Client server WiFi structure

Question

We have 3 measurement system and a master computer. Only the master computer have access to the Ethernet because of this we would like to use WiFi connection, but for this purpose I have no idea which part fits the best. The measurement systems are 500m to 2000m away from the master. I found this solution but one of this is cost around 850Euro.

I have no idea where can I buy the antenna and the modules and which does it fit to this purpose.

Answer 1

Good directional antennas at both ends can extend WiFi to 2 km, but it won't be legal. Maybe it's not a issue if you're in the middle of nowhere or your beam is well focused such that it doesn't spill past your own installation. But that only means you are unlikely to get caught, not that it's legal.

Directional antennas are said to have "gain", but what that really means is gain in some direction as apposed to either a isotropic radiator or a dipole. With a properly designed antenna, the WiFi circuitry can't tell the difference and it will send out the same power RF signal. The high gain antenna will concentrate this power in a narrow beam, thereby increasing the radiation levels along that beam compared to a normal antenna. However, the legal limit is specified as a particular maximum field stength (Volts/meter) at some specified distance from the transmitter. This limit must be met in all directions, hence making conentrated beams with the same transmitted power as a dipole illegal.

In practise, the FCC (in the US) doesn't go looking for such violators. It would take a complaint by a neighbor or someone to get them to look into it. If you can focus the beam such that it stays on your property or ends at your installation by hitting the ground, going up, or getting blocked, then in practise you'll be OK. Just don't go around advertising the fact that you're doing this.

Answer 2

We aren't able to tell you which parts to buy - we're not omniscient (well, most of us aren't).

But we can spark off some ideas of how best to tackle the problem.

From what I understand you have 3 sensors at different locations with a central device which records the data from these sensors.

There is some more information you're going to have to give us for us to give a top-notch answer, such as:

• Are the sensors within a line-of-sight zone of the base station, or do they surround it? ie:

          s
                            s
  b   
                 s
  

or

                   s


      s                b    

                                      s

(b = base station, s = sensor)

• What kind of data rate is required to communicate the data in?

Are we talking data that can be transferred at say 56Kbps, or does it require megabits of data?

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Some thoughts on what you have given us to work with so far:

WiFi over 2Km

While this is possible, it is not easy. WiFi, in its standard form, can only really cover a few hundred meters at best. With a better aerial you can boost this to maybe 500m. To get the long distances you are talking about you would need a very special aerial - such as a Yagi Array, a Waveguide, or a parabolic dish. All of these are directional, so will only be able to cover a small line-of-sight area.

Note that all "improved range" aerials don't boost the power - they just focus it differently. Omnidirectional ones sacrifice vertical coverage in favour of horizontal coverage. Directional ones focus all the energy into a single beam or cone.

Have you considered other solutions instead of WiFi?

Other possibilities:

1. Use the ISM band at a lower frequency (say 868MHz or 433MHz) that will give a longer range (due to longer wavelength) for the same amount of power.
2. Use a third-party carrier. Mobile phone communications are getting cheaper and cheaper. With 3G coverage getting more and more prevalent, what's to stop you having a 3G connection on each sensor and using the internet to transfer the data?