WiSense node antenna orientation

In this post, I am going to talk about how antenna orientation has an effect on a node’s range.

The WiSense CC1101 base nodes utilize a whip antenna. See the pic below.  This is a quarter wave, reduced height, omni-directional helical whip antenna. It connects to the RF board through an SMA connector which allows the antenna to be removed.

As mentioned above, this antenna has an omni-directional radiation pattern. The radiation pattern of an antenna determines it’s coverage.  Let me quote Wikipedia’s description –

“In radio communication, an omni-directional antenna is a class of antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna’s axis. This radiation pattern is often described as “doughnut” shaped.”

omni_pattern

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Imagine this doughnut (in the pic above) placed on a table with the hole pointing towards the ceiling. As you can see, the radiation pattern is uniform in all directions in the horizontal plane (parallel to the ground).  An antenna with this radiation pattern will have the same range in all directions parallel to ground but it’s range in the vertical direction (perpendicular to ground) won’t be as much. Let us compare it with the isotropic radiation pattern shown in the figure below.

Isotropic 1

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You can see that the isotropic radiation pattern is symmetric in all directions which means any antenna with this radiation pattern will have the same range in all directions. Unfortunately, such an antenna exists only in theory but it serves as a good reference for our discussion.

node_pattern1

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The Linx antenna used by WiSense nodes have an omni-directional pattern. Imagine the antenna at the center of the doughnut hole pointing towards the ceiling.  These antennas are what you need if you need good coverage in the horizontal plane (parallel to the ground). For example, you have a bunch of WiSense nodes (all antennas pointing skywards) spread out over a soccer field.  and the nodes are more or less at the same height above ground.  In this scenario all the nodes should be able to talk to each other. Now, suppose you place one of the nodes at some height above the ground (say 10 meters),  this node may go out of coverage of the other nodes.

Suppose you have two WiSense nodes at either ends of the soccer field. Assume they are sending packets to each other. Note down the RSSI (received signal strength) in dBm of frames received by each node. If the nodes (and therefore the antennas) are pointing up, then they will be able to talk to each other. Now suppose you tilt one node (and therefore the antenna) parallel to the ground, you will note a significant drop in signal strength of received packets. You might loose connectivity altogether. Now suppose you tilt the other node so that it is also parallel to the ground. You will loose connectivity completely since the radiation patterns of the two nodes are now vertical to the ground and parallel to each other.

How about two WiSense nodes close to each other (say a meter apart) with one node pointing towards the ceiling and another node pointing sideways (parallel to the ground).  The RSSI will still be less (compared to the case where both are pointing towards the ceiling) even though both are pretty close (relative to the maximum range of WiSense nodes (> 300 m in the open). This has to do with antenna polarization. Electromagnetic waves radiated by an antenna constitute an E field (electric field) and an H field (magnetic field) perpendicular to each other as well to the direction of propagation of the wave. The electric field or “E” plane determines the polarization (orientation) of the electromagnetic wave. An antenna is vertically polarized (linear) when its E field is perpendicular to the Earth’s surface. The Linx whip antenna on a WiSense node is a vertically polarized antenna when the node is pointing towards the ceiling.  It is important that two wireless devices (which want to communicate) with each other should be identically polarized. That is, the E-plane of the radio waves produced by the two antennas should be parallel to each other. In a linearly polarized system, a misalignment of polarization by 45 degrees will degrade the signal up to 3 dB and if mis-aligned by 90 degrees the attenuation can be 20 dB or more*.

You can easily see the effect of polarization using two WiSense nodes. Take one RFD/FFD and let it connect to a coordinator node. Keep the nodes a meter or two apart. Configure the RFD/FFD to send data periodically to the coordinator and monitor the data on a laptop (to which the coordinator is connected).  The sample UI app (from WiSense) prints out the signal strength (RSSI) of each packet received from a remote node (in addition to sensor data). Start off with the RFD and coordinator pointing to the ceiling. Note down the RSSI of say 10 packets sent by the RFD. Now tilt the RFD so that it’s antenna is parallel to ground. Now note the drop in received RSSI.

References:

Posted on December 12, 2014, in Uncategorized. Bookmark the permalink. Leave a comment.

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