Best of the ISP-Lists

Glass That Cuts Signals

Members of the ISP-Wireless list share the secrets of tinted glass. Although most glass will not affect a radio frequency (RF) signal, certain special types can cut down or even cut off a signal completely.

[September 19, 2001]

On the ISP-Wireless list in August, EW asked,

"I have a wireless Point of Presence on the roof of a building behind a window. Does anyone know how much loss I can expect to get with tinted glass?"

A number of respondents suggested that there are a lot of factors to consider:

[KS noted] "It depends on the glass and the tint material. If it's reflective at all on either side, you will likely not get much of a signal at all coming through in that direction."

[JX added] "It depends on how thick the glass is. I did a test on normal window glass, and the loss is 3dB."

[CW offered] "At the Sears Tower in Chicago, there is a special floor dedicated to Point-to-Point microwave about 100 feet below the rooftop and enshrouded in fiberglass: the loss is about .5 to 1 dB."

[NP added] "It depends to a large extent on the method and amount of tinting; you're going to create your own multipath environment with signals bouncing off the inside of the window. Positioning is also an issue: I put up two PtP links on the fifteenth floor of a building that had tinted windows. The first one worked great; the second one didn't. The only difference was that on the second one the antenna was angled in the window."

AG shared a problem with non-tinted glass:

"I have been told that non-tinted glass has no loss. On a current installation, when I take the antenna behind the glass, I get zero signal. Raise the window so there is no glass, and the link quality goes to high. Am I missing something?"

Others explained that there's clearly more to that glass than meets the eye:

[MS observed] "You're not going through non-tinted glass: I've got two installs that go right through a window, and it works great."

[MC offered] "It sounds like you have tinted, coated, or High-E glass; High-E is energy efficient and is usually double paned, coated, and filled with gas in between."

[DO laughed] "I don't know what's on that window, but I want some of it for my radio test room shielding!"

[MO explained] "A lot of new construction uses glass that is designed to hold heat out, and often it's not tinted. If your glass has this coating, you will get zero signal—it's a real bummer when you have a customer with LOS through a window."

[RB added] "Heat Mirror glass is commonly double paned and filled with argon or some other inert gas, but many times there is a thin reflective plastic or Mylar film in the middle between the panes that reflects outside heat back out and reflects inside heat back in."

[BC observed] "Most, if not all, of this glazing is tin oxide (SnO₂) coated. It's a brick wall to RF."

—End

Related articles:
	[Sep. 18, 2001]	FCC Tries To Clear The Air
	[Aug. 31, 2001]	Research Says LOS Issues Tie Up MMDS Future
	[Oct. 13, 2000]	Optical Wireless

Online resource:
	Choosing a Wireless POP Site