On June 23, 2017, WNJU Telemundo 47 officially began broadcasting from One World Trade Center (1WTC) in New York, becoming the first station to return to this site since Sept. 11, 2001. Other New York City stations will be joining WNJU there soon—WCBS, WNBC, WPXN and WNET—sharing the UHF and VHF panel antennas mounted on the spire.
The UHF panel antennas, RFS models PEP40 and PEP96, contain two crossed dipoles (in an “X” configuration) with the +45-degree and –45-degree elements fed with two transmission lines. Transmitter power is split evenly between the +45 and –45-degree elements. The phase of the two feeds is varied to change the polarization from horizontal polarization only to full circular polarization. Transmitting only vertical polarization would be possible if the FCC allowed it. polarization from horizontal polarization only to full circular polarization. Transmitting only vertical polarization would be possible if the FCC allowed it.
|John Gonzalez (L), WNJU director of technology and operations, and John Lyons, assistant vice president and director of broadcast communication of the Durst Organization, push the button to fire up the WTC1 broadcast antenna.
TESTING FIRST FULL-POWER SIGNALS
Prior to the official launch of WNJU’s signal from 1WTC we conducted test transmissions. In my RF Technology column “An Inexpensive Setup for Rapid DTV Field Measurements” (Jan. 11, 2015), I described the system used for testing coverage from test transmitters and antennas installed at 1WTC. These tests were done using circular polarization and showed the site was capable of penetrating the “midtown mountain” of high-rises to provide coverage on the north side of Manhattan.
When the WNJU transmitter installation at 1WTC was completed enough to allow overnight on-air testing at full power, we conducted measurements as close as possible to a circle with an 8-mile radius centered on 1WTC. Unlike the 2015 measurements, this time we used a WNJU ENG van with a 38-foot mast and two Blonder Tongue broadband log-periodic antennas mounted on top of the mast at the same height. One antenna was horizontally polarized and the other vertically polarized. Measurements were made using a Rohde and Schwarz ETL analyzer with drive test software and the same Hauppauge Aero-M tuners, LNA, splitter arrangement used for the 2015 measurements.
The WNJU 1WTC facility was transmitting 340 kW ERP, circularly polarized. In addition to a quick check on antenna performance, this gave me an excellent opportunity to see how vertical polarization performed in the field at outdoor antenna heights under various conditions. Where possible, measurements were made where we could see the 1WTC spire from the test site. In these locations, measured channel power was around –21 dBm on both antennas (vertical and horizontal polarization).
THE BENEFITS OF VERTICAL POLARIZATION
In some locations, particularly to the east in Brooklyn and Queens, it was difficult to find a site free of obstructions at the required angle and radius from 1WTC. I was surprised to see that in these locations—with clutter from surrounding high-rise apartment buildings—the horizontally polarized signal was down 5–6 dB from the unobstructed sites while the vertically polarized signal only lost 1 or 2 dB; this after peaking the antenna on the horizontally polarized signal.
This behavior was seen in multiple locations. At two sites in Staten Island on the same radial, we got strong, approximately equal signals on both polarities at a site by the water with a clear view of 1WTC. This site was too close to 1WTC, so we moved inland, but were only able to find a location partially obstructed by a building. As observed at the other sites, the vertical polarization had a 3–4 dB advantage over the horizontally polarized signal.
I haven’t had a chance to analyze all the data we collected, but the field observations indicate that in urban areas with clutter, the vertically polarized signal appeared to propagate much better than the horizontally polarized signal. It’s possible the receive antenna pattern had something to do with this, but as I noted, the received power was the same on both antennas when we had a clear, clutter-free, line-of-sight to 1WTC.
When repack work slows down, I’ll take a look at the echo plots from the ETL to how echoes (reflections) differed between the two polarities. Perhaps we should start looking for receive antennas that are circularly polarized or at least designed to be mounted for something other than horizontal polarization. Would a receive antenna tilted at 45 degrees work better than mounted for horizontal polarization?
HOW MUCH VERTICAL ERP IS ENOUGH?
One of the questions I heard frequently at the 2017 NAB Show and again at the ATSC Next-Gen TV conference in Washington, D.C., was, “How much of my power should I put in a vertically polarized signal?”
Many stations will have to change antennas, and often transmitters, as a result of the incentive auction. With at least part of the cost reimbursed by the government from auction proceeds, it may be worth spending some additional money to add or increase the amount of vertically polarized signal transmitted.
During the last round of maximization, I was able to increase vertically polarized ERP up to 50 percent of the horizontal ERP when using directional antennas at some stations with good results. With less directional antennas, getting sufficient transmitter power to transmit 1,000 kW horizontal and 500 kW vertical just wasn’t practical in 2009. Today it isn’t that difficult to achieve high power in a relatively small space.
The WNJU Rohde and Schwarz THU-9- EVO transmitter at WNJU is capable of producing up to 106 kW ATSC 1.0 power in six racks. Solid-state transmitters from Comark and GatesAir can now provide similar power density.
IMPLICATIONS FOR REPACK ANTENNA AND TRANMITTER DESIGN
Even though the benefits of additional vertically polarized ERP are clear, unless an antenna like the RFS PEP or Dielectric APT panel array is used the horizontal polarization (h-pol) gain and vertical polarization (v-pol) gain has to be set before the antenna is shipped. Adding v-pol ERP will require additional transmitter power to maintain the h-pol ERP, which is what determines the FCC contour.
Many stations had their ERP reduced as a result of being moved to lower channels in the FCC repack. See my column “Repack Complexities Abound” (February 2017), for a chart showing the amount of reduction for channel changes.
I expect many stations will be filing for power increases in the second priority filing window. If it isn’t possible to install enough transmitter now to make the maximized contour with the desired v-pol ERP, consider installing an antenna for the new channel with additional v-pol and hold off a bit on building the maximized construction permit until the transmitter can be upgraded.
With solid-state transmitters, this could be as simple as adding another rack of amplifiers and combiner, but it is important to let your transmitter vendor know your upgrade plans when making the initial purchase. Picking the right antenna and transmitter configuration now, even if it requires some short-term compromises, could pay off in the long run.
Doug Lung is vice president of Broadcast Technology, NBC/Telemundo stations. He welcomes your comments and questions. Email him at firstname.lastname@example.org.