We got to semifinal of hackaday prize(yay!) , which also means some extra founds to pour in to the project. One of the things that cost money but it is well worth of buying one over building one is proper BIG outdoor antenna for the gateway which can cost around $100.
- By proper I mean designed to withstand elements(UV, snow, ice...) while maintaining performance at set frequency through years.
- By BIG I mean, the larger the better because there are either small OR good antennas. Similarly to camera optics where bigger aperture means better images in the dark/dusk in RF bigger antenna has better performance receiving very week signals send by low power SLoRa nodes from far away.
But there is another parameter to keep in mind directivity(misleadingly called gain) of antenna. Since antenna doesn't have built in amplifier the Gain is not actually amplification but directivity or ratio between radiating with same power in perfect sphere(0dBi) or radiating more sideways(in this case) and less up-down.
Luckily for me, the list for only 868MHz antennas is not long and I have 5dBi(~40°) and 8dBi(~27°) to chose from.
Before choosing antenna I have to know where it will be positioned. Since I want a gateway to be placed as high as possible I marked the potential spots, checked the ownership of the land at possible spots to know who to ask for permission for placing the antenna "tower".
We have managed to find the location that matches all the above criteria and even has a line of sight to our house which will enable us to use ESP-07 module with directional antenna to upload data to the internet. Using Ruler Tool in Google Earth I can make a cross-section of the hill to see elevation profile of the hill.
Cross-section of the hill with slope converted to degrees(in green)
On the left we have the average slope of 10°(14° max) and on the right 17°(21°max). On one side we don't want the antenna radiating to much in to the ground because of reflection and on the other hand we don't want to have any blind spots.
In my case I am estimating that relative vicinity to the antenna will be enough to get LoRa packet through even from the spots on the right of above cross-section, while in both North-South and East-West cross-sections I have mostly elevation profile around 12-14° x2 = 24-28°(times 2 because omnidirectional antenna radiates equally upwards as downwards). Therefore I will chose the 8dBi model to reduce reflection from the ground and also increase the long range sensitivity.
When antenna arrives and we set up the gateway I hope for 20km line-of-sight range and at least 2km non-line-of-sight(hopefully).
(even though 2G antennas are designed to operate at same frequency ~900MHz + 1800Mhz, they are usually smaller since they are meant to work with higher power over shorter distances)
For better feeling how omni antenna radiates just google: omni antenna radiation pattern