How to Use a Wi-Fi Grid Antenna to Boost your 3G Cellular Reception

By Aaron Weiss

November 29, 2007

Sick of diminished or nonexistent 3G coverage at home? Follow this step-by-step DIY guide to boost your EVDO service.

For some people, being offline can be described as either like breathing without oxygen, a mild irritant, or a relief. If the first two symptoms sound familiar, then cellular broadband might be right for you.

Sometimes known as 3G, major U.S. cellular brands Verizon, Sprint, and Alltel now sell broadband access using EVDO technology. Originally aimed at corporate road warriors, cellular broadband can make the difference between dial-up and staring at the wall for people in homes and offices beyond the reach of cable and DSL.

While cellular data service is generally fast and reliable within a few miles from EVDO-enabled towers, many people find themselves in so-called "fringe areas"—just out of reach of strong signals, whether due to distance, topography, or both. Being stuck in the fringe doesn't necessarily doom you to dial-up. With some modest parts and labor, you might be able to boost a lifeless, anemic signal into a healthy connection.

Signal matters

The strength of your cellular signal is measured in dBm units, a decibel-based power unit. If you are running the connection software that comes with your cellular modem, it probably shows your signal strength in the typical "four bars" graphic, which roughly correlates to pre-defined dBm levels.

Verizon customers can see the precise signal strength in dBm units by hovering their mouse over the "four bars" graphic at the bottom of the Verizon Access Manager window [below]. Other providers offer similar methods for viewing signal strength with more precision than the bar graph.vzam.jpg

Measurements are in negative integers, so the closer your dBm is to zero, the stronger it is. Generally speaking, a very strong signal strength would be greater than approximately -70 (again, greater meaning closer to zero). A very weak signal strength would be lower than -100. Worse than -105 and you will probably experience erratic, unstable performance, while worse than -117 may drop you offline.

Because dBm units are logarithmic, a +3 dBm increase represents a doubling of signal power while a -3dBm decrease represents a halving of power. Therefore, even a "small" change in dBm can represent a significant change in actual broadband performance.

Later when you experiment with an antenna, you will want to view the dBm reading as it changes. There is a short lag between movements of the antenna and updates in readings. It is also typical for the dBm to fluctuate, sometimes significantly, so you will want to look for an average reading over a minute or two.

Finally, you may receive the same broadband performance over a range of dBm readings. For example, you might find that a signal strength of -75 does not produce any faster speeds than a reading of -82, but that -82 is much faster than -90. This will vary by tower. You should test your network speeds at any of the many broadband speed test sites to find the most realistic balance of performance and signal strength for your location.

You do know jack

Cellular data modems are sold in two form factors—PC cards for insertion into laptop computers, and USB "sticks" which can be plugged into either laptop or desktop machines. The most important thing for boosting reception is that your cellular modem feature an external antenna jack. Many models do, but without one your signal-enhancing options are rather limited.

Although USB modems might be a tad bulkier for laptop use, these modems offer more flexible positioning options. At the very least, you might find that plugging a USB modem into a USB extension cable can let you place the modem near a window for a slight boost in some situations.

While many cellular modems do include external antenna jacks, the jacks themselves differ in shape. Before connecting any external antenna apparatus, you need a cable that plugs into your particular modem's jack at one end, and terminates in a more standard connector at the other end.

These short cables, sometimes known as "adapter cables" or "pigtail cables" can sometimes be ordered directly from your cellular vendor, or else found at online sources, such as 3gstore.com and, of course, eBay. You might also look for dealers who sell products by Wilson Electronics, who make adapter cables for a wide variety of cellular devices, and whose wares are widely available online (and at highway truckstops!).

All about cables

Shortly, we'll look at a variety of antennas, from small desktop spires to large outdoor contraptions. But, no matter which kind of antenna you ultimately need to boost your cellular signal, some basic rules apply across the board.

