Intermod Results
Q: I ran an intermod study and it generates all possible results of my 85 receivers and 80 transmitters. How do create an intermod study that produces a study that will generate results that are more helpful?
A: A big part of a successful intermod study is (as you discovered) filtering out as many of the least likely problems so you can focus on the most likely.
There are several FAQs that you should look at to get the "big picture" of the intermod program. Probably the best place to start is Finding, Solving and Preventing Intermodulation Problems.
Several others describe the details of operating the program. These are summarized in the TAP Training Questions.
Several suggestions can help trim down the output to focus on the most likely problems:
(1) Number of harmonics: In theory, looking at the 9th harmonic of every transmitter might seem like a good idea, but since most of the problems will result from lower harmonics you might want to be sure you are not looking at unnecessary information. Setting the harmonic range down to 5th or even 3rd might be a good start.
(2) Intermod order: Once again, it may be mathematically possible to find a potential intermod product of the 5th harmonics of three different transmitters. But the practical likelihood of a 15th order product actually causing a problem at a real life transmitter site is pretty low. So setting the maximum order to, say, 5th or maybe 7th, will weed out all those possible but improbable results.
(3) Diagnostic or Preventative? What are you trying to find with the intermod study? For example, if you have an existing site where everything is working pretty well and you want to check to see if adding a new user will cause a problem, you can use the Intermod program in the "Preventative" mode. This means that you can ignore all possible mathematical intermod products that contain only the existing transmitters at the site (since there are no apparent existing problems with these transmitters). You can set the program to only consider transmitters you flag as "reference" frequencies. The program will only look at combinations that include the "new guy" to see what things might be stirred up by the addition of the new transmitter. On the other hand, if there is a problem you are trying to solve (the "Diagnostic" approach), you can limit the receivers used in the study to only include the receivers that are experiencing a problem. Then only the transmitter combinations that fall within the bandwidth of the affected receiver(s) will be included in the program output. Naturally, there may be situations that don't fit neatly into either of these categories, but the idea is to eliminate those factors at the site (existing transmitters or unaffected receivers) that don't seem to be a part of the problem you are trying to solve. If you do need to run a study that includes all transmitters and looks for possible problems at all receivers, be aware that the output (by definition) will be all possible combinations.
All of these suggestions boil down to the same idea - eliminating as many of the factors as possible (extreme harmonics, unreasonably high intermod orders, certain transmitters and receivers) to let the program focus in on the most likely culprits that are causing or might cause a problem.
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