Earlier this year TV TechCheck reported on field experiments
performed by Dielectric Communications (Raymond, Maine, www.dielectric.com),
which suggest that use of circularly polarized transmit antennas
at UHF may offer performance advantages for mobile reception (see
the July 13, 2009 issue of TV TechCheck). In a recent presentation
to the Association of Federal Communications Consulting Engineers
(AFCCE, www.afcce.org), Kerry Cozad, Senior Vice President, Broadcast
Engineering with Dielectric provided an update on this topic,
including new information on how the use of circular polarization
("Cpol") compares to horizontal ("Hpol") and vertical ("Vpol")
polarization at VHF.
The new VHF performance data (obtained at 210 MHz) resulted from
field experiments conducted in a manner similar to those used
to obtain the UHF data (obtained at 700 MHz) discussed previously.
In the UHF tests, Dielectric utilized three transmit antennas
(with Hpol, Vpol, and Cpol, respectively) mounted side-by-side
and operated one at a time, but at the same frequency, ERP and
azimuth and elevation patterns, so that the results obtained from
each would be comparable. For VHF, Dielectric used a single dual-feed
patch antenna and simply varied the vertically and horizontally
polarized feed components (while keeping constant ERP) to establish
the three polarization conditions tested.
For both UHF and VHF tests, signal reception into an "electrically
small" linearly polarized receive antenna was evaluated for each
of the three transmit polarizations in five different reception
environments: open areas, wooded areas, office building, house,
and small vehicle. The first graph (above) summarizes the average
margin improvement obtained using Cpol compared to Hpol and Vpol,
for both VHF and UHF cases. The second graph (below) illustrates
(for VHF only) the margin improvement in each reception environment
using Hpol as a baseline, where the dashed lines in the graph
represent the average margin improvement for Cpol (red) and Vpol
(blue) compared to Hpol.
In analyzing these results, Mr. Cozad noted that the difference
between the Vpol and Hpol margin improvement is smaller for VHF
(1 dB) than for UHF (2.5 dB), and suggested that one reason for
this is that the electrically small receive antenna provides less
polarization discrimination for VHF than it does for UHF (note
that the wavelength of the 700 MHz carrier is 17" while that of
the 210 MHz carrier is 56", and that these different frequencies
were being received by the same receive antenna). Regarding the
improvement offered by Cpol, Mr. Cozad said that while linear
polarizations are subject to "nulls" in coverage, circular polarization
"fills in" those nulls resulting in improved reception.
For additional information on this study, contact Kerry Cozad
at Dielectric, at Kerry.Cozad@Dielectric.spx.com.
ATSC Digital Television 8-VSB Transmission System
Fundamentals & Measurement Seminar
January 20-21, 2010
Milwaukee Area Technical College
A 2-day seminar will be presented on the ATSC's digital television
(DTV) vestigial sideband (VSB) transmission system fundamental
concepts and measurement methodologies by Gary Sgrignoli, of Meintel,
Sgrignoli & Wallace. This seminar is a combination of the original
VSB fundamentals and the VSB measurements presentations. This
updated seminar will help you develop an understanding of the
8-VSB transmission system basics as well as measurement techniques
in the laboratory, at a transmitter site and a remote field test
site. To sign up or if you have questions contact Kevin Kukowski,
MPTV, (414) 297-7576, kukowskk@matc.eduor Gary Sgrignoli, (847)
259-3352,gary.sgrignoli@IEEE.org.
