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!
Vehicle Mounted Mobile Device |
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.
Cheers,
Andrew
Great post Andrew. I never heard about dB-MEG before as well.
ReplyDeleteAs for the antennas. I have great experience with these: http://www.luxulmilitary.com/products/
If you want I can give you the direct contact, I am sure they will send units to you for testing.
Regards, Gregor
Thanks Gregor, I'll check those out.
ReplyDeleteAndrew
Nothing against Laird (formerly Antenex) but that antenna pattern seems pretty poor. Not really close to omni, and looks like you could easily be 15dB down if the vehicle was turned the wrong way. If you NEED dual band your options are fewer.
ReplyDeleteRaxrad and Laird are pretty much industry standard for mobile antennas. You could try a MaxRad MLPVDB2458 (though I cannot find an antenna pattern) which has the same form factor as the Laird. Otherwise a 12” PcTel MMO24580608 would perform ~15dB better than your Laird but it is pipe mounted not mag mounted.
Cisco offers several outdoor omni antennas but most are single band.
What is a typical distance from the AP to the client, and what is the AP height? The 12" antenna is more appropriate for greater distances (so the angle between AP and client is small and you are in the main beam of the antenna)
http://www.antenna.com/apg_products.cgi?id_num=11060
http://www.maxrad.com/artifacts/201141pctel_broadband_mobile_antenna_brochure.pdf
http://www.cisco.com/en/US/prod/collateral/wireless/ps7183/ps469/product_data_sheet09186a008022b11b.html
Hi Deuce,
ReplyDeleteThat is the same conclusion that I have come to. Thanks for the links to additional antenna options.
The mobile vehicle mounted device is roaming throughout a large trailer yard, and could vary in distance from 20' directly underneath the AP at the dock doors to over 800' away at the end of the yard. The infrastructure APs which are mounted on the exterial wall of the warehouse with semi-directional antennas. The APs are approx. 30' high and the mobile device antenna on top of the cab is approx. 10' high.
Thanks,
Andrew
FYI, just saw another one on a coworkers desk this morning. Belair BNCKG0083 (N-Type connectors, 11" omni, dualband)
ReplyDeletehttp://www.tessco.com/products/displayProductInfo.do?sku=304088&eventPage=1