Broadcasters have watched the number of people receiving TV service over the air drop in comparison to the number of people receiving TV over cable or satellite. Interest is increasing, however, in receiving TV on portable devices such as laptops, media players, cell phones, in-car displays. Broadcasters formed the Open Mobile Video Coalition to promote broadcast TV to portable/handheld devices and ATSC M/H is well on its way to becoming a candidate standard. Conventional transmission system designs that worked well for fixed reception may not be the best way to get RF to these mobile/handheld devices.
FEW USE ELLIPTICAL OR CP NOW
Most stations today transmit signals using horizontal polarization only. Of the 1,237 licensed DTV stations listed in my CDBS TV Engineering spreadsheet for Aug. 27, 2008, only 6.2 percent use any vertical polarization, while 13.7 percent of the 699 post-transition construction permits have at least some power vertically polarized. As FCC interference and allocation studies are based on horizontal polarization only, it is relatively easy to modify an existing construction permit to specify elliptical or circular polarization, so we may see this percentage increase as more broadcasters examine how their over-the-air signals will be used.
Before discussing the benefits and tradeoffs in using elliptical or circular polarization (EP or CP for short), it is important to note that not all antennas with horizontal and vertical signals are CP or EP, but may instead may be "slant" polarized. A slant or diagonally polarized antenna transmits a signal that's polarized somewhere between vertical and horizontal, in one plane only. Use of such an antenna appears to be a violation of FCC rules Section 73.682(a)(14) that require CP or EP antennas be right-hand polarized. A station with an antenna transmitting a slant polarized signal at 45 degrees from vertical might claim 100 kW horizontal polarization ("h-pol") and 100 kW vertical polarization ("v-pol") when the signal was really 200 kW in the diagonal plane.
While a slant polarized antenna will provide more coupling into vertically polarized receive antennas and twice the power of a CP antenna into a dipole antenna oriented to the same polarization angle, on a line-of-sight path it is easy to see it the signal will drop off significantly as the dipole is rotated 90 degrees from the transmit polarization. For a true CP or EP antenna, there should be no angle where the receive antenna loses signal. Picture the transmitted signal as a rotating vector, as compared to a vector in one plane only for slant polarization. A slant polarized antenna will not provide the same advantages as a CP or EP antenna.
Chart 1: Experiment results—heavy scatter, depolarized environment, CP provides margin improvement of all possibilities of service.
Designing an antenna for CP or EP is more complicated than splitting the transmitter power between a horizontally polarized antenna and a vertically polarized antenna. Antenna Concepts, in their Blaster line of antennas, uses stacked helical antennas as radiators. They generate a true CP signal.
Dielectric Communications uses crossed dipoles with the power split controlled by a divider to achieve variable amounts of true EP or CP in its new TUM broadband panels. This ensures the phase relationship of the split dipoles remains constant at different frequencies and requires only one feed line for each panel. In slot antennas, the vertical slot creates an h-pol signal. Adding an external dipole to the slot allows generation of a v-pol signal, which when properly phased with the h-pol signal, creates the EP or CP signal.
One thing I've learned working with both ERI and Dielectric in the design of post-transition EP antennas is it can be very difficult to get the v-pol azimuth pattern to match a directional h-pol azimuth pattern. A small change in the h-pol pattern may allow a much better match with the v-pol pattern. Antenna manufacturers have software that can accurately model both patterns, allowing them to modify the physical design of the antenna for the best match.
Transmitting EP or CP requires some tradeoffs. If h-pol ERP and pattern directivity aren't changed, additional transmitter power will be needed. CP operation will require double the transmitter power, increasing both capital and operating cost. Increasing over-all antenna gain by narrowing the azimuth or elevation patterns reduces transmitter requirements. Using a more directional v-pol azimuth pattern may help. It won't change FCC coverage. A more directional elevation pattern adds gain, but is likely to reduce field strength in some areas while requiring more tower space. Worst case, it may be necessary to reduce the h-pol coverage to add v-pol. Is it worth it?
