Chapter 7 SCA BROADCAST COVERAGE DISTANCE (Can Anyone Out There Hear You?) I don't remember ever meeting a radio station manager who didn't claim his station covered at least half of the state. This remarkable coverage distance seems to have no relationship to the size of the station or the type of service. If this appears to you to be in conflict with the laws of physics - you're right!! In FM broadcast service, the transmitted energy is in what we refer to as the VHF (Very High Frequency) band. VHF signals tend to travel in direct straight line paths, neglecting the multipath reflections which occur due to part of the signal bouncing off of buildings, etc. If you were to visualize the FM broadcast signal as being a beam of light from a flashlight leaving the antenna on top of the transmitting tower and heading out toward the horizon, you would have a fair picture of what happens. The signals strength gets weaker the farther out it goes, just as the flashlights beam would become dimmer. Also, since the earth is a big sphere, sooner or later that beam would continue on it's straight line path out past the horizon and into space as the earth curved away beneath it. A fair "rule of thumb" to use to find out how far this radio signal horizon is from the station is by assuming it is about one and a fourth times the square root of the antenna height. (i.e....A 400Ft transmitter antenna yields about 25 miles.) So if you are a listener out some distance from the stations transmitter, the signal you are trying to receive is very small and may well be way up above your head, and your antenna , and you will get little or nothing to listen to. The solution to improving this situation is to increase the power level of the transmitted signal; or, increase the size of your receiving antenna; or, improve the "sensitivity" of your radio (this is not necessarily the same thing as adding an antenna amplifier, which usually will not help); or, by raising the height of the transmitting antenna or your receiving antenna, if you can get it up high enough to find the signal path; or probably doing all of these things. VHF signals, like light beams, are also fairly easily blocked by objects. Even if you live reasonably near to the transmitters site it is possible that an object, such as a building, between you and the stations tranmitter can cast a "radio shadow" over you and cause weak reception. This is especially true for listeners who live in river valleys and other low locations. In Radio Talking Book services it is usually not practical for the service to provide and install roof top mounted VHF FM antennas like is often done for TV reception (also a VHF service). This is unfortunate because not only is multipath crosstalk hard to avoid without such an antenna use, but also the "whip" type antennas usually used on the Radio Talking Book SCA receivers are about l5Db to 20Db less sensitive than the "model antenna" used by the FCC to predict how far a station will cover. It is apparent, therefore, that your broadcast coverage distance is a variable which is dependent upon power level, antenna heights, equipment employed, and environmental conditions as well. Also, do not forget that the effective distance of an SCA is, in addition, affected by the 67KHz injection level and the modulation level and bandwidth and audio processing effectiveness. And you thought I was going to give you a simple number of how many miles your service would cover? Well, I'll try to! But you will have to see to it that your Radio Talking Book service is operated correctly and also make adjustment for unusual terrain conditions to make use of my nominal distance predictions. Oh yes, before I forget, I must mention that when it comes to distance coverage predictions, I have very often been "scoffed at" for being an extreme pessimist. (Remember I told you that all station managers like BIG coverage maps!) Well, I prefer to believe that rather than being a pessimist, I am a realist. I fully appreciate that radio signal distance predictions are far from being an exact science unless you exactly calculate all the factors, and even then, unexpected variations occur which tend to make your results look like you were not sober when you determined them. So if the predictions indicate reception might not be good at some distant location, try it anyway, unless it is at a ridiculous distance, and find out how well it might be working. Just don't promise service you can't predict to work or you will only achieve in making potential listeners mad at you and your "crummy service". Keep in mind also that in providing a broadcast service you must be able to provide good listenable reception to your users. Unlike "C.B." or other radio communication services, your listeners will not patiently put up with noisy and intermittent reception. They will use and enjoy your broadcast service only if it is reasonably noise free and available to them mostly all of the time. If your listeners turn on their radio and your broadcast isn't there they won't try it again very many times! VHF signals go through wide variations in signal strengths over periods of time due to atmospheric effects. Do not be fooled by brief observations that the reception at distant locations is what you saw once. It may be either much worse or much better most of the time. We engineers try to allow for this effect by including a factor termed "fade margin" into our prediction calculations. Now that I've expressed most of the "disclaimer" comments, let's get on with how to calculate distance coverage for an FM SCA broadcast service. If we were removed from the "real world" and were floating around in space, the determination of signal coverage becomes rather easy. For one thing, we wouldn't have to concern ourselves with atmospheric effects. For another, without the earth's surface to interact with our signals radiation patterns, we can assume all of our transmitted power originates from a single point in space, and that it radiates out from that point equally in all directions. (Engineering terminology for this condition is that it is an "isotropic" source.) As the energy which is radiated by the point in space expands outward, like a bubble which keeps growing in size, the amount of energy available on a given distance of the bubble's surface gets proportionally less as the "energy bubble" (field) gets bigger. This is the amount of signal your antenna would be subjected to, and is measured in micro-volts per meter (uv/mtr). We call this the "field strength" of the transmitted signal. When an electrical conductor, such as your radios antenna, intercepts such a signals field strength at some point in space, electrical energy is delivered to do useful work such as "driving" into your radio. Electrical work is termed "watts", but the power levels we get from the antenna into the radio are much smaller than "watts" so we measure them in units of "milliwatts" (l/lOOOth of a watt). Actually, the signal levels are usually quite less than even one milliwatt, so we express them as Decibels relative to one milliwatt ("Dbm"). I could add that about six years ago a new standard term was agreed upon called the "femtowatt", but let's not go any further with that for now, it would only make this story even more "murky". What all this discussion has been leading up to is that if you are in free space and know the power level and frequency of the transmitted signal, and if you know the distance to the place of reception, then you can calculate the available field strength (uv/mtr). From that you can determine how much signal power will be delivered to the radio (Dbm). The mathematical equation for that calculation is: ( 2 2 ) ( ( Eo ) ( K Wl ) ) Dbm = 10 Log ( (120 Pi) ( 4 Pi ) ) ( -3 ) ( 10 ) Where: Eo = Field Strength (v/m) available. K = 1.00 (isotropic antenna) Wl = 3.06 meters (assuming 98MHz) Pi = 3.1459...... For example, at the lmv/meter Field strength locations (the limit of the so called urban coverage area), this equation reveals that there is -57Dbm available. Converting this power level (-57Dbm) to determine the voltage level which would exist across a 50 ohm load, discloses that there would be 315 microvolts (uv) input at the receivers 50 ohm antenna terminal. Knowing how much signal power is available (Dbm) and having knowledge about how well the receiver performs at various signal power levels, we now know if the system can be expected to provide usable service for the conditions of transmitted signal power and the distances involved. Well, that's not too hard! But, unfortunately the real world doesn't work quite so predictably as do our physics theories. Fortunately, however, many people, not the least among them is the F.C.C., have wanted to know the answer to these distance coverage questions for quite a long time. Therefore, a great deal of study, which includes lots of empirical data, has been gathered and compiled into what we, and the F.C.C., call the F(50-50) coverage prediction curves. These curves are found in part 73.333 of the F.C.C. rules and regulations. The term"50-50" means that at least 50% of the people will get the predicted signal strength 50% or more of the time, when using a receiving antenna at 30 FT above ground. However, when using these curves to predict coverage, remember that the SCA coverage of the usual Radio Talking Book service will be about 10 to 20Db below this main channel FM performance due to it's being a subcarrier of the main carrier and because of the typical use of whip antennas which are mounted on the receiver. To use the F(50,50) curves you must first determine your operating parameters, such as the transmitted power level and transmitting antenna height above ground. Next, using the sliding scale provided for these curves, line up your actual power level with the "40" on the left edge of F50,50 curves, and the sliding scales right edge vertically in line with your transmitting antenna height, which is across the bottom of the F(50,50) curves. The sloping lines of the curves now will intersect the right edge of the sliding chart, which reads the predicted Field Strengths at the mileage represented by the curved lines. The field strengths obtained from this chart are in terms of Dbu (Db relative to luv/mtr) and also directly in microvolts per meter (uv/mtr). Please read FCC rules sections 73.311, 73.312, and 73.313 to get further understanding of this subject. With the predicted Field Strength at various mileages now available to you from the F(50,50) curves, it is a fairly simple matter to calculate the expected signal level into the receivers at those locations, using the equation previously presented or similar methods. At this point I can offer some advice and observations. We are fairly well aware what the limits of acceptable performance are for FM and FM SCA service in terms of Field Strength. We know, for example, that the limit of the "good" urban coverage of an FM station exists out to the distance where the Field Strength has become 60DBu (lOOOuv/mtr). The limit of the "fair" rural coverage exists out to the distance where the Field Strength has become 50Dbu (300uv/mtr). And the absolute distance of any reasonable hope of usable reception extends out to about 40Dbu (lOOuv/mtr). Using this as a guide we can create a "typical" coverage chart for various classes of service. Let's look at some examples. A common low power station on a college campus might use 100 watts at 100 feet. A full facility class "A" FM station is 3KW (kilowatts) at 300 feet. A full facility class "B" FM station is 50KW at 500 feet. A common facility class "C" is lOOKW at 1000 feet, and a full facility class "C" station is lOOKW at 2,000 feet. (The "feet" listed is the transmitting antennas height above average terrain, and the power levels are the effective radiated power ("ERP") from the antenna.) This yields the coverage predictions for FM main channel performance shown in Table 1. However, what we have here are the predictions of how well the main channel FM service should work. Considering that the SCA service will be at least lODb worse than the main channel due to the previously listed factors, we can "subtract" that degree of performance from the prediction data. This suggests that the SCA should be "good" out to 70Dbu, "fair" out to 60Dbu, and "poor" to "unworkable" out at 50Dbu and beyond. This creates the chart for typical SCA Radio Talking Book service performance shown in Table 2. Table 1: FM MAIN CHANNEL PERFORMANCE Miles to Field Strength of: Station Class 60Dbu(good) 50Dbu(fair) 40Dbu(poor) Low Power (lOOW @ lOOft) 3.3 5.8 10.5 "A" (3KW @ 300ft) 15 24 37 "B" (50KW @ 500ft) 32 46 61 "C" (lOOKW @ lOOOft) 45 59 78 "C" (lOOKW @ 2000ft) 57 75 95 Table 2: FM SCA PERFORMANCE Miles to Field Strength of: Station Class 70Dbu(good) 60Dbu(fair) 50Dbu(poor) Low Power (lOOW @ lOOft) 1.9 3.3 5.8 "A" (3KW @ 300ft) 7 15 24 "B" (50KW @ 500ft) 20 32 46 "C" (lOOKW @ lOOOft) 31 45 59 "C" (lOOKW @ 2000ft) 42 57 75 What this data tells us is that except for "shadow areas", listeners up to the first listed column should all get very good reception. Listeners between the first and second columns will generally have usable service, but they are in your "fringe" area and some of them, especially those near the second column distances, may not be able to receive your programs unless they install outside antennas on the roofs of their homes. People who live between the second and third column distances will probably usually have to use outside antennas if they get any reception at all. People living beyond the third column will not normally be able to use your service at all excepting exceptional conditions and antenna installations. You should now be able to respond with a good answer when someone asks you if anyone out there can hear you!