The annual IEEE
Broadcast Symposium
is one of the
best forums to find out
about new broadcast RF
technology, whether it
be antennas or transmitters, radio or television.
The symposium, of course, is not limited
to RF and this year there was a tutorial
on IP in broadcasting focusing primarily on
studio applications and a session devoted to
audio in radio and TV.
This month I’ll describe some of the
broadcast RF presentations at this year’s
event.
DTV COVERAGE
Wednesday afternoon’s tutorial, “Computer
Simulation Tools for Broadcast Engineering”
included two papers about one of
my favorite topics: RF propagation and coverage
analysis.
Bill Meintel from Meintel, Sgrignoli and
Wallace is known for his work on the software
used to determine DTV coverage and
interference in accordance with FCC Office
of Engineering and Technology Bulletin
69 (OET-69). The law allowing the FCC
to conduct incentive auctions requires the
use of OET-69 to determine coverage and
interference as a result of the planned repacking
to free up spectrum for wireless
broadband.
While some of the paper reviewed topics
I’ve previously discussed, Meintel spent
some time explaining the complications
that will come with the repacking. Before
the FCC or an engineer can run a new coverage
and interference study, they will have
to know whether the desired station is on a
new channel and the channels of interfering
stations, which will change after repacking
and perhaps even during the auction.
PROPAGATION ANALYSIS
Sid Shumate from Givens and Bell presented
a tutorial on “Propagation Analysis.”
As with Meintel’s presentation, some of the
information wasn’t new, although I found
his discussion of empirical and deterministic-
based propagation models useful and his
detailed explanation of the methods used to
determine path loss in deterministic models
very interesting.
Shumate discussed the limitations of
ITM/Longley-Rice, the basis of OET-69, with
regards to the way it handles diffraction.
He noted that the Kirchoff-Fresnel model
used in ITM is now depreciated for shadow
area diffraction. “New shadow-area diffraction
methodology based on a better understanding
of the Huygens principle, has
lead to advancements
in stealth technology,
fractal antennas, and
metamaterials, even to
a patent [to the Fractal
Antenna Corp.] for electromagnetic
clocking,”
Shumate said.
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Broken coax connector |
ITM has not been updated
to a new diffraction
model, but Sid has
been working on updating
his ITWOM model.
Perhaps sometime next
year he’ll release a new
version of ITWOM that
improves accuracy over
the version currently
available with the SPLAT propagation software.
For anyone looking for a review of the
basics of RF propagation calculations, Shumate’s
paper was included in the IEEE 2012
Broadcast Symposium Proceedings.
INTERMODULATION ISSUES
Intermodulation can propose problems
for receivers at and around crowded
antenna sites. There were two papers presented
on this topic, one by consulting
engineer Greg Best covering methods for
calculating intermodulation (IM) at a site
and another by Merrill Weiss of the Merrill
Weiss Group showing the real-world impact
of passive intermodulation (PIM) on
a site at Mount Diablo in northern California.
Best gave an overview of site interference,
explaining it can either be conducted
(intrasystem or self-interference, as in a
diplexer), or radiated interference (occurring
between different systems with their
own antennas). IM can be created in receiver,
transmitter or passive components.
Much of the simulation work on IM has
been for the military, mostly shipboard applications
with a number of RF systems
in a confined area. The use of computer
simulations is complicated by the need
to create a mathematical model of the
actual equipment to determine their potential
to create interference. For example,
real-world filters have bandpass regions
outside their design region that can make
them susceptible to IM. In reality, computer
simulation of IM will be limited in the
number of transmitters that can be studied,
the order of IM products calculated—
Best pointed out some can only calculate
up to third IM products—file size limits on
the computer used and the program’s assumption
that devices are linear.
Best said ComSite Pro is the only
computer simulation he found that was
up-to-date and commercially available.
Check the ComSite Software website
(www.comsitesoftware.com/cs_pro.aspx)
for several useful and free tools. Unfortunately
Best’s paper was not included in the
proceedings from the symposium but may
be available from him or the IEEE BTS.
Weiss described how he measured PIM
for his Mount Diablo project, complete
with graphs and photos. He observed
that PIM cases are likely to become more
prevalent with repacking of stations into
the low end of the UHF band and with an
increase in the number of FM IBOC operations.
He found that the assumption that
a mask filter controls emitted spectrum
isn’t always correct. PIM can be caused
by any passive component in the RF system—
filters, cables, connectors, even incorrect
choices of metals and plating.
Beyond the RF system, PIM can be
generated by corroded or contaminated
elements, dissimilar metals in contact, and
rust acting like a diode on or around the
tower. When RF current flows, bolt extensions
can work like antennas! Weiss
showed a video indicating the effect of
weather on PIM; the change was dramatic.
He presented a huge amount of data in a
short period of time. If there is interest, I
will cover his presentation, including the
how and why, in a future RF Technology
column. His presentation is available in
the proceedings.
Next month, I’ll take a look at some other
presentations, including Mobile DTV,
the future of broadcasting and two papers
from FCC engineers—Martin Doczkat’s
presentation on RF exposure simulation
and compliance and Bob Weller’s incentive
auction update.
Let me know the topics you would like
me to cover in more detail. Email me at
dlung@transmitter.com.