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How a TV station tower will affect your wireless network

The company that I work for is going to be building a new building very close to a TV station tower. What do I need to take into consideration to make sure my wireless network functions properly being so close to a tower? Where can I find more information on a TV tower's effect on my overall network?

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Broadcast television signals will not have any direct impact on your Wi-Fi wireless. Although both are radio frequency (RF) technologies, they use very different parts of the electromagnetic spectrum. Spectrum allocations determine which technologies can transmit on each radio frequency band. Here in the US, that job falls to the FCC.

According to FCC rules, VHF broadcast television channels can be transmitted between 30 and 300 megahertz (MHz). For example, Channel 3 is centered at 55.25 MHz while Channel 13 is at 211.25 MHz. UHF broadcast television channels use slightly higher frequencies, up to about 800 MHz. (Visit http://www.tech-faq.com for a compete list of TV channel frequencies). These are the current NTSC (analog TV) channels, soon to be replaced by ATSC (digital TV) channels.

Wi-Fi wireless devices transmit at much higher frequencies. In the US, the ISM band used by 802.11b/g devices starts at 2.4 gigahertz (GHz), while the UNII band used by 802.11a devices starts at 5 GHz. New 802.11n devices can transmit in either the ISM or UNII band.

The purpose of spectrum allocation is to avoid interference between transmitters. TV broadcasters purchase exclusive licenses for specific channels in each serving region (e.g., metropolitan area). This is how ABC, NBC, and CBS avoid interfering with each other. However, both the ISM and UNII bands used by Wi-Fi are non-licensed bands. That means that you and your neighbor can both have your own Wi-Fi network, without purchasing an exclusive license to do so from the FCC.

But this is also why you and your neighbor can end up interfering with each other, if you both tune your Wi-Fi access points to the same channel. Devices that operate in non-licensed bands usually depend on low-power, short-range transmissions to avoid interference. For Wi-Fi, so long as you and your neighbor are several hundred feet apart, your APs will not interfere with each other in any significant way. However, there may be other radio devices in your office -- cordless telephone handsets, baby monitors, Bluetooth speakers -- that also transmit at 2.4 GHz and will interfere with your Wi-Fi network. Worry about those things -- not about the TV broadcasting tower.

This was first published in December 2008

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