Are Sears/Willis Tower Signal Strength Variations In High Wind Observable?

[Schroedingers Cat]

Has anyone measured signal variations of signals from the Willis Tower in high winds?

Since the FM antennas are only two bays as I recall, I wouldn't expect much variation. But particularly UHF signals with many bays and narrow beam width, it would possibly be observable. This article says it sways three feet in high winds.

This World-Famous Chicago Skyscraper Can Sway Up to 3 Feet in the Wind

Constructing this tower was a major feat of engineering.

www.discovery.com

 

[davideduardo]

Has anyone measured signal variations of signals from the Willis Tower in high winds?

Since the FM antennas are only two bays as I recall, I wouldn't expect much variation. But particularly UHF signals with many bays and narrow beam width, it would possibly be observable. This article says it sways three feet in high winds.

This World-Famous Chicago Skyscraper Can Sway Up to 3 Feet in the Wind

Constructing this tower was a major feat of engineering.

www.discovery.com

The issue would be the downward angle which could be aimed slightly upwards in a high wind. If it has enough energy at, let's say, 15° to 20° above vertical, the loss of coverage at that height is only going to be for a mile or two in central Chicago where there are few single family homes and nearly everyone has cable. Outside the loop, it won't make any difference and even in the farther reaches inside of the loop along the North Shore like Gold Coast and north of downtown towards Oldtown, it does not look like it will make much difference.

 

[Brian Bowers]

Has anyone measured signal variations of signals from the Willis Tower in high winds?

Since the FM antennas are only two bays as I recall, I wouldn't expect much variation. But particularly UHF signals with many bays and narrow beam width, it would possibly be observable. This article says it sways three feet in high winds.

This World-Famous Chicago Skyscraper Can Sway Up to 3 Feet in the Wind

Constructing this tower was a major feat of engineering.

www.discovery.com

The tower with the FM antennas mounted to it has 3 faces (I don't know if the opposite tower is also 3 faced). On initial installation, there was an FM bay installed on each face (I don't think that has never changed), and there are four levels of 3 bay (sometimes referred to as baskets on this tower) antennas. Upon initial installation, each level of bays was delegated to a specific radio station frequency. Since then, I've read that numerous stations are now combined into a few of the bay levels.

I don't think anyone has ever done signal variation measurements due to high winds, but numerous studies have been done over the years related to multipath, and signal coverage (using field strength measurements). Personally, I think that if there was any signal degradation due to building sway from high winds, it would be very hard to measure, and probably be negligible (and not worth the money to do the study), but that's my own opinion.

I've read that as of 2019 (and possibly even before then), the roof area is off limits now. The door is interlocked to all transmitters. OSHA rules the roof. With all of the transmitters up there, that must be quite an extravagant interlock system.

 

[PTBoardOp94]

I've read that as of 2019 (and possibly even before then), the roof area is off limits now. The door is interlocked to all transmitters. OSHA rules the roof. With all of the transmitters up there, that must be quite an extravagant interlock system.

Presumably there are only a handful of antennas, and a bunch of combiners on the upper floors. If so, pretty easy to put a sample coupler on the combiner(s) and tie that to the interlock.

 

[Schroedingers Cat]

I tried another approach, calculating the Arctangent of the sway divided by the height above the ground. It looks like about 1/10 of a degree variation, which wouldn't make much difference. Changes in the refractive index of varying air masses would probably be more significant.

 

[Brian Bowers]

This is the type of CBR (Cavity Backed Radiator, hence the basket reference) FM antenna mounted on the Willis Tower.

https://www.dielectric.com/wp-content/uploads/2015/11/CBR.pdf

Again, four 3-bay levels, with each 3-bay level devoted to a particular station frequency, or station owner or cluster (when using a combiner system). The initial installs are quoted as being Harris CBR's, but it's possible Dielectric was the original manufacturer, and Harris just put their name on them, for vendor or distribution purposes (I don't really know for sure).

 

[Kent]

Has anyone measured signal variations of signals from the Willis Tower in high winds?

Since the FM antennas are only two bays as I recall, I wouldn't expect much variation. But particularly UHF signals with many bays and narrow beam width, it would possibly be observable. This article says it sways three feet in high winds.

This World-Famous Chicago Skyscraper Can Sway Up to 3 Feet in the Wind

Constructing this tower was a major feat of engineering.

www.discovery.com

From what I've always heard, 18 inches to 3 feet is standard sway on towers in general. Brian sounds like he has an engineering background and probably knows more than I do on the subject, but I can't imagine the standard sway on pretty much any tower causing any measurable signal variations.

While I can't tell you what the wind and sway would have been in downtown Chicago, my experience has always been, traveling north into Chicagoland on I-55, the stations on Big Willie start coming in just past Dwight as you're entering Grundy County. Over the last dozen years or so, that's been consistent.