Antennas for Wireless Sensor Networks

[pucca]

Hi, I have an issue with antenna implementation in a WSN. Imagine that we have tree nodes that want to communicate. Each node has two antennas of the same type. The signal is splitted through a splitter towards the two antennas. So the first node n1 can communicate simultaneously with the other two nodes n2 and n3. If the n2,n3 transmit simultaneously, how can I separate the signals to the n1 (without conflict)?

 

[pucca]

Is there some sort of buffer for the received signals?

 

[VEBill]

Two antennas with a "splitter" forms a two-element antenna array with directive properties related to wavelength and spacing. Details are lacking, but it is very likely that this is not what you want for this sort of application.

The reason many 2.4GHz system have two antennas is for spacial diversity. It allows at least one antenna to not be in any multipath signal null. But this is accomplished with dual receivers and complicated voting circuits, not a splitter.

Two nodes transmitting at the same time is called a collision. Some systems (TCP/IP) will uses acknowledgments to manage the flow of data. For sensors, it is common to simply miss an update until the next cycle. Other than steerable nulls and extremely complex antennas, managing collisions is not in the domain of antennas.

 

[pucca]

The only way to get two different signals is to have two different receivers? If I want to have the two antennas is there some way, perhaps with a switch that I could choose the mode between them?

 

[VEBill]

That's not what I wrote.

You should provide more information about what you're up to before I start making suggestions.

Also you should use Google (and even Wiki) to research further into the keyword topics I mentioned.

 

[pucca]

Hi, I will eventually use two antennas. When transmitting both of the antennas will be used, for the receive mode the antennas will work by turns. Is it better to use a splitter to separate the signal for the two antennas or is it better to just have two different paths on the pcb (how can I switch the mode between the two antennas more easy ?)

 

[VEBill]

Please review my previous posts.

 

[Comcokid]

What you are after is called a "diversity antenna" arrangement.

For small low data rate devices, a pair of antennas will be used. The antennas will be seperated generally around a 1/4 wavelength apart, and if possible for the enclosure, will be oriented at right angles to each other. The idea is if one antenna is in a null of the propagation path, the other antenna will probably not be. The device will switch to one antenna at a time, usually with a RF switch.

Diversity can be done for both transmit and receive, but to make maximum use, the choice of which antenna to use will be dependent upon information about the signal strength of each antenna on receive, and data from the other node receiving the data about which antenna worked best for the transmitted data. There are no standard algorithms for this, you will have to develop one that works best for your sensor system.

Transmitting with two antennas does not 'double' your signal - it just makes for some non-onmidirectional gain pattern. Likewise, receiving with two antennas using a splitter does not double your signal strength - it just cuts the signal level by half or slightly more. This is why antenna diversity in low data networks is usually done with a RF switch.

 

[pucca]

Thanks a lot about the reply. What would you consider about using MIMO antennas?

 

[Comcokid]

The MIMO approach takes the multiple antenna approach to the next level in performance, and complexity (both hardware and software).

In my previous statement, I indicated that: 'with two antennas does not double your your signal - just makes for a non-onmi gain pattern'. MIMO begins to exploit multiple antennas to deliberately create a non-onmi pattern, because you increase the gain in a particular direction, both transmit and receive. This is achieved with a lot of added complexity. But, for most low-data rate systems, such as wireless sensors, the last thing you want is complexity because it comes with increased problems with installation, maintainence, and troubleshooting - three things that can result in a high field return rate and unstasified customers.