Installed at three locations in Haiti during the summer of 2014, this modem has become an object of some interest. Many of its features are attractive within the developing world:
- Easily available on Ebay
- Operates over 11-15V DC in and is therefore compatible with direct connection to deep cycle lead-acid batteries.
- Antenna input for operation in low signal strength areas.
- Browser based information available about signal strength, and data band used by the Internet Service Provider.
The 3g modem was tried in the Bay Area of California on the T-Mobile and ATT cellular data networks. The signal strength on both networks was marginal at my location –in the -95db area.
I played around with the ATT signal more, mostly as a result of my inexperience. The first thing I did was to look for, and purchase a high gain antenna. I assumed that operating in the 900 MHz range was what was required. The antenna arrived in pieces. But it turned out, when I looked the connection details, on the information page provided by the Netgear router, that the ATT signal was indeed on the 900Hz band, but the T-Mobile signal was using 1800MHz. I really should have ordered a dual band antenna (more like http://www.amazon.com/gp/product/B00A4EK44Y/). But the exploration I did with ATT was instructive.
(a very inexpensive way to start learning about 3g modems is to buy a pay as you go data plan from “H2O” –$10 to get started).
One of the first things I wanted to find out about was how much current the Netgear box needed. I fired up my adjustable power supply. What I discovered was that the Netgear has switching regulators that demand more current as the voltage is lowered. It needs about 5 watts and any voltage between 8 volts, and 15 volts ( at nominal 12v input, it draws about 400mA).
There is a configuration check box, labeled “use internal antenna” which lets the user select the external antenna if the box is unchecked.
One of the interesting discoveries was that the internal antenna is relatively poor at responding back to the cell tower. I was using “speakeasy.org/speedtest” to verify the modem’s function as I varied the power supply voltage. I discovered quickly that the upload test would fail at any power supply voltage less than 12v (though the download test worked down to 8v, the lowest voltage I tried). My cell phone measured -97db, and the netgear measured -95db, on the ATT signal, so the tower is not close.
But when I switched to the external antenna, upload performance was unchanged at the lowest (8v) level input. Both directions are essential for internet access to function properly. So the ability to connect an external antenna becomes a strategic advantage for the Netgear device.
The coax connection between the antenna and the Netgear needs careful attention. The directional antenna I bought has an advertised gain of 15dbi, which is equivalent to 12db gain over a dipole. The relatively inexpensive low-loss coax I bought has a loss of 16db per 100 feet at 1800MHz. So an antenna high on the roof, connected by a long run of coax, might not do much good. But this is where Power Over Internet (POE) cable can be used effectively. The Netgear can be mounted on the antenna pole, inside some sort of protective case (plastic food storage container comes to mind).
The coax can be a problem in another way. Even if the Netgear is mounted on the antenna pole, and is only 6 ft long, it needs to have the correct ends on it. In this case the antenna was type-N female, and the netgear was type-tnc male (also called reverse-polarity tnc). Specifying the correct ends for a ready made cable can cause grey hair.
The simplest POE is achieved by passive power inserters. Currently Amazon has them at http://www.amazon.com/HCP05-Passive-Injector-Splitter-Connector/dp/B00DZLSRJC/ for less than $10.
With POE, the concern becomes the amount of voltage drop because of the resistance of the ethernet cable. Used in this way, cat5 ethernet cable has a total loop resistance of 3 ohms per 100 feet. Since I measured the Netgear current in the 400mA range for 12v input voltages, this implies that it would receive 10.5v at the end of a 100 foot run. I saw normal performance at 8v, so probably 10.5v would have sufficient margin to be reliable.
The main advantage of 12v passive POE is that everything can run off of a 12v deep cycle battery, and stay powered during the frequent interruptions in grid power that are found in most developing countries. Kids using XO laptops, android tablets, can still do their work when the power goes out.