Willis/Sears Tower FM Coverage Estimate

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R. Fry
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Willis/Sears Tower FM Coverage Estimate

Post by R. Fry » Sun Jul 28, 2019 6:37 am

For anyone who might be interested, below is a clip showing a section of a Longley-Rice coverage study for the non-D, ~4kW ERP FM stations on Willis/Sears Tower in Chicago. The study includes the effects of terrain elevations (300-ft horizontal resolution).

Image

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kkiddkkidd
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Re: Willis/Sears Tower FM Coverage Estimate

Post by kkiddkkidd » Sun Jul 28, 2019 2:42 pm

Thanks Richard,

What effect does the other tall buildings in the downtown area have on real world coverage? I have been up to the radio suites on Hancock and was wondering if the effects of those other tall buildings was predictable or even noticeable.

Later,
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RGORJANCE
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Re: Willis/Sears Tower FM Coverage Estimate

Post by RGORJANCE » Sun Jul 28, 2019 4:19 pm

Kevin:

I sold the original CBR antenna units to a number of stations in Chicago. The first configuration was one level of three bays---one on each face---for each of the stations, and if I can remember correctly, there was not a great deal of multipath. Coverage seemed pretty good all around the area, including downtown.. Many years after the installation, I was involved in field strength msmts, which as I recall were pretty consistent all over the area on about a 20 mile radius. Obviously, the car, trailer and mast wouldn't float, so I haven't got any data for the "Big Pond".

Since that first installation, they have added several more stations, and I believe they are doing combined feeds.

Also, Richard will be able to provide more exact detail that you are questioning. Just thought I'd add a bit of history.

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Re: Willis/Sears Tower FM Coverage Estimate

Post by radio_guru » Sun Jul 28, 2019 4:46 pm

There are a couple combined operations up there. Iheart has two and Entercom has at least one, if not two combined systems each operating on a single set of CBR's. A set of CBR's can handle 20+ KW and each antenna has 3" line.

The CBR's offered the least amount of multipath throughout the metro early in the site's life, and most especially coming in on I-90/94 from the south where multipath was fierce. Hancock stations suffered greatly as a result of the then original Alford antenna (still used as a back-up) way down the east mast. Since the ERI two bay went up at the top of the east tower and the combiner replaced a few years earlier, multi-path dropped precipitously.

Multi-path was also determined to be the result of group delay through the filters and poor tuning of the then tube TX's causing lots of AM noise. Warren Shulz (SK) did a bunch of ground breaking measurements and adjustments in the 80's to remedy the TX tuning problem into the filters and later quantified the original Dielectric filters used there could not be adjusted for uniform and low group delay.

Today, lines are trimmed with a trombone section into the filters/diplexers to minimize mis-tuned zero-cross reflectivity. Much the same as cusp rotation for AM TX's.

RG

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Re: Willis/Sears Tower FM Coverage Estimate

Post by kkiddkkidd » Sun Jul 28, 2019 7:02 pm

RGORJANCE wrote:
Sun Jul 28, 2019 4:19 pm
Kevin:

I sold the original CBR antenna units to a number of stations in Chicago. The first configuration was one level of three bays---one on each face---for each of the stations, and if I can remember correctly, there was not a great deal of multipath. Coverage seemed pretty good all around the area, including downtown.. Many years after the installation, I was involved in field strength msmts, which as I recall were pretty consistent all over the area on about a 20 mile radius. Obviously, the car, trailer and mast wouldn't float, so I haven't got any data for the "Big Pond".

Since that first installation, they have added several more stations, and I believe they are doing combined feeds.

Also, Richard will be able to provide more exact detail that you are questioning. Just thought I'd add a bit of history.

Fossil
That is very interesting history and addresses a few curiosities that I have about FM coverage into large concrete jungles.
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Kevin C. Kidd CSRE/AMD
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KK Broadcast Engineering
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R. Fry
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Re: Willis/Sears Tower FM Coverage Estimate

Post by R. Fry » Mon Jul 29, 2019 5:09 am

Bob Gorjance and radio_guru both had good comments in response to Kevin Kidd's question.

In my job before I retired as the FM Applications Engineer at Harris I was the technical interface between customers, consultants and various Harris engineering groups, and involved in the FM transmitter and antenna installations at Sears Tower. I still have some of the documents from that era, so I put together the graphic below.

The Harris FM CBR antenna earned a good reputation in the field. I suspect that much of the reason for that is its radiation characteristics, compared to those of conventional FM transmit antennas.

It was relatively expensive, but as they say, "performance is remembered after price is forgotten."

Image
Last edited by R. Fry on Mon Jul 29, 2019 8:27 am, edited 1 time in total.

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Re: Willis/Sears Tower FM Coverage Estimate

Post by radio_guru » Mon Jul 29, 2019 8:17 am

Even after the field modeling, some additional parasitic elements were added to smooth out the minor variations in the as shown graph. It was found by a now long passed engineer there were some "dead" spots around the metro. His station (I forgot which one) added them and solved the problem without adding any new problems. Eventually, most stations have added then over the years.

RG

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Re: Willis/Sears Tower FM Coverage Estimate

Post by R. Fry » Sat Aug 03, 2019 5:35 am

As for the "building penetration" of an FM station using reduced ERP with an overheight, low-gain transmit antenna, below is a comparison of the nearby radiated fields from Class B FMs using a 1-bay antenna at 1550 ft HAAT and a 6-bay antenna at 500 ft HAAT.

The 50 kW at 500 ft station wins, but even so — the 1-bay system generates a peak field intensity of about 2.6 V/m per polarization near the antenna site, and has no nulls in its elevation pattern.

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Re: Willis/Sears Tower FM Coverage Estimate

Post by RGORJANCE » Sun Aug 04, 2019 6:39 am

Richard:

Thanks for that detail. First time seeing this info. I remember "back in the day" looking at the good axial ratio charts.
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Re: Willis/Sears Tower FM Coverage Estimate

Post by RGORJANCE » Sun Aug 04, 2019 6:46 am

There is one other detail I will add.....

One of the stations going up in the early days got stopped by the FCC as it was discovered that the station would be technically interfering with a station to the Southeast. Simple solution was to reduce power to the basket on the SE face of the tower making the antenna directional and it worked out perfectly. Just so happened it was almost the exact azimuth needed to do the job.

I have talked about this in the past and if I am duplicating a previous post, please excuse senior moment. (been showing up with some regularity these days :roll: )

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Re: Willis/Sears Tower FM Coverage Estimate

Post by RGORJANCE » Sun Aug 04, 2019 7:46 am

Here is a picture I took on gtop of the Sears Tower waaayyy back when. Shows the 4 levels Of CBR . Each station had a level of three around.
CBR AT SEARS.jpg
The roof area is off limits now. The door is interlocked to all xmttrs. OSHA rules the roof.

I hope the pic opens.

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Re: Willis/Sears Tower FM Coverage Estimate

Post by NECRAT » Sun Aug 04, 2019 10:43 am

The mast looks better without the radome.
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