Q: When I run an intermod study, the program produces hundreds of pages of output. Is there a way to focus the output on the problem areas?

A: An intermodulation study investigates how various combinations of transmitter frequencies (and their harmonics) may generate resultant "intermodulation products" that fall on (or in a specified range or "guard-band" of) receiver frequencies. The study produces a list of potential problems where multiple transmit frequencies may interfere with a receiver in the same area.

The goal of the study is not to simply generate every mathematical possibility of frequency combinations, but rather to provide a tool to help find the most likely combinations that present a practical probability of an interference problem.

When many transmitter frequencies are included in the study, it is all too easy to generate more output than can be realistically evaluated. Producing hundreds of pages of printed output listing every possible combination is not as useful as a few dozen pages that represent realistic concerns. When using an intermod program, it is important to know how to specify input parameters that help limit the output to practical limits.

In addition to the fundamental frequency for each transmitter, spurious harmonic frequencies from that transmitter can also contribute to intermod problems. Therefore, the program allows you to specify the harmonics to be considered for each transmitter. While it is possible to specify up to the ninth harmonic, this would usually be far beyond any practical significance, since the radiation at the higher harmonic frequencies is normally adequately suppressed by filtering equipment already installed. Running an intermod study which specifies all transmit frequencies up to the ninth harmonic may generate mathematically possible problem combinations, but most will be of no practical use.

The number of harmonics to be considered for each transmitter frequency can be customized. For example, you may want to specify the third harmonic for the 100-Watt two-way radio transmitters at a site, but for a large 10000-Watt broadcast transmitter, you could specify a higher harmonic, such as the fifth, to account for the likelihood of higher harmonic levels for the higher powered transmitter. In the same way, if you are suspicious of an older piece of equipment, or one that is poorly maintained, a higher harmonic value might be appropriate, because of the unusually high power level in the higher order harmonics.

The other side of the coin from the transmitter harmonic specification is the guard band specified for each receive frequency to be protected as a result of the study.

The intermod study shows not only computed intermod products (i.e., combinations of transmit frequencies and harmonics) that fall exactly on a receiver carrier frequency, but also those near enough to that frequency to be a potential problem, such as within the channel width of the receiver input.

While it is possible to specify a wide guard band for each receiver, this will again result in the program generating many intermod products that are of no practical significance. For example, if a receiver has a bandwidth of +/- 5kHz from the carrier, that is probably a reasonable guard band for the