Last month I described how certain weather conditions can lead to inversions that change the refractivity of the atmosphere, allowing the signals from TV stations to be seen well beyond the radio horizon. As more DTV stations crowd into the TV spectrum, the chance these weather conditions will result in co-channel interference increases. This month I'll look at station characteristics that make some stations more likely to experience enhanced propagation and offer some suggestions on designing facilities to help overcome co-channel interference. Finally, I'll look at modes of enhanced propagation that do not depend on ducting.
As you may remember from last month's column, inversions form above the earth's surface when a warm, dry air mass slides over a cooler, moister air mass. Looking at an actual upper air sounding from UCAR of Oakland, Calif. (Fig.1), you will notice that the temperature does not change in a straight line. There is an inversion starting around 910 millibars and stopping at about 850 millibars (4,800 feet). An antenna in this region is able to propagate energy into the inversion layer, while antennas above and below it will not. For that reason, antennas located on mountains above the inversion are not likely to experience enhanced propagation nor are ones below it. Note that in most cases the antenna has to have some energy above the typical radio horizon to couple into this region. An antenna high above average terrain with a large amount of beam tilt will have little ERP at low depression angles and will have a much weaker signal over the horizon.
(click thumbnail)Fig. 1
When looking at a Skew-T diagram like Fig. 1, pay attention to the height and size of the inversion. As mentioned last month, signals can travel through a duct created by a strong inversion the same as they would travel through a leaky waveguide transmission line, with less attenuation than propagation through free space. Consequently, larger inversions will propagate lower frequencies.
Upper air charts like the one shown here are available on the Web at http://www.rap.ucar.edu/weather/upper/ and at http://weather.uwyo.edu/upperair/sounding.html. The first URL includes charts for heights up to and above 300 millibars; the second stops at 700 millibars. Note that pressure (in millibars) is shown instead of altitude.Table 1 shows the conversion based on the U.S.StandardAtmosphere (1976). See http://meteora.ucsd.edu/wx_pages/stuff/Std_Atmos_US_Low.pdf for a graphic of this.
Sometimes it's easy to determine the height of the inversion layer without upper air soundings. Flying into Los Angeles it can be quite obvious. Photo 1, taken on a recent flight into LAX while fires were burning in the Angeles National Forest, shows Mount Wilson sitting above an inversion layer.
(click thumbnail)Photo 1Stations likely to experience enhanced propagation, therefore, are ones with antennas at heights where inversions often occur and with antenna elevation patterns that have a relatively large relative field at small depression angles. Note that mechanical beam tilt will affect this. Antenna location is also a factor. Inversions occur more often near the coastline or bodies of water.
What can you do if enhanced propagation is causing interference for your viewers? The best approach is to look at methods for strengthening the desired signal. Changes in surface refractivity not only affect the interfering station, they can affect the desired station as well. Signal strengths calculated using the default 4/3-earth curvature will not be accurate if surface refractivity is changed by an inversion. Obviously the best way to reduce interference is to increase the strength of the desired signal. Increasing power is one option. If interference is occurring in Grade A and City Grade coverage areas, using an antenna with a fatter elevation pattern (less gain), more electrical beam tilt and/or more null fill will help. Studying where interference complaints are coming from may provide some insight where the desired signal has to be improved.
As a last resort, most UHF DTV stations are using horizontal polarization only. Switching to a circularly polarized or even elliptically polarized antenna will give viewers another way to maximize the desired signal while rejecting the interfering signal. The two polarities often propagate differently and while it can be difficult to discriminate polarity at the edges of a station's coverage area, the signal will be stronger if horizontal ERP is not reduced to obtain the vertical component and may be stronger even if it is, due to differences in propagation. In an earlier column I reported on research by Bob Plonka at Harris to demonstrate the advantage of having some vertically polarized signal.
OTHER MODES OF ENHANCED PROPAGATION
Although tropospheric propagation is the main topic of this discussion, it wouldn't be fair to low-channel VHF stations to ignore other modes of propagation that can be troublesome. These problems are familiar to ham radio operators ("skip" in the ionosphere and sporadic E "clouds"). Most readers of this column are familiar with "skip" and know its connection with the sunspot cycle. Engineers at Channel 2 stations have a lot of stories to tell about distant reception, often from viewers expecting to see their station!
Sporadic E-layer propagation is not as commonly known. It can affect higher frequencies than typical F-layer skip, up to and including TV Channel 13 and occurs most frequently during late spring and early summer. Sporadic E propagation does not last very long and is not likely to be a significant source of interference. Wind shear appears to be one of the causes of Sporadic E. The winds cause ions in the atmosphere to be concentrated into the "clouds" that reflect signals back to earth. The earth's magnetic field also plays a role in this.
If these articles on enhanced propagation have piqued your interest, there are two publications from the American Radio Relay League (http://www.arrl.org) that you may be interested in: "Beyond Line of Sight -A History of VHF Propagation from the pages of QST", edited by Emil Pocock (1992) and "The ARRL UHF/Microwave Experimenter's Manual"(1997).
This year's IEEE Broadcast Technical Symposium had several interesting papers on DTV reception and transmission, including the design and use of single frequency networks. Look for a full report next month. Comments are always welcome. Drop me a note at email@example.com
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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