as best I can in the order you asked them as I understand them, feel free to correct my understanding of your question.

  1. N5-X20 vs N5-X25 no difference in performance? Yes, but with a large dosage of not exactly. While you could get the same modulations on each end of the link, you will most likely need to use more power on both sides of the link to achieve the same SNR (Signal to noise ratio) with the N5-X20 vs the N5-X25. You also will be more susceptible to noise with the N5-X20 which will hurt link performance a lot. My recommendation is to use as high gain of an antenna as you can wherever you can to help with these and other issues I have not mentioned for the sake of brevity.

  2. What we think about using an X12 or X16 antenna on one side and an X25 on the other? Overall you are loosing some amount of signal strength, which will hurt the performance of your link as compared to using two X25s. That said, if you used an X16 antenna and an X25 antenna you should have 1dB greater signal strength on each end when compared to the link just using two X20s. Signal strength is a factor of how good your alignment is, how far you are shooting, antenna gain (both on the transmit and receive sides) and transmit power. So, with all other things being equal, by having a 25 dB antenna on one side and a 16 dB antenna on the other side you will have a slightly better performing link, in theory, over having two 20 dB antennas.

  3. Is is better to match the gain on antennas that are communicating with each other, ex. an X16 on each side of the link? No, if you can get a higher gain antenna you will almost always have better performance on the link overall using it. There are some exceptions, but they are not very relevant to this situation in my opinion.

  4. Would an X12 work for the link if paired with an X25. Probably, depending on how much noise you have in the area and as long as your fresnel zones are completely clear. (AKA, you have absolutely no obstructions on the path of the link whatsoever.) If you have much radio noise in the area it will be very difficult, your signal to noise ratio will go down which makes it hard for your radio to tell the difference between noise and data and forces the whole link to talk slower to keep the link up. If you can, use the X16, 4 dB doesn’t sound like much, but it might make a world of difference in the view of the amount of noise you would have to deal with. If you have to use a X12 antenna, then use as small of a channel as you can, 20 MHz would be best.

#3

Thank you for your opinions, so
1- Message received, i will use as big as they let me put in each roof
2 - Ok, i will consider X12 nearly as a no-go
3 - Ok, the bigger, the better (exceptions you refer maybe could be clients very close to the base station, below 500 meters or so, no relevant here as you said)
4 - same as 2, by now considering X12 as no-go. I also have checked the radiation patterns, and X12 and X16 are much wider (not a surprise) than X20 or X25 that are the narrow ones.

Conclusions:
I also have noticed that the X20 or X25 dishes are only sold in packs of 2.
So, i will buy:

  • 2 C5x units
  • 1 pack of 2 X25 dishes, since they are not sold separately as far as i know

I will go up both roofs and install them, trying as discreet as i can, I will try to hide it withouth comprimising the fresnel zone.

I promise some pictures and results

#4

Where are you located? Here in the USA the FCC has made a ruling that if you are dealing with an HOA they can’t limit communications antennas so you can hold that up to them if that is what you are dealing with.

That said, it’s freqently not a great plan to piss people off especially if they live around your equipment and you don’t.

If you can get a clear shot with no fresnel zone obstruction with a smaller antenna, but you can’t do so with a larger antenna you could be hurting yourself more then you are helping.

To clarify, I oversimplified things in #2, signal strength is a product of of transmit power + antenna gain - path loss (distance + obstructions, aka stuff that gets into the fresnel zone) + receiving antenna gain - alignment issues (because bad alignment means the signal doesn’t get through). Still a gross oversimplification, but gives you the idea.

Also you have the advantage of the antenna where you presumptuously want most of the data to be flowing toward can be larger. If you are using mismatched antennas the larger one will have the better receive signal (because it’s able to collect more of the weak radio waves) which means the data throughput will most frequently be higher going towards it then away. Most people download most of the time and upload rates can be significantly slower then the download rates and not cause issues.

That is guessing that the house that doesn’t have the antenna restrictions is where you are going to, if it’s the other way around then you don’t have that advantage.

All that said, if you can’t get a single N5-X25 and your stuck with a 2 pack and don’t want to have a spare laying around, then as my father said, “it’s easier to beg forgiveness then ask permission”…