Dataworld Tracks AM Night Skywave Signals

(published in Radio World May 26, 1993)

By Timothy Z. Sawyer
T.Z. Sawyer Technical Consultants

CHEVY CHASE, Md. As a frequency allocation and antenna design consultant, I can appreciate the complexity of the computer programming skills needed to provide our industry with a computer program that is quick and cost-effective in determining the ability to add a new nighttime AM operation, or modify an existing one. Dataworld's new AM Nighttime Study program fulfills this need and more.

The program may be accessed remotely via a computer modem or the staff of Dataworld will run it for you, sending you the results, if you so desire, via an overnight mail or delivery service. All that is needed to run this program are two items of information: the frequency of interest, and the station's antenna location (geographical coordinates).

Quick retrieval

If you are examining an existing facility, the program can retrieve the geographical location of the antenna system via its' extensive database of operating stations or pending applications.

Once you enter the information, the program will compute the maximum allowable radiation values toward all stations requiring protection by the proposed operation. The program computes these values for both domestic and international stations located in our hemisphere.

The FCC Rules and Regulations require that we consider the impact of our proposed operation on co-channel stations and first adjacent domestic channels, plus, co-channel international stations. This is a considerable amount of information to process and evaluate, which is why this computer program is a valuable resource to the frequency allocation engineer.

Night Limits

One term mentioned a number of times in this article is RSS Nighttime Limit. RSS is simply the square root of the sum of the squares (root-sum-square) of each value added. For example, if we add 4 + 3 we get 7, the same addition as a RSS value equals 5.

We use RSS to add the individual levels (contributions) of the interfering signal arriving from each station to calculate the nighttime limits of each station requiring study. There are two values that are of interest to us: the 50% Exclusion RSS Nighttime Limit and the 25% Exclusion RSS Nighttime Limit.

The actual RSS is composed of all the interfering signals and is called the 0% RSS Exclusion value since no signals are excluded from the calculations. Applying a 50% (or 25%) exclusion principle to the RSS calculation means that each succeeding signal is compared to the Running RSS (subtotal), and if it is less than 50% (or 25%) of the Running RSS the value is excluded from the RSS.

Station interference

As an example, we have four stations that contribute interference to a station. To determine the amount of signal we can add to a station's RSS Nighttime Limit, we must first find the 50% Exclusion RSS Nighttime Limit and the 25% Exclusion RSS Nighttime Limit of the distant station.

We have determined that the signal levels (after applying the appropriate protection ratios) from other stations to the station that we are interested in are as follows: Station A 3.65 mV/m, Station B 1.28 mV/m, Station C 2.75 mV/m, Station D 1.15 mV/m.

We first rank them in descending order of magnitude, in this example the order is A, C, B, then D. We next add them in a RSS fashion:

Station    mV/m   RSS    %  

---------  ----  -----  ----

Station A  3.65  3.650  100% 

Station C  2.75  4.570  75.3%

Station B  1.28  4.746  28.0%

Station D  1.15  4.883  24.2%

Calculation of a station's nighttime interference free service is accomplished by adding the squares of the values and extracting the square root of the sum, excluding those signals that are less than 50% of the RSS values of the higher signals already included.

We excluded the contributions from stations B and D to arrive at a 50% Exclusion Nighttime Limit for this station of 4.570 mV/m. This type of calculation is known as the Exclusion Method, and is how we determine the Nighttime Interference Free (NIF) contour value for a station.

For the 25% Exclusion Nighttime Limit we exclude those signals that are less than 25% of the RSS value of the higher signals already included for a value of 4.746 mV/m (Station D is excluded). The 25% Exclusion Nighttime Limit is now considered in all domestic studies as the value new stations cannot exceed as a contributor to the Nighttime Limit of existing domestic stations.

Further explorations

If you are interested in exploring this in more detail, Section 73.182 of the FCC Rules and Regulations and Mass Media Docket 87-267, Appendix C, provides examples of sample calculations.

Consider the following: to compute the maximum allowable radiation toward a distant station, we must first find the present 50% RSS Nighttime Limit, and the 25% RSS Nighttime Limit of the station.

That's not too bad, but if this is a domestic station, don't forget to add in all the signals from both foreign and domestic first adjacent channels before you begin your calculations.

If it's a foreign station, the nighttime limit of the station is calculated according to the pertinent international agreements between the United States and the foreign country.

Next step

Now that we have the 50% and 25% RSS Nighttime Limits for each station that will require study, it's time to compute the maximum allowable radiation toward each station. The program now branches to three different calculations for each station requiring study.

The first task is to determine where our present operation ranks in the nighttime limit of each station. If our proposed operation is a new facility, no problem, we're not contributing to anyone's nighttime limit.

If we wish to improve the facilities of an existing station, we must decide if we are above the 50% RSS Nighttime Limit or below the 25% RSS Nighttime Limit of the station. If we are already at the 50% RSS Nighttime Limit or greater of a domestic station, we must reduce the amount of radiation we presently radiate toward that station by 10%.

If it's a foreign station, we can remain at the same level of radiation. If we are somewhere between the two limits (50% and 25%), no increase in radiation is allowed, nor is a decrease required.

If the station is a foreign station we can increase radiation toward that station up to one-half of the 50% RSS Nighttime Limit or the value of the smallest contributor to the present RSS Nighttime Limit, whichever is the lesser of the two.

If we are below the 25% RSS Nighttime Limit of a domestic station, we can increase our level of radiation toward the distant station up to the 25% RSS Nighttime Limit of the station under study.

Dataworld's program handles this very nicely for you, flagging each station as to whether we must remain the same or must reduce our radiation in the direction of the station being studied. If we can increase the radiation toward the station being studied, the program will compute the amount of skywave radiation that may be radiated by our proposed operation in the pertinent azimuth and vertical elevations.

The amount of information provided by this program could overwhelm the non-engineer, but it is presented in an orderly manner and should be understandable to those that have a basic knowledge of AM nighttime allocation procedures.

For further information on Dataworld products and programs, contact Hank Brandenburg at Dataworld, in Maryland at 800-368-5754. Fax: 301-656-5341. You can also reach the Dataworld Web Server at http://www.dataworld.com