The
Broadcasters’ Desktop Resource
… edited by Barry Mishkind – the Eclectic Engineer
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High Powered History
Building the Sears Tower Site – Part 2
By Warren Shulz
[June 2014] It was the early 1970s when radio
antennas were first employed on the Sears
Tower (now: the Willis Tower) in Chicago, the
tallest building in the US for over 40 years, until
the One World Trade Center building was topped off last year. But at that height, overcoming
problems was part of the job. Warren Shulz continues the story of the Sears Tower site.
ly. The CCA grounded-grid 10 kW transmitter
just kept the power applied to the WLAK feed
line without interruption – and then the arcing
took out all four feed lines, burning everything
to a molten mass and taking all four stations
down with it.
LIGHTNING STRIKE!
Flex line is not recoverable after a fire, although
rigid can be serviced, cleaned, and restored if
the soot was not excessive and confined. At
Sears Tower, repairs required replacement of
much of the flex line and cleanout of the rigid
lines in use.
We left off last time with a lightning strike that
crippled the transmission systems of all four of
the original FM stations to transmit from the
Sears Tower.
The four stations, WLAK 93.9, WCLR 101.9,
WFYR 103.5, and WXFM 105.9 were operating
off separate single-bay antennas mounted on a
40-foot pole attached to the West cylinder on
the tower. For the last 100 vertical feet from the
transmitter rooms, the four original stations’
flex feed lines shared a common messenger cable. Everything was tied together with the steel
messenger cable.
Then, during severe lightning WLAK, the
uppermost antenna on the 40-foot mast, took a
hit. However, the VSWR protection circuit in
the WLAK transmitter was not working correct-
TOTAL LOSS
The lesson learned was not to put all the cables
on the same messenger cable.
The new placement plan was to provide separation by a few diameters. The layout has held up
well since then, with the WFYR 103.5 now
probably being the oldest working line at the
site. It has been in place since 1974, having
been sold to ABC for its 94.7 auxiliary antenna,
and has given more that 40 years of service.
And that was the way things stood from 1972 to
1982 – the 12-foot diameter tubes were the end
of development from the building. Finally, in
1982, a group TV broadcasters decided to develop the site.
area: if I placed my foot on the mounting base
of the 20-foot pole with two FM antennas, significant RF heating could be felt via whole body
exposure in the near field of the antenna. It was
an obvious RF hot spot until the antennas were
moved away from the area.
TV GROUP DEVELOPS A PLAN
It was Fred Eychaner’s plan to move Channel
50 (WPWR-TV) from Gary, IN to Chicago that
provided the pressure to develop TV antenna
support towers. (Eychaner bankrolled the project, which would net him $425 million when he
sold WPWR to Fox in 2002.)
These four temporary antennas operated in this
location for at least 18 months. Coverage to the
East was near zero as the metal skin of the 110th
floor penthouse provided a solid shield.
A group formed by Channels 7, 11, 26, and 50,
along with the FM stations soon got the project
green-lit. In 1982, two 256-foot towers were installed on top of the Sears Tower, lifting its total
height from 1,451 feet to 1,707 feet (520.3 m).
The plan was to stack tower sections using a
helicopter.
AN FM MASTER ANTENNA
A committee of the four legacy FM broadcasters
and a new arrival – a ABC/Cap Cities station
WYTZ - 94.7 (engineered by the late Harry
Preister) was added to the original group of four
– began work on an FM Master Antenna.
Other stations were contacted, but as costs were
in the area of $300k to develop a transmitter site
and most FM's operators already had reasonable
sites at Hancock that had cost a fortune, most
decided not to spend any more money.
This five-station group committed to developing
the new FM antenna plan for the site. The TV
broadcasters arrived at a total aperture value in
1982 of $336,000, or an aperture fee of $42,000
to own one of the eight sections of the tower for
FM station antenna slots.
During construction, the TV group needed to get
the 40-foot FM pole out of the way of the tower
build-out. To that end the pole was removed and
broken into two sections of 20-feet each, then
welded to the lift anchors on the 110th floor’s
roof. The four FM stations then were permitted
to resume broadcast operation from the 110th
floor roof under an STA.
To make everything fit, the original group of
five stations had to notch all eight mounting
locations as the vertical braces spanned a 20foot space. The fiberglass I-beam had to be cut
back in critical fit areas to clear the cavity back
radiator (CBR) on each tower face and stiffened.
Each fiberglass I-beam was supported with Oak
wood laminated into the support arm brackets
but not near the baskets. As it turned out the inside of the radome should have been not 11 feet
but 11 feet, 6 inches for full clearance.
Again, recall that this was before RF exposure
limits. You had four stations each with a nominal 4.5 kW ERP at a location where you could
actually walk right up to it. It truly was a hot RF
To avoid any future disruptions in station operations when the three empty slots were eventually filled, the original five FMs bankrolled the
cost (about $10,500 for each unoccupied slot) to
The 256 foot towers were placed by helicopter
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modify the radomes for the three unassigned apertures, including the vertical fiberglass I-beams
that support the radome cover attachment to the
tower and the vertical bars that attached the
Harris CBR antenna to the tower face. The
money would be recouped when new stations
eventually arrived.
radome enclosure. It was later proven that an 11
foot, 6 inch clearance was needed.
This call to Harris effectively defined the antenna supplier because of the radome fit. It would
have been possible chose another antenna and
custom fit the radomes – that is what was done
by analog Channel 5 – but, as expected, it was it
ended up being quite an expensive affair.
The three open slots then were ready to accept a
CBR antenna without the need to remove the
radome covers to remove the fiberglass I-beams
for undercutting at that future date. Ultimately,
WFMT, WJMK, and WBBM-FM took the slots.
LIMITED CHOICES
The process for the FM antenna selection was a
little backward: like having a shoe looking for a
foot – the fit was dictated by radome enclosure.
PLACING THE MASTER ANTENNA
Eventually some nine different manufacturers
and/or designs were considered. As noted, the
choice of the Harris CBR module was basically
pre-determined by the structural engineer’s
specifications for the radome arms and radome
clearances.
At the time, RCA was also in the mix to develop
a master antenna plan. The late Dr. Matti Siukola was the Antenna Engineering Manager from
RCA but they never gained traction for a plan
for the master site.
However, Siukola did establish that it was best
to put all the UHF antennas on the West tower
and the VHF antennas on the East tower. Why?
It was thought the VHF antenna field was better
able to look through an object as opposed to
having reflections come from an object. Thus
most UHF antenna would null to the East (over
Lake Michigan) without getting reflection from
the East tower, which was just 90 feet away.
The costs of the single antenna approach were
high. On the other hand the cost of a diplexer
also was high – and no particular space was at
hand for a large diplexer. Furthermore, in that
period diplexing had failure issues.
The FM engineers questioned the potential effects of the East mast being that close. But in
the end it was decided to just let it go – nothing
could be done except to acknowledge there
would be a pattern distortion.
SIZE MATTERS
At the time there was no suitable FM antenna in
hand. However, Asrow (the structural engineer)
spoke with Harris.
Based on a casual telephone call, they
determined we needed an eight-foot triangular
tower and a Harris Cavity Back Radiator (TV-6
antenna) (CBR) which required an 11-foot
This photo shows the wooden oak bracing added to the connection points of the radome cover.
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Once the construction was finished, the ongoing
maintenance costs were allocated by the vertical
aperture percentage compared to the total of all
vertical apertures. This was one of the reasons
why WLS-TV passed on space for a sidemounted horizontally-polarized antenna for the
ABC FM station; it was to reduce the aperture
maintenance cost.
In our next installment we will take a closer
look at the FM Master Antenna, how it was chosen, manufactured, and the pattern we were able
to get from it.
Inside a radome on the Sears Tower
The sheet metal flaps seen are de-coupling
chokes on the radome support arms. These were
used in different lengths depending on antenna
frequency. Apparently this was found to be
necessary during full-size pattern testing to
obtain the +/- 1.5 dB circularity specification.
--Warren Shulz is enjoying the retired life now,
after being Chief Engineer at WLS AM-FM for
22 years after 15 years at WFYR-RKO. Now, he
is out enjoying RVing and riding his eBike.
By doing this preparation work at the same time
the first five slots were built, it avoided a total
removal of the vertical fiberglass I-beams for
notching and setting the mounting brackets for
the three future antennas.
When he is not out having fun, you can contact
him at wshulz@cs.com
--If you would like to know when Part 3 of the Sears Tower history is posted,
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