Doug Lung / 08.07.2012 08:22PM
Recalculating DTV Coverage
Back in the days of analog
TV, the Grade B
contour map was the
measure of a station’s coverage.
It was plotted using a set
of curves developed with a
combination of field measurements
and free space path loss
based on the station’s channel
height above average terrain for the radial studied
and effective radiated power on that radial.
The set of curves for the predicted threshold
(example: 64 dBμV/m for UHF Grade B), for 50
percent of the locations 50 percent of the time
was used for determining coverage, and another
set, based on 50 percent of the locations 10 percent
of the time, was used to determine the interference
The coverage contour for ATSC DTV used a
combination of these two curves (or their mathematical
equivalent), to determine the DTV service
area contour based on a predicted threshold
(example: 41 dBμV/m for a UHF station on
Channel 36) at 50 percent of the locations 90
percent of the time.
Field measurements show the FCC curves are
fairly accurate for most station locations—the
central valley area between Fresno and Sacramento,
Calif., being one noted exception—but
they didn’t account for terrain beyond 16 km
(about 10 miles) from the transmitter site.
They also didn’t predict reception at any
given location inside the coverage contour. As a
result, when the FCC was looking to provide every
analog station with a channel for DTV, they
used the Longley-Rice Irregular Terrain Model
to calculate station coverage at cells inside the
Mt. Harvard antenna tower in Southern California
Since the early days of TV, coverage has been
based on the use of an outdoor antenna 30 feet
above ground. While outdoor antennas are still
used today, particularly in rural areas, reception
on indoor antennas has become popular.
Since the DTV transition we’ve seen several
companies join established firms such as Winegard
and Antennas Direct in offering indoor antennas for over-the-air reception—the Mohu Leaf and Walltenna antennas
are two examples.
Coverage studies based on signal strengths for outdoor antennas at 30
feet aren’t much help with these antennas.
PICKING APPROPRIATE SIGNAL LEVELS
What level is appropriate for indoor reception? In “DTV in the House,
Part 1” (RF Technology, Sept. 5, 2007), I showed a field strength of 67
dBμV/m would be needed at UHF Channel 36. For my coverage studies,
I’ve been using 68 dBμV/m (20 dB above the nominal community grade
signal level) for UHF indoor reception.
Strong signal coverage is becoming more important as people eschew
outdoor antennas in favor of small indoor antennas. In addition to the
nominal FCC UHF field strengths of 41 and 48 dBμV/m, I also look at the
population predicted to receive a signal at 68, 88 and even 98 dBμV/m.
Increased field strength increases the chance of reception. Remember
those FCC curves? For DTV they are based on 50 percent of the locations,
90 percent of the time. In a controversial filing regarding an FCC Notice
of Inquiry on the Technical Standards for Determining Eligibility for Satellite-
Delivered Network Signals Pursuant to the Satellite Home Viewer
Extension and Reauthorization Act of
2004, Hammett and Edison, on behalf of
EchoStar, showed a 17.5 dB increase in
UHF DTV signal strength was necessary
to change the reliability statistic from 90
percent of the time to 99 percent of the
A you can see, there is a big difference
in the signal level required for reception
if a viewer is willing to take some effort,
which could include installing an outside
antenna or simply wants to plug in one
of the new, flat, wall-mounted indoor antennas
and call it a day. When designing
transmission systems, it is important to
take this into account.
For most markets, the transmitter site
is already defined, so the key considerations
are antenna pattern and transmitter
power. The ideal solution to providing
the maximum signal to the most number
of people would be to use an isotropic
(equal power in all directions) radiator or
at least one that had equal power at all
angles below the radio horizon.
Obviously this isn’t practical. Using an
omnidirectional antenna with low elevation
gain and a lot of null fill may be practical
and will achieve similar results, but
due to transmitter and transmission-line
power limitations it may be difficult to
achieve any significant amount of elliptical
polarization, which is also important
for reaching indoor antennas.
In the end, the final solution is likely
to involve some compromise— a more
directional antenna and, if the site height
above average terrain allows, use of mechanical
beam tilt to put the strong signal
where it reaches the most people.
USING ‘WEIGHTED’ POPULATION
One way to determine the best combination
of azimuth pattern, elevation pattern
and mechanical beam tilt is to use a
“weighted” population count to compare
First, calculate the population covered
at each of the desired signal levels (example:
48, 68, 88 98 dBμV/m) for an isotropic
antenna. Give each signal level a weight—
for example, 0.1, 0.5, 0.3 and 0.1 for 48,
68, 88 and 98 dBμV/m, respectively.
Multiply the population receiving each
signal level by the weight and sum the results;
this is your reference. Do the same
for each antenna pattern, seeing how
close, on a percentage basis, each one
comes to isotropic antenna.
Comparing a large number of antenna
variations will take a long time. I’ve come
up with a way to convert the output of
the SPLAT propagation analysis program
into a Python NumPy matrix instead of
the flat text files I previously used. By putting
the elevation and azimuth angles to
the receive point or obstruction and path
loss from SPLAT in a matrix and combining
this with a matrix containing antenna
pattern data, I should be able to easily generate
field-strength matrices for many different
Once I figure out how to combine
these results with gridded population
data, it will provide the population coverage
needed to determine the optimum
antenna orientation. Look for more on
this in a future column.
Comments and questions are welcome.
Email me at email@example.com.