BENEFITS OF CP
ERI and Dielectric presented papers outlining the benefits of CP at NAB this year. Senior RF Engineer Myron Fanton of ERI showed that vertically and horizontally polarized signals are uncorrelated because the major propagation effects—line of sight reception, diffraction over obstacles, reflection and refraction (to some extent) are polarization sensitive. Where a null occurs in the h-pol signal it is unlikely to occur in the v-pol signal and vice versa. With the appropriate diversity receive system, Fanton cites studies showing diversity gains of 9 to 11 dB for Rayleigh (non-line-of-sight) propagation. In this case, reducing the h-pol by 3 dB to generate CP would still result in a 6 to 8 dB overall gain in system performance.
In his presentation, Kerry Cozad, senior vice president for engineering and technology at Dielectric, noted that field tests conducted in the early 1990s showed adding 10 to 25 percent v-pol that led to significant improvements in picture quality. However, DTV testing at WRAL-TV found no improvement with the addition of a v-pol signal. Dielectric's shipments of EP antennas peaked in 1995 and dropped off until last year, when they saw a sharp increase in sales. One hundred percent of the mobile media (700 MHz) antennas Dielectric has sold have been circularly polarized. To determine how CP compared with linear polarizations, John Schadler, director, advanced antenna systems development at Dielectric arranged a transmit antenna capable of h-pol, v-pol or CP at one end of a test chamber, a number of scatter objects in the middle, and, at the other end, an antenna on a rotor representing a cell phone or portable media device that would be moved through a variety of positions. As the phone was rotated, the field strength was recorded for different transmitting antenna configurations. The results showed that CP provided a 4 to 5 dB improvement in signal variability compared to h-pol or v-pol only antennas. It was interesting to see that v-pol signal alone was about 1 dB worse in variability than the h-pol only signal. Reducing variability is equivalent to increasing the SNR margin. Dielectric's tests indicate that for handheld reception, it may be worth sacrificing some h-pol ERP to obtain CP or EP.
MUST HAVE V-POL
There is another reason to include v-pol in new antenna designs. In urban areas, the mobile/handheld device will probably not have line of sight to the transmitting antenna, and even if it does, surrounding buildings will provide reflections that depolarize the signal—reflections will change the polarity of the signal. This means a receiver is likely to find a path that provides reception even if the receive antenna and transmit antenna are cross polarized. Move outside the urban area to an elevated expressway with line of sight to the transmitter and there may be few reflections to provide the vertically polarized signal the vehicle's antenna requires.
By now, it should be obvious that some amount of v-pol effective radiated power (ERP) is desirable if indoor, mobile or portable reception is important. The tough decision is how much ERP is needed? Full CP would be best and should not be too difficult for VHF DTV stations. Indeed, going full CP is one way to get more RF in the air if interference or FCC power/height restrictions limit h-pol ERP.
For UHF stations operating at 1,000 kW ERP, it is harder to justify doubling transmitter size and power bills. In that case, evaluate where the audience is. Don't forget highways if mobile TV is important. Will a more directional azimuth pattern provide coverage to major population areas, with distributed transmission used to cover isolated communities in other directions? For example, an omnidirectional pattern may not be the best choice for a Los Angeles UHF station on Mount Wilson, as coverage to the north is blocked by mountains in all but a few directions. One medium power distributed transmitter on the other side of the mountain is a better way to serve this population. If the transmitter is located in center of the area being served, it may be better to hold off maximizing to 1,000 kW ERP h-pol and instead reduce the h-pol ERP to 750 kW and put the other 250 kW into the v-pol signal or even drop to 500 kW and go full CP, if mobile/handheld/portable reception is the station's core business model. The benefit of this approach is that in the future, if more power is needed, amplifiers cabinets can be added without the expense and disruption of changing out the antenna.
One cautionary note—a station that depends on its over-the-air signal to reach distant cable headends, translators or rural viewers with outdoor antennas will not want to sacrifice h-pol ERP for more v-pol ERP. At least not without first making sure the headends and viewers can either use a circularly polarized receive antenna, which will offset the reduction in h-pol ERP if the antenna gain is the same, or are able to accept the reduction in h-pol signal strength.
What is the optimum amount of v-pol in an elliptically polarized antenna? New research from Dielectric provided some surprising results. I'll have the details next month.
Comments are always welcome! E-mail me at [email protected].