Coverage holes

[Gregg Lengling]

BY DON MARKLEY

Broadcast Engineering, Mar 1, 2004

 
The terrible truth is that you can build a beautiful transmitting plant using the latest state-of-the-art transmitters and antennas and still find areas where the signal is less than optimum. That is another way of saying that some of your viewers won't be viewing, causing them to complain to the front office. Even worse, they may mention the problem to advertisers. The result will inevitably be a reaction varying from management memos to loud discussions at staff meetings. As chief engineer, you have to determine what the problem may be and then determine if it is fixable.

First things first
The first thing to do is check the transmitter plant itself. The problem probably doesn't lie there, but it's always a good idea just to verify that it's working properly. And, if the vast majority of your predicted service area is receiving good signals, the problem probably doesn't lie with your antenna system, either. Transmitting antennas are so finely tuned that any failure will probably show up very quickly at the transmitter. The exception is an antenna with lots of power dividers, individual lines and pieces that can fail, such as a panel or bat-wing type of antenna. There, failures can occur well down the power-division chain without showing up on the transmitter VSWR meter. But such failures aren't going to affect just the signal out on Moose Breath Road; they'll affect everything in the direction of Moose Breath Road and in some of the area on either side of that path. An antenna malfunction cannot create a hole in signal coverage. That phenomenon is caused by path blockage or terrain variation.

Propagation study
Next, contact that oft-maligned helper, the station's consulting engineer. He should perform a Longley-Rice propagation study of the area involved to see if the problem is terrain-related. The study looks at the terrain all the way from the transmitter to the individual receiving point. But the current method uses relatively large blocks of area and checks within those blocks for signal. If the consulting engineer finds a signal anywhere within the block, the entire block is considered to be receiving service. If problems seem to be occurring in small areas, have the consulting engineer perform the study with areas only a few tenths of a kilometer on a side. This is much the same as adding more pixels to a display — it offers more detailed results. If the study indicates that the signal in the problem area should be good, it's time to break out the meters and visit the area.

On-site investigation
When you visit a site, pay particular attention to man-made obstacles such as buildings. An obvious example of such a problem area is the north side of Chicago. Similar to other cities, Chicago has an area of tall apartment and office buildings separated by concrete canyons. The signal quality down in the lower parts of those areas is poor, and can reasonably be cured only by (pardon the foul language) cable TV service. Besides blocking a signal's direct path, a large, flat-sided building can create horrendous multipath problems. For example, the Sears building in Chicago has long caused an apparent ghost in signals originating on the John Hancock building and being received northwest of the loop. The ghost is a reflection off the side of the Sears building, which is acting like a big billboard reflector.

If there is a problem, it probably isn't going to be limited to a few houses. But there are always exceptions, usually where homes are located in a comparatively low-lying area or on the wrong side of a steep hill. In hill country, there can be large areas, including cities, that simply are blocked from receiving the main signal. For years, analog broadcasters have treated these problems by using either boosters or translators. That isn't going to change with DTV. But just how DTV broadcasters will do it is still uncertain, pending rulemaking and the accumulation of more experience.

One approach to solving such a problem is using on-channel, synchronous transmitters. For example, Penn State Public Broadcasting has proposed using such transmitters to cover local communities that can't receive the direct signal from the main transmitter. For more information, check out the Axcera Web site at //www.axcera.com.

In fact, broadcasters have put a considerable amount of effort into investigating the use of several lower-power transmitter sites instead of one main site to cover the desired area. That may be a good solution for servicing hilly or mountainous areas. So far, though, the investigators haven't amassed enough experience to provide conclusive answers.

For more even terrain, ATV services are turning out to work well. As a general rule, if a station has had good analog service in an area, the digital signal will be good there as well. DTV service is turning out be robust. A lot of the old fears that many broadcasters expressed concerning signals over the horizon, shadowing, etc. are turning out to be unwarranted. In other words, DTV is working well. In fact, many stations are finding that their DTV service is better than their analog service ever was.

Antenna problems?
If there is no apparent reason why the signal propagating in one or more directions from a station is not providing the desired service, it is possible that the transmitting antenna system, together with the structure on which it is mounted, isn't providing the desired performance. Unfortunately, a lot of stations try to save money by buying a new antenna without commissioning a study to determine how it will perform in the total installation. There is no such thing as an omnidirectional antenna in a side-mounted configuration. But you can predict the effects of the tower and, as a result, attain a reasonably accurate analysis of the predicted coverage. If you haven't commissioned such a study, or if you have and the study reveals a problem, it is possible to do some measurements to confirm that the antenna is doing its job.

Ground measurements
In previous articles, we have discussed measuring television signal strength. Here, the task isn't as much to determine the exact value of signal strength as to determine the antenna pattern — that is, the relative pattern of the transmitted signal. You can sometimes spot severe problems by using a field-strength meter on the ground. The procedure is to properly extend and adjust the meter's antenna, take measurements at a number of points within a small area, and average the measured values. You can repeat this procedure at a number of locations, all of which you should carefully select to be at the same distance from the antenna and to be relatively clear of obstacles. If you suspect there's a nasty problem, such measurements may confirm its existence. The problem with such measurements is that it is extremely difficult, if not impossible, to separate the effects of terrain and path obstacles from the performance of the antenna itself. Don't expect this method to identify small changes or problems in antenna performance.

Mid-air measurements
Measurements made from an airplane or helicopter, when properly performed and analyzed, can give a more accurate representation of antenna performance. Most consulting engineers have their favorite methods of performing such measurements based on experience. There are a couple of firms that perform measurements from an airplane to analyze antenna performance. The author has seen the results of some such measurements that were fairly good, but he has also seen some that were useless. Don't simply take such measurements on your own. You can get much more meaningful results by working with the station's consulting engineer and the measurement firm. It's not difficult to obtain huge amounts of data. The real challenge is analyzing the results to provide answers to your questions.

An imperfect world
Finally, you may find that there just isn't any way to get signals to a particular area. The station's management may not like this conclusion, but they will likely find it preferable to buying and installing a translator for four homes.


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Don Markley is president of D.L. Markley and Associates, Peoria, IL