Long range WiFi science experiment

Posted: 3rd March 2011 by Seth Killey in WiFi

WARNING: The following post is really dry so it’s doubtful this will interest anyone other than the extreme nerd.

About a year ago I had only slightly more WiFi knowledge than the average consumer. I knew about the various protocols…a,b,g,n. I knew about using different channels…1,6,11 and how to choose the one least used in the area to reduce interference. I knew basic mechanisms for securing your hot spot with WPA2. I even experimented with high gain antennas to boost signal. However, now I support a WiFi link between two satellite offices at work that are about 400 feet apart and in different buildings. Now certainly this is nothing special to achieve, but it does require a little more thought than your typical, get an 802.11n router, set it up and forget it mentality. In our situation, we don’t own either of the buildings. In fact, one building is a courthouse so we have a wireless access points situated inside each building and I’m not sure we’d get approval to mount a directional antenna on the outside wall or rooftop. Right now we have 2 Linksys WAP200 802.11g access points in each location with omnidirectional antennas. Recently I replaced the stock antennas with high gain 8 dBi indoor omnidirectional antennas to boost signal strength. With a bit of tweaking in the settings, firmware updates, and antenna adjustments I think we’ll be OK. However, this set me down a path recently where I researched a number of different scenarios on long range WiFi. I’ve read articles where people have bought long range parabolic dishes and received a signal 60 miles away from their campsite. This got me wondering just how much range I could get inside the city without the aid of a directional, outside antenna. I found a couple of intriguing access points that tout long range capabilities. Perhaps the most economical is EnGenius ECB3500 http://www.neweggbusiness.com/Product/Product.aspx?Item=N82E16833168065.  With this device you can adjust the TX power up to 28 dBm or 620 mW.  By comparison, the two units in place in my branch offices are capable of about 14 dBm or 25 mW.  What I like about the ECB3500 is you can also replace the stock antennas (5 dBi) with an even greater high gain antenna http://www.streakwave.com/Itemdesc.asp?ic=IN24%2D9RD%2DRSMA&eq=&Tp=).  If you really want to get crazy, I found a couple more powerful access points like the Renasis SAP36g which has 820 mW of TX power or the Ubiquiti PowerAPN which has a ridiculous 1000 mW of TX power.  However, the Renasis unit is twice as expensive and the PowerAPN doesn’t clarify if the antennas are detachable.  In any event, you also have to factor in FCC regulations.  I haven’t read thru all the FCC regulations regarding the 2.4 GHz frequency band, but I believe for an unlicensed 2.4 GHz signal you are limited to about 4 watts.  You can use an EIRP calculator to determine how much power is legal http://www.wifinerd.com/wifi-calculators.html.  For the EnGenius ECB3500 that means you can max out the 620 mW TX power and put up about 8 dBi antennas and be right at the limit.

In practical terms, this gets my brain wondering just how far I could go with an indoor omnidirectional antenna utilizing the full 4 watts allowed for an unlicensed signal.  I would love to see if I could make it to my mother-in-laws house which is about 1 mile away.  It’s almost worth buying the equipment just for the fun of seeing how far I can go.  There’s also some nice calculators that let you determine range based on TX power, antenna gain, cable loss, etc.  I found the most useful (IE…easiest to follow) here http://huizen.deds.nl/~pa0hoo/helix_wifi/linkbudgetcalc/wlan_budgetcalc.html.  Granted, these calculations are based on line of sight, ideal conditions with no interference…which is never the case in reality.  I’m pretty sure ISP’s would frown on this, but I’m betting I could share my internet connection with the entire neighborhood if I wanted, but who wants to play tech support for all your neighbors?  One last note, if you want to see which WiFI channels are in use in your neighborhood I just started using inSSIDer which is a nice little tool since netstumbler is not available on Windows 7.

  1. Tony V says:

    If you don’t have clear line of sight between your place and your mother in law’s, you can forget that idea, but if you have, it can easily work and with much less than a whole 1000mw of power. Some time ago, I set up a wireless link between one work building and another. We used 18mw access points and 18db grid antennas, mounted outside. We had a VERY solid link. Later, after one of the units was damaged by a nearby lightening strike, the people then working there, replaced the 18mw damaged unit with a 72mw one. They called me in it check over the installation, and my barefoot laptop could connect to the master unit a mile away, with me just standing on the roof of the distant building. I had about -72dbm of signal like that, and -88dmb inside a sort of wooden shed that was on the top of the building.

    In conclusion, you don’t need the massive power of these units you are talking about. Most long range microwave data links in this country (UK) operate with a few milliwatts of transmit power. Most of the gain is provided by good antennas, and there is no substitute for that. Good high gain antennas also help by HEARING the other end really well and providing directional sensitivity which screens out interference and radio noise.

    Point a grid antenna out of the window towards the mother in law’s place, even if it is behind the glass. It will work a whole lot better than large amounts of microwave power radiating in all kinds of useless directions. A 24dbi grid antenna on a 72dbwm access point will produce the same eirp as an 18watt access point on a dipole antenna. In reality, you always lose some of your transmit power in cables, but you could mount your cheap wrt54g access point running dd-wrt at the focal point of a dish antenna and get a massive eirp out of it. Look how antenna gain affects eirp. See below:

    72dbm barefoot access point. The gain figures are +db gain of antenna.

    144dbm + 3db
    288dbm + 6db
    576dbm + 9db
    1152dbm + 12db
    2304dbm + 15db
    4608dbm + 18db
    9216dbm + 21db
    18432dbm + 24db

    Good luck..

    Tony V

    • Seth says:

      Thanks for the feedback Tony. Yeah, if I was serious about connecting to my mother in laws house I would definitely go the route of directional, outdoor antenna. I was more curious how much distance you could get with an indoor omnidirectional just out of curiosity, but I’m not sure I’d want 1000mw of power pulsating through my house regardless. Good to know that in your experience antenna strength has proven more useful