- Protecting Our Spectral
Neighbors
- By Jim Totten, Transmitter Supervisor
– WKBD/WKBD-DT
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WKBD TV first signed-on in 1965 as a Kaiser
Broadcasting station, operating on Channel 50, serving the Detroit,
Michigan market. The station is presently part of the Paramount
Stations Group of the CBS Television Stations.
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Our studios, transmitter and offices are collocated
in a recently built, state-of-the-art facility covering 56,000
square feet, including a 10,000 square-foot technical center.
Our NTSC antenna sits atop a 1,056-foot tower where we radiate
2.2 megawatts ERP from a newly installed transmitter. But
this story isn’t about our analog facility. It is about
the extraordinary challenges we faced as we implemented our digital
television service.
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We, along with some 36 other stations across
the country, were assigned to operate on Channel 14 (470-476 MHz)
as our digital allocation. At first glance, “no big deal,”
but the Land-Mobile (LM) Service occupies a 20 MHz (450-470 MHz)
slice of spectrum adjacent to Channel 14. Because of this
LM spectrum adjacency, these Channel 14 DTV allottees face challenges
beyond the normal DTV implementation problems. As an example,
one LM station is located atop a building just a half-mile away
from our transmitter site. It is assigned to 469.975 MHz,
just 25 kHz away from us, and “looks” into our main beam.
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Compare our digital transmission system, which
is currently radiating 50 kW ERP, to the maximum permissible output
power for most LM units operating in their band (100 Watts) and
you can see the potential for problems. Most LM receivers
are designed to operate with signals as low as 0.35 microvolts
for 12dB sinad (0.5 microvolt for 20dB S/N). Add a 10dB
gain antenna, as many LM stations use, and it really becomes a
challenging situation, which will only worsen when we increase
to our maximum DTV power (200 kW ERP). Even a microscopic
amount of our DTV signal, operating within normal limits, could
overload LM receivers, causing intermod and desensitization.
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The WKBD Engineering team, under the guidance
of Chuck Davis, our Chief Engineer, and John Viall, DTV transmission
implementation guru for Paramount Television, commissioned our
engineering consultant, John F.X. Browne & Associates, to
initiate an aggressive program to evaluate potential interference
from our DTV signal to the LM services and to implement solutions
to prevent any interference. Browne Associates determined
that there are 3,000 land-mobile licensees operating in the 450
MHz band within 30 miles of our transmitter site, including 649
operating between 468-470 MHz. We also found 95 hospitals
and medical centers, along with 115 county and city services (police)
licensees operating in this band.
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Len Eden (Browne Associates) contacted a land-mobile
field technician and set up a test bench of typical LM radios.
Using our test equipment, we spent several days collecting data
to determine how the radios would be affected in various LM/DTV
signal level scenarios. Once the information was compiled
and evaluated, we determined that it would be necessary to attenuate
our out of band signal more than 50 dB below the FCC’s DTV emission
mask. Browne Associates contacted Dielectric Communications
to address the problem, and Dielectric’s Jim Stenberg and Comark’s
Henry Fries joined our interference avoidance team.
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The initial design of the filter incorporated
the required sharp roll-off, but also introduced group delay in
excess of 4,000 ns. Design enhancements by Dielectric reduced
this to 2,200 ns and Comark was able to correct for this magnitude
of distortion. Once the filter was installed, and we began
operational testing, Eden conducted field tests with the LM receivers
and demonstrated that the design parameters of the filter had
been fully met, even at full power.
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WKBD-DT signed on for the first time on December
4, 2000, and we held our breath. We checked with previously
set up monitoring stations and determined that there were no discernible
problems. So far, so good. We’ve had no interference
complaints, to date, from either the LM folks or the medical telemetry
community, who we also considered as a possible point of interference.
It just goes to show that a dedicated team of engineers can avert
what could have been a serious problem.
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