You will find two styles of cable connectors commonly used in cellular gear: N-type connectors and FME-type connectors. The N-type connectors are large and heavy, and typically connect to thicker gauges of cabling. The FME connectors are smaller and connect to thinner, more pliable cabling. You may need both types of cable in your antenna setup, which means you may be using both N-type and FME connectors, possibly requiring adapters between the two.

Both N-type and FME connectors come in either gender, male and female. Obviously, only connectors of opposite genders can be directly plugged into each other. Alas, electronics cabling is not politically progressive.

A stock pigtail cable that you connect to your cellular modem will probably terminate with an FME male connector.

Gain and loss

No matter what kind of antenna you connect to your cellular modem, it will be connected by cable—possibly more than one cable. It may be ten inches of cable or it may be 75 feet. All cable carries with it some signal loss. Generally speaking, the longer the cable, the more signal you lose.

Be wary of so-called booster antennas, which claim to offer gains of +3-5dBM, but which include ten or twelve feet of integrated cable. It is possible that the cable itself, possibly of low-grade, will induce enough loss to cancel out any gains from the antenna.

When looking for cable to run from the pigtail to your antenna, it is best to use the lowest-loss 50-ohm cable that is practical. For most situations, this means either "9913" or "LMR400" cable. With these, you will lose about 6.5 and 5.8 dBm respectively per 100 feet at the typical EVDO frequency around 1900 MHz. You can go online to calculate signal loss for a variety of cables.

Both cables are relatively thick and not as flexible as, say, TV cable. Although thinner, more flexible cable like "LMR200" is available, because it carries nearly three times the loss, it only make sense for very short lengths of just a couple feet.

Pre-made stock cables of these kinds typically come with N-type connectors already attached to both ends, often N-males. You can find adapters to connect the N-male connectors to FME-female termination for connection with your FME-male pigtail. Some vendors will carry stock cables with different terminations, or will even customize cable with any connectors you want, though this often costs extra.

All about antennas

Like people and gourds, antennas come in all shapes and sizes. But unlike people, not all antennas are created equal. (The jury is still out on gourds.)

The major cellular providers in the U.S. may use two separate frequency ranges for PCS data service--the 800 MHz band and the 1900 MHz band. Some antennas are designed and sold as "dual band," with reception at both bands. But unless you plan to move your antenna to other locations, it is likely that in your area, your cellular provider uses just one of these bands for broadband. Furthermore, it is likely that band is 1900 MHz, by far the most common band for EVDO signals.

Although most people in most places will need only 1900 MHz reception, you can verify this a few ways. One, call your provider. They may or may not be helpful. Two, access the "field test" screen of either your cell phone or your cellular modem. Instructions vary widely by model. Users of Verizon Access Manager software can press CTRL-D and enter the password diagvzw, then click "Field Test." A window will open with lots of statistics about your connection. It does not specifically show which band you are on, but you can usually infer this from the "Channel" reading. Looking at charts for PCS channels used in 800MHz and 1900MHz ranges, you can probably find your channel and thus deduce your frequency band.

Besides frequency, antenna reception is defined as either "omni" or "directional." An omni antenna can pick up signal coming from any direction and therefore doesn't require precise aim. Directional antennas, obviously, do need to be aimed at the signal tower, but offer the advantage of potentially offering much larger gains.

Omni antennas are most useful for people who can receive a usable signal and want a small boost up to +5 or so dBm. Popular omni antennas include "magnetic mount" models with bases that are self-standing on desktops or car roofs, or the venerable "Wilson Trucker" all-weather omni antenna, which must be clamped onto a host surface for support.

Beyond this level of gain, it makes more sense to look at directional antennas. The "yagi" [below] is a very common directional antenna that comes in a variety of sizes. It should be aimed toward the tower for best performance. A 16-inch yagi properly aimed can offer a boost of as much as +15 dBi, which represents as much as a 500% increase in signal power.

yagi.jpg

Get out…side

In the real world, a variety of factors can reduce the performance of the yagi, or any directional antenna--or any antenna at all, for that matter. Building construction, hills, and trees will all reduce signal strength from the tower.

