|Typical Dipole Antenna Radiation Pattern|
In Wi-Fi we learn a lot about how decibels measure relative, not absolute, power output in relation to some other reference point. We have dBi (isotropic radiator), dBm (1-milliWatt), dBd (standard dipole antenna), etc. These are known quantities, and most Wi-Fi professionals worth a lick can run circles around antenna azimuth or elevation patterns and specifications using these units of measurement. We get them, we know them, we are "comfortable" with them.
Then something comes along and throws us off our game, and pushes us to question our own basic understanding of the universe of Wi-Fi (Hey, that's kinda catchy: "universe of Wi-Fi". I might have to start using that phrase). Such was the case last week for me. It all revolves around the db-MEG. "dB-what?" That was my first reaction too!
|Laird Phantom Antennas|
The dB-MEG is defined as follows:
Decibels Mean Effective Gain is used by one manufacturer. To establish the Mean Effective Gain for their antenna, received power is first measured and averaged using a ¼ wave whip (0 dBi) in a real mobile or reflective environment.Here are sample specifications from one such antenna that we have in-use in our environment:
The whip is then replaced with their antenna, and again measured in the same environment.
The MEG gain is then the ratio of the two received powers. On average their antenna receives twice the power of the ¼ wave whip in a mobile or reflective environment. Alternatively said, their antenna has 3dBi Mean Effective Gain.
|Laird TRAB24/49003P Sample Antenna Specifications|
Intermittently, we have problems with these devices dropping connection during roaming events. After we verified adequate coverage, ruled out interference, ran active tests and debugged the connection, I am ultimately questioning adequate signal reception by the client. Given two things really leads me to this conclusion: 1) The mobile device does not support diversity with external antennas, and 2) these Phantom antennas have lower gain than dipoles (when measured in dBi) and not quite omni-directional beamwidths. I can understand why our partner team chose these models, they are seemingly a good fit for mobile vehicles. However, their intended application in urban environments with high signal reflectivity and multipath is not even close to the same use-case as our warehouse shipping and receiving yards in rural American locations with vast wide-open spaces, fluctuations in trailer density in the parking lot, and minimal multipath. I am questioning whether the antenna radiation patterns (shown above) are causing the intermittent signal drop-out.
The problem isn't licked yet, but running across something new like dB-MEG presents a previously unknown variable to consider. I wasn't aware of this measurement unit in the market, and wouldn't have been unless I had run across this issue with an already deployed product in our environment or worked on an urban outdoor mobile vehicular project of some sort.
I'm still learning about these Phantom antennas, and have many more questions. However, based on these specifications and our performance issues, I'm not sold on their application in our environment.
I've added dB-MEG into my arsenal of Wi-Fi acronyms, even if it's categorized in the "nice to know, but not really useful" section.