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Everybody Talks About Fog

Members of the ISP-Wireless list report from the field on the effect of fog on 2.4 GHz and 5.8 GHz fixed wireless broadband Internet service. Fog effects laser more than it effects RF signals, but it can degrade RF signals too.

[July 10, 2001]

Everybody talks about fog, but what can we actually do about it? On the ISP-Wireless list in June, PH queried,

"Will very thick fog interfere with 2.4 GHz or 5.8 GHz signals?"

Some respondents contended that it's a possibility worth worrying about:

[RB offered] "Sure it'll interfere. I sat one time with a 940 MHz point to point link and watched an RSSI voltmeter as the morning sun burned off the thickest fog I'd ever been in. The signal went up 6 dBm as I watched. Fog thick enough to do that will likely affect higher frequencies as well: plan on it happening."

[DR added] "Normally the fog itself will not have much effect, but the inversion layer that causes the fog can make some really weird things happen. I've seen the signals increase and completely disappear."

Others suggested the effects should be minimal:

[MH recalled] "We've had fog so thick we couldn't see cars in the parking lot, extending as high up as our antennas, and we still had good signals with 2.4 GHz DSSS. There does seem to be a type of rain with very small drops that causes problems, the kind that's like a very heavy fine mist."

[BA observed] "We had fog today. Visibility was less than a quarter mile. We saw our ten-mile link signal quality drop a little, but local clients seemed to be right at the same RSSI levels as on a clear day."

[JA added] "Here in the hills, we have some very thick fog. So far, we have seen no signal degradation due to fog. We have had some heavy rain over the last couple of weeks, and throughput stayed consistent through that as well."

Still others claimed that fog can actually improve the signal:

[JM noted] "We've actually seen that fog, rain, snow, or any other inclement weather seems to make things work better. Go figure."

[BB agreed] "The funny thing is, during rain, fog or the like, our 2.4 GHz signals seem to improve a bit. Noise drops a little and SNR goes up. We had a tornado come right through downtown just over a year ago with hail and all the trimmings: never missed a beat."

[RC added] "Fog is generally associated with improved range on VHF and UHF bands. It is a sign of an inversion layer in the atmosphere which bends the signals over the normal radio horizon. At 144, 432, and 1296 MHz (Ham bands) it is known to send 1 Watt signals hundreds of miles. For WLAN, though, it can cause problems such as interference from nearby cells on the same frequency."

—End

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