You can't do much about hills and trees, but you can avoid the negative effect of walls and siding by moving your antenna outdoors. Generally speaking, all antennas will return better results outside. But—and this is a big ‘but’—the key is not to use so much cable that its losses negate your gains.

In a perfect world, your antenna should be outdoors, but as close to your cellular modem as possible, to keep the cable short. But, the world isn't perfect, and "as short as possible" could be 5 feet for one person and 50 feet for another. (Cellular modems aren't weatherproof, so these must stay indoors, even if that means your attic.)

Remember that a USB cellular modem can be plugged into a USB extension cable. If necessary, you are better off with 10-15 feet of USB cable rather than 10-15 extra feet of antenna cable, because the USB cable won't incur performance loss (assuming it stays within USB cable length specifications).

A Wi-Fi find

One of the best performing antennas for PCS cellular actually isn't a cellular antenna at all--it is a Wi-Fi grid antenna designed for 2400 MHz reception. Sold online by HyperLink Technologies, the HG2424G 24 dBi High Performance Die Cast Reflector Grid actually loses only a small amount of gain at the EVDO 1900 MHz frequency. Although there are similar grid antennas for Wi-Fi with similar specs sold by other vendors, I can vouch only for this model from personal experience. Don't assume that all antennas rated for 2.4 GHz Wi-Fi will also work for PCS—it is possible that competing products would perform similarly well--or not. Anecdotal reports on the ‘Net, however, do report success with other 2.4 GHz antennas.

I purchased the HG2424G after failing to get a usable cellular signal with other antennas. My location is surrounded by trees over 70 feet tall and blocked by a 200 foot hill, in addition to being a couple of miles outside Verizon's official coverage area for service.

By aiming this parabolic grid antenna into the forest, but precisely toward the tower I determined to be closest to my location, I managed to boost an unusable signal to an average of -92 dBm. In real world performance, this has translated to download speeds of about 1.2 mbps and upload speeds just under 300 kbps.

anatomyofantenna.jpg

D-I-Y dishes

The principle behind the success of the HG2424G antenna is that it collects and reflects signal toward a "feed," the tuned bit that receives the signal frequency and sends it down the wire. Some enterprising types have endeavored to build their own signal reflectors, commonly using re-purposed satellite dishes.

An oblong parabolic dish like that used for the old Primestar service can make a particularly effective reflector for an EVDO signal, but you can also achieve some signal gain with a smaller round satellite dish. For the feed, you can use an off-the-shelf  "flat patch" antenna or build your own biquad antenna.

Get high and amplify

Even with the help of a powerful outdoor antenna, your signal may still have room for improvement.

In some situations, elevation can work wonders. Mount your antenna on a roof if you can, or else put together a mast. Because these antennas can weigh more than a few pounds, you'll need a relatively sturdy and stiff mast. While very tall masts can be expensive and complicated to setup, you can cobble together a cheap and strong 12-15-foot mast using just pipes found at your big box building supply depot. Two threaded six-foot plumbing pipes joined with a coupling nut gives you a stable mast, albeit one that needs to be secured somehow—perhaps with wall mount brackets or a satellite dish tripod.

And don't forget to ground your mast if it is in a position to attract lightning. Signal? Good. Massive power surges? Bad.

If your signal needs major life support, you can pump up the volume with a powered amplifier. At around $300 or more, a PCS amplifier [below] may well cost more than your antenna and mast combined, but can boost your signal by as much as 800%.

pcsamp.jpg

Persistence and patience

Obtaining a good cellular broadband signal in a signal-challenged area is not always easy. But if there is a cellular tower within about 15 miles of your location, there's a reasonable possibility you can get some signal with the right combination of antenna, elevation, aim, and possibly amplification.

Many of the principles involved in cellular reception overlap with both Wi-Fi networking and even ham radio, both of which enjoy active online communities.

Good luck doesn't hurt, either.

Aaron Weiss is a freelance writer and editor based in upstate New York.



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