Enhanced Propagation of TV Signals

[Gregg Lengling]

RF Technology: Doug Lung

Part I


Interference between DTV and analog stations sharing the same channel was back in the in the news this summer when WBOC in Salisbury, Md,. reported problems with interference from WHRO-DT in Hampton Roads, Va., (TV Technology, Aug. 7). You may remember the problems with DTV interference to WOOD, Channel 8 in Grand Rapids, Mich., from WMVS-DT Channel 8 in Milwaukee, Wis., almost two years ago. In the WBOC case, the interference occurred on Channel 16, proving the problem isn't limited to VHF stations.

With the FCC's November 2002 DTV construction extension deadline approaching, the number of DTV stations on the air will increase significantly. As a result, I expect to see more reports of DTV-to-analog co-channel interference. Last month I showed how the default parameters specified by FCC Bulletin OET-69 for calculating signal strength and interference with the Longley-Rice model do not always represent real-world propagation or signal strength. Changing the Longley-Rice parameters for each interference analysis to accurately represent the specific climate and tropospheric weather for each path would, of course, be extremely difficult. If such an effort were actually attempted, the number of channels available for DTV use would drop. Some of the conditions that cause interference may only exist during certain times of the year.

Even though it is unlikely the FCC will offer any solution for co-channel interference if its calculations show the amount is acceptable, it is a good idea to become familiar with the conditions that can lead to interference and know when to expect them. This month I'll describe the atmospheric conditions that can propagate TV signals beyond the horizon.


WHAT IS "ENHANCED PROPAGATION?"

If you've read Bulletin OET-69, you know that the Longley-Rice studies are to be done using a surface refractivity of 301.0 N-units (parts per million). Look at most microwave path plots and you will notice a K factor of 1.333 is used, representing 4/3 earth curvature. Both of these numbers represent the typical refractivity and effective earth curvature seen in the U.S. Although the two numbers are related, the difference is that the K factor represents the effect of the refraction on RF propagation rather than the amount of refraction. K factor makes it easy to plot propagation over earth as it changes the effective earth radius to represent the path a signal will take through an atmosphere, over the earth, with a certain amount of refraction.

With no refraction (K factor = 1), a signal would travel until it hits the horizon. As the K factor increases, the atmosphere bends the signal so that it curves toward the earth, allowing it to go past the horizon. This is why the "radio horizon" under typical conditions is farther away than the actual horizon. As the K factor increases toward infinity, the radiation that would normally go into space is bent back to the earth, well beyond the horizon.

I ran some plots with different K factors over smooth earth using a height of 365 m at one end and 10 m at the other end over earth 4 m above sea level to see how it affected propagation. (Click Here for Figures 1-3.) Fig. 1 shows the path with a K factor of 1. As you can see, the signal begins to approach the ground at about 65 km from a 365 m tower. Increase the K-factor to 1.333, the typical value, and the signal travels an extra 10 km to approximately 75 km (Fig. 2). When the K-factor reaches 20, a UHF signal can propagate 300 km, as shown in Fig. 3. Note that while the graphs look similar, the scale is much different, reflecting the change in effective earth radius with the change in K factor.


CLIMATE EFFECTS

Atmospheric refraction varies widely, depending on the local climate. Microwave paths that demand high reliability are often studied not only at a K-factor of 1.333, but also at 0.5, 1.0 and 20. The Radio Propagation Handbook by Peter Saveskie, published by TAB Books (Editor's note: TAB Books is now the TAB Electronics group of McGraw-Hill), has an appendix showing the K factor at various locations during different times of the year. Interestingly, areas around the Great Lakes are especially prone to unusually high K factors certain times of the year.

It would take more than one column to describe how to calculate surface refractivity and K factor, but it is important to understand that the refractive index of the atmosphere is determined by the temperature and humidity of the air. Normally the atmosphere becomes cooler and drier as altitude increases. The normal 1.333 K factor is based on the refraction during this condition. However, refractivity increases significantly when there is a temperature inversion - cooler, damper air underneath warmer, drier air. As expected, K factor increases as well. Under these conditions, there usually aren't clouds in the sky but the air is hazy due to the humidity near the surface. Humidity plays an important role in increasing refractivity, which is why this enhanced propagation mode is usually seen near water and seldom seen in mountains or dry areas.


WEATHER CONDITIONS THAT FAVOR ENHANCED PROPAGATION

The BBC published a paper (available at http://www.bbc.co.uk/reception/factsheets/docs/reception_weather.pdf)  describing four different types of inversions: Subsidence, Advection, Nocturnal and Frontal. I'll summarize the how and where these inversions form; you can refer to the BBC's online Adobe Acrobat document for more detailed information.

Subsidence inversion can occur when a large high-pressure system moves away from an area and allows cooler, more humid air to move in underneath it. If this results in a sharp vertical difference in humidity and temperature where the two air masses meet, refraction and K-factor will increase significantly. The BBC paper points out "This type of inversion can happen at any time of the year but is more common in the spring and autumn. It can produce abnormal propagation for extended periods over large distances of several hundred kilometers."


COMMON NEAR WATER

In the summer, advection inversions are common near bodies of water. These inversions occur when warm dry air above hot land is blown over cool water. The contrast here comes from the hot air forming a layer above a humid foggy layer near the water. The BBC notes that the same condition can happen in the winter if warm moist air from the sea is blown over cold, frosty or snow-covered ground.

If you scan the TV channels late at night or very early in the morning, you may notice many distant UHF stations are receivable that usually aren't seen during the day. This enhanced propagation is probably due to a nocturnal inversion, which occurs when the ground and the air above it cool rapidly while air higher up remains warm. As with advection inversions, fog can form. Reception distances are limited, however, because the inversion usually stays fairly close to the ground.

The last type of inversion is the frontal inversion. The BBC description notes "this type of inversion occurs during low-pressure rather than high-pressure weather." The inversions occur when temperature and humidity change dramatically along or near an advancing front. As expected, these conditions are usually transitory but they can lead to strong inversions.


DUCTING

The weather conditions I described aren't that unusual in coastal areas or in humid areas with flat terrain. What has to happen to cause the K-factor to increase to levels that allow propagation over 300 km or more? When an inversion occurs at much higher levels above the ground than usually seen with fog, at heights between 450 m and 1,500 m, tropospheric ducting can occur. Note that there can be multiple inversions stacked on above each other. In the area where the inversion occurs, the signal propagates more like it is in a leaky waveguide than traveling through free space, resulting in less loss and stronger signals. As one would expect, the thickness of the inversion determines how low a frequency will propagate through the tropospheric duct. Some ducts will support microwave or UHF propagation but not VHF propagation.

Tropospheric ducting occurs due to subsidence, frontal or advection inversions. Nocturnal inversions happen too close to the surface to form effective ducts, although ducting usually strengthens at night. When a large high-pressure system over the Pacific shifts south or north in the winter and summer months, respectively, a tropospheric duct can form between Mexico or California and Hawaii. Amateur radio operators have successfully communicated over the path from Mexico to Hawaii at distances of 4,151 km at frequencies as high as 1,296 MHz!

Highly directional receive antennas and the use of antenna rotors can often mitigate the effects of ducting. However, this kind of receive equipment is not very common.


COMING UP

Next month I'll explain why some TV stations are often seen quite far from their transmitter sites, while other stations in the same area on nearby channels are not. There are some steps stations can take to improve their signal that may reduce interference from distant stations. If you have experience with recent examples of enhanced TV propagation, please let me know. I'll need the call letters of the station, the location of the receiver and dates and times of reception to see how well reality matches theory. Drop me a note at dlung@transmitter.com.

If you are interested in learning more about tropospheric propagation, I found a Web site that offers forecasts on the location and strength of inversions and ducting that lead to enhanced propagation. Visit http://www.iprimus.ca/~hepburnw/tropo.html  for forecasts and more information on how weather affects VHF and UHF propagation.