Monday, June 13, 2011

Measuring RF Antenna Gain in dB-MEG

Typical Dipole Antenna Radiation Pattern
Isn't it fun when you think you know the ins-and-outs of a technology, then run across something that fundamentally has you scratching your head in confusion? You've been working with a technology for quite some time and you think you've seen it all, or at-least heard about it all if you haven't had the need to work with specific pieces.

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
Let me provide a little bit of background. The retail organization where I work owns and operates dozens of Distribution Centers (warehouses) that accept bulk product, process, and ship it out to hundreds of stores apiece. These warehouses have yard hostlers, essentially trailer cabs that ferry semi-trailers to/from parked positions and dock doors. These cabs are outfitted with rugged vehicle mounted mobile devices with a cable lead for an external antenna mounted on the roof of the cab. Since the mobile device in question only provides a single external antenna lead, signal reception (or lack thereof) becomes an important consideration given the lack of antenna diversity. Now, these devices have been around for a while and mobile device selection and ownership are owned by a partner team, not the Wireless Engineering team directly. Additionally, they were deployed before I joined the team.

Laird Phantom Antennas
The vehicle mounted mobile devices in the yard were installed with Antenex / Laird "Phantom" antennas (we also have more of these mobile devices inside the warehouse, but they are in operation with the built-in omni-directional antennas). These antennas are manufactured for use in mobile vehicular environments, typically in urban environments with high amounts of RF signal reflection, refraction, diffraction, and scattering. Additionally, these antennas don't use typical units of measurement, they use a new gain specification called the dB-MEG. It appears that this technology has actually been around for a while, referencing an IEEE paper from 1990. The vendor (Antenex / Laird) claims that these Phantom antennas provide "a radiation pattern tailored to the mobile environment" by tuning the directivity and polarity of the antenna.

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.

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.
Here are sample specifications from one such antenna that we have in-use in our environment:

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.



  1. Great post Andrew. I never heard about dB-MEG before as well.

    As for the antennas. I have great experience with these:

    If you want I can give you the direct contact, I am sure they will send units to you for testing.

    Regards, Gregor

  2. Thanks Gregor, I'll check those out.


  3. 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.

    Raxrad 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)

  4. Hi Deuce,
    That 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.


  5. FYI, just saw another one on a coworkers desk this morning. Belair BNCKG0083 (N-Type connectors, 11" omni, dualband)