Transmission & Distribution: Just what is my signal strength?
Just what is my signal strength?
By Don Markley
It seems that everyone wants to know what the received signal strength is for their station. The manager and sales department want a nice neat map of coverage to use in their sales pitches. They also want population and household numbers to convince clients just how they can reach buyers economically. The FCC wants coverage contours or areas to make sure that the station is meeting their requirements. Some non-viewers want the signal strength so they can bypass over-the-air reception and use their satellite service. The engineering department is concerned about whether the antenna is doing what they want, and the owner simply wants a map they can feel warm and fuzzy about.
Potomac Instruments’ FSDAQ-DT system is focused on the DTV broadcaster’s need to accumulate a large amount of field strength data to determine a station’s coverage. It was designed to be used to collect data while traveling at highway speeds. Photo courtesy Potomac Instruments.
The problem is deciding just how to determine the signal strength either at a particular point or over the total service area. Making that determination is difficult at best, whether one uses calculation schemes or measurements. It is always an education for an engineer when taking a field strength meter out for the first time. It is amazing, especially at UHF, to see the variation in the signal strength when the test antenna is moved even a few feet. That is why the Commission has defined a method to be used in measuring field strengths. It involves moving the antenna over a distance while recording the measured values, then determining the mean of those values to come up with a reasonable final value (See Section 73.686).
To be at all meaningful, field measurements must use a calibrated field intensity meter together with a calibrated antenna system. The antenna system would normally consist of a dipole and a connecting cable that has been checked for attenuation across the band. To do the measurements the dipole needs to be extended to the proper length for the frequency involved and raised to the proper height. Then the indicated field strengths must be adjusted by a correction factor that should have accompanied the meter and antenna system from the factory. When the meter is recalibrated periodically, such calibration should be done with the antenna and cable to be used for measurements. If this sounds like a lot of trouble, it is. Measurements must be done right if they are to be more useful than walking around with a portable TV set and a coat hanger antenna.
The gist of the measurement problem is that it is best done by someone with the proper experience and equipment. There are several firms that provide such service. These firms have vans equipped with the extendable antennas, meters, recording equipment and the support electronics needed. Unless an individual station contemplates doing a lot of measurements for some reason, the use of a measurement service is probably a good investment.
However, measurements are not the norm for determining coverage for a TV station. In fact, the Commission has narrowly defined those times when they will even consider measurements. The usual method is by calculations using one of several propagation models. The most common method in the past used the published FCC charts known as the F(50,50) and F(50,10) curves. There are three sets of those curves to be used for low-band VHF, high-band VHF and UHF frequencies. The curves are based on a statistical analysis of thousands of field strength measurements done in the fifties. Roughly, the curves were intended to allow the determination of predicted field strength for any radiated power at any distance for 50 percent of the locations at that distance at either 50 percent or 10 percent of the time.
There are several limitations that affect the use of these curves. For instance, the height of the antenna is considered to be determined from an average terrain calculation. This becomes a problem when atypical terrain is involved. In that case, actual coverage may differ substantially from predicted coverage.
The use of the F(50,50) or F(50,10) curves has some problems, but one must remember that its use was the best thing available for many years. The more accurate propagation methods that are now in vogue all require a significant amount of number crunching. That is relatively easy to do with the fast and inexpensive microprocessors available today. However, one didn't want to even think about such calculations 30 years ago. It would have meant spending weeks (at least) with slide rules, math tables, adding machines and so forth, not to mention days and days pulling elevations off topographic maps. It used to take one full day to get eight radials from topographic maps, plot the elevations and determine the average — a process that is now done in milliseconds. The FCC curves allowed a reasonable calculation of field strength that could be done simply with the calculation techniques that were readily available. The method could even be almost understood by attorneys, making its acceptance at the Commission a fact.
Now that everyone has reasonable computers available, propagation studies can be done by more rigorous methods. The biggest example is the allocation and interference studies that have been done by the Commission for digital television. Those studies are based on what is commonly known as the Longley-Rice Propagation Model or Tech. Note 101. The entire publication is not really a note, but a nice-sized book. You can find this, along with a lot of subsequent and related studies, through the Department of Commerce Web site at www.Commerce.gov.
But, be forewarned before you dig into its pages — the implementation of the method on a computer is a massive undertaking. You not only need to write the code for using the method, you will need a complete terrain elevation database.
The FCC has its own program written in Fortran. Several firms have been running that program on a machine such as a Sun workstation with results that essentially duplicate the FCC's calculations. Both EDX and V-Soft Engineering have developed programs for PCs. Of the two, the V-Soft programs are, in the author's opinion, more user-friendly. In addition, V-Soft has an in-house computer running the FCC's program that is available to online users for a reasonable fee.
Be warned, none of these programs are inexpensive. To do your own studies you will need to purchase the basic software program, a terrain elevation database giving elevations at 3” intervals, and a usable copy of the FCC database if interference studies are desired. Obviously, that database will have to be updated regularly, which can be done commercially. It is possible to download the database directly from the FCC. However, it is relational in nature and utilizes many different sets of data. Unless you are a very well-qualified programmer, don't mess with it — just buy it in the format you need. All of this will set you back enough that it probably won't fit on your Visa card, especially at holiday time. You can obtain the Commission's full program directly from them and modify it to run on a decent workstation, again, if you are a very experienced programmer.
So, what does it do? Basically, the method looks at a single point and determines the field strength at that point. It starts by calculating the free space field strength at that point. Then it reduces that field strength by all the factors that might be an influence. It starts by looking at the terrain for all the distance from the transmitter to the point and deciding what signal reduction the terrain will cause. It also considers the terrain coverage such as sparse or heavy vegetation. Signal absorption and clutter are evaluated and a final anticipated signal level is established. It does this very fast for the entire area involved and does it by breaking the study area up into small increments. If the study point in an increment is found to receive a certain signal level or interference, then the entire increment is considered to have that signal level or interference.
That is a broad explanation of a very complex study. However, it should be enough to understand the complex nature of the study involved. Running a Longley-Rice field strength study on a typical TV or FM station on a good 1GHz computer will take a few minutes. That translates to a whole bunch of number crunching. It also results in a good method of determining what a station's coverage area really may be.
There are other models that can be used for service predictions. Perhaps the most common of those methods is one named TIREM for Terrain Integrated Rough Earth Model. This method has also been developed by NTIA, along with various Department of Defense groups. The argument can be made that TIREM is better than Longley-Rice. TIREM is available from EDX for those who may wish to try it. However, the FCC has clearly stated that the Longley-Rice method must be used for DTV interference.
The message is simple. If you want to predict field strength, the availability of fast computing technology has made otherwise overly cumbersome methods practical. The Longley-Rice model is the method of choice for FCC work. However, unless you are going to do a lot of such work, the better programs are a bit expensive. The good news is that there are plenty of firms out there who have the software to conduct your studies. Check with your consulting engineer. Any engineering firm worthy of the name has the necessary capability.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
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