The FCC TVStudy
includes the ability
to run studies using
azimuth and elevations
patterns, but it wasn’t
until release of Version
1.2.8 that there was a
way to apply mechanical beam-tilt only
to stations with real antenna patterns.
This is critical because applying mechanical
beam-tilt to horizontal plane radiation
patterns that usually have no relationship
to the signal on the ground leads to bogus
results. This month I’ll show the impact
of using the TVStudy
with the use of real antenna patterns. The
results surprised me!
MECHANICAL BEAM-TILT EXPLAINED
Most UHF and high VHF TV transmitting
antennas located on tall towers or mountain
tops use some form of beam-tilt to
push the signal down to the ground. Sending
the signal straight out perpendicular to
the antenna would result in much of the
signal being lost in space. The beamwidth
of the signal from the antenna narrows
with increased elevation gain.
|Fig. 1: TVStudy Pattern Editor Plot tab for KNBC
A station on Mt. Wilson, more than
5,000 feet above Los Angeles, would have
very little signal on the ground if it used a
high-gain antenna without beam-tilt.
A transmitter site located near the
center of the population can use electrical
beam-tilt (EBT) to provide the same
amount of down tilt in all directions
around the antenna. In this case, the horizontal
plane azimuth pattern will be the
same shape as the main beam azimuth
pattern with EBT.
However, in markets such as Los Angeles
where the antennae are at high elevations
near the edge of the population center,
mechanical beam-tilt (MBT) is often
used with EBT to put more energy into
the populated areas and reduce the power
wasted over sparsely populated mountains,
deserts and ocean.
|Fig. 2: TVStudy Pattern Editor Plot tab for KVEA
|Fig. 3: This shows the KPXN FCC azimuth pattern and the main beam pattern.
Like most Mt. Wilson stations, KNBC
uses EBT and MBT. The MBT results in
the horizontal plane azimuth pattern
show less energy over Los Angeles and
more over the mountains and high desert
to the north. KVEA on Mt. Harvard also
uses EBT with MBT and the differences
are even greater there.
The patterns shown in Figs. 1 (KNBC)
and 2 (KVEA) are from the TVStudy Pattern
Editor Plot tab.
USING REAL ANTENNA PATTERNS
During the DTV transition I received
a number of emails from companies trying
to use FCC antenna patterns to display
coverage of Los Angeles area stations.
They wondered why the antennas were
pointed in the wrong direction. MBT and
the resulting horizontal plane radiation
pattern are the reason. Let’s look at what
happens to the coverage and interference
predicted by TVStudy when we use real
antenna patterns instead of the FCC horizontal
While stations have to include real azimuth
and elevation pattern data on their
FCC CP applications, many do not include
tabular data in a format easy to convert
into a .cvs file for import into TVStudy.
The KNBC and KVEA pattern data can be
cut and pasted from the data in the engineering
ION Media was kind enough to provide a
PDF with tabular data on the Dielectric TFU-
26DSC-R C170 used at KPXN-TV. KPXN-TV
is an adjacent channel to KVEA and at the
same site so it makes an interesting case to see the effect of real vs. bogus antenna
patterns on coverage and interference in
TVStudy. Fig. 3 shows the KPXN FCC azimuth
pattern and the main beam pattern.
|Table 1: TVStudy V 1.2.8: Default vs. Real Patterns
My studies for KNBC, KPXN and KVEA
were done using TVStudy
Version 1.2.8 on
Manjaro Linux. Default Study Parameters
were used except for real pattern studies
where “Use mechanical beam-tilt” in the
“Patterns” tab and “Use real elevation patterns
for contours” in the “Contours” tab
In creating scenarios, for station selection
I selected “Prefer operating facilities” and
“Search by service.” Selected services were
DT, DD, DC and CA. “Search by center point
and radius” was selected using a 320-kilometer
radius around the KNBC tower. “Allow
editing” was selected in the scenarios to allow
entering real antenna pattern data for
the stations, and the box on the edit screen
allowing default patterns to be used in some
cases was unchecked for scenarios/stations
where the real pattern data was used.
Table 1 shows noise-limited coverage,
terrain-limited coverage and interference-
free coverage for four scenarios: 1)
TVStudy default patterns and parameters;
2) Real antennas patterns with mechanical
beam tilt for KNBC, KPXN and KVEA;
3) Real antenna patterns for KPXN and
defaults for KVEA; and 4) Real antenna
patterns for KVEA and defaults for KPXN.
More details are available on the spreadsheets
and files I’ve made are available on the
file menu at xmtr.com/rf242.
The results surprised me. I did not expect
coverage to decrease when accurate
antenna models were used. The shrinkage
in a noise-limited area (contours) with
real patterns was not expected.
Next month I’ll see how the contours
from “Use real elevation patterns for contours”
match contours plotted at the radio
horizon per Part 73 and how some of the
other settings affect the results for these
I’d like to do a test with a pair of adjacent
channel stations using mechanical
beam-tilt in another market. If you
can provide real azimuth and elevation
patterns in a text-extractable format for
the stations drop me an email at email@example.com and if time allows I’ll
run the same comparisons on them.