It all started on a pleasant summer day.
I was driving along minding my own business in my hometown of Brainerd,
Minn., when suddenly there it was: a new 180-foot wireless Internet tower at
the Consolidated Telecommunications Co. building in the Brainerd Industrial
It had all happened overnight, so to speak. One day there was nothing; the
next day a crane had lifted the new tower into place for everyone to see.
wireless tower has 700 MHz antennas, mounted near the top, that tilt down
slightly to control the coverage area.
The tower is just 0.87 miles (1.4 km) west of the KLIZ(AM) three-tower
directional antenna system. KLIZ operates on 1380 kHz, which makes the new Consolidated
tower almost exactly one-quarter wavelength in height at KLIZ’s frequency.
KLIZ is one of my regular clients so I immediately went out to take AM
directional monitor point readings.
The new tower had indeed become a significant re-radiator of the KLIZ
signal. It was picking up and retransmitting KLIZ in a fashion that raised the
field intensity levels in two nearby monitor points from a comfortable 50
percent of their limits to just at their limits. Ouch! I could even plainly see
most of the Consolidated tower from one of the monitor points.
What to do
I went into the Consolidated
building and asked for the manager. Fortunately we had met before; I was not
some “nut” walking in the door claiming they’d ruined the world by putting up a
The project manager, Bill
Stroot, found my story interesting but mostly unbelievable.
How was he to know about
AM directional antennas? He isn’t an RF guy, though he knows a little more now.
Bill had done almost all of the proper research, making sure the tower was not
near an airport or flight path and that it complied with all city building
codes. He showed me an FCC license for the 700 MHz band wireless Internet
facility. He inferred that if the FCC said it was OK, it shouldn’t be a
Stroot finally agreed to
do some homework while I went to my office to look up FCC Rule 73.1692, which I mentioned in an RW article a few months
back. The rule about broadcast station construction near or installation on an
AM broadcast tower reads:
The unipole detuning box sits at the center of the inside of the tower.
Where a broadcast licensee or permittee
proposes to mount a broadcast antenna on an AM station tower, or where
construction is proposed within 0.8 km of an AM nondirectional tower or within
3.2 km of an AM directional station, the broadcast licensee or permittee is
responsible for ensuring that the construction does not adversely affect the AM
Telecommunications license is for an “area of coverage” and does not have any
specifics on tower location or locations to accomplish the task. It all makes
sense unless there is an AM tower nearby.
The FCC normally looks
out for AM stations by issuing licenses or construction permits to two-way,
cellular and other wireless facilities with “special operating conditions or
restrictions” when they are near an AM. I wonder how many more of these “area”
licenses slipped through the cracks at the FCC.
When we met again, Bill acknowledged
that he had talked with someone else who had experienced a similar situation
and that we should do something about this one. That something came to about $12,000,
which Consolidated paid.
Consolidated is a
cooperative telecommunications utility that started out as a rural telephone
company many years ago. They seemed to have no serious problem finding money.
The project turned out to
involve a unipole detuning system from Nott Ltd., tower climbers to install the
unipole and me to tune (detune) it.
I tried something different on this one. The three
near-vertical 145-foot unipole wires are on the “inside” of the tower rather
than the outside. This was made possible because the tower is a self-supporting
structure 22 feet across at the base. The unipole detuning box sits at the
center of the inside of the tower. The vertical unipole wires turn horizontal
at 8 feet above the ground and meet just above detuning box.
Bill Stroot stands near one of the unipole wires.
In the end
The project when smoothly
and it worked as predicted to detune the tower. This made the structure electrically
invisible at 1380 kHz so it would not pick up and re-radiate signal on that
frequency. The KLIZ(AM) directional monitor points returned to normal and we
all walked away smiling. Bill Stroot, now retired, is in one of the photos near
one of the unipole wires so you can understand the size of the tower base.
A letter of agreement was
drawn up between the radio station and Consolidated allowing reasonable access
for checking the tuning of the unipole during regular business hours. Remember,
Mark Persons WØMH is certified by the Society of
Broadcast Engineers as a Professional Broadcast Engineer and has more than 30
years’ experience. His website is www.mwpersons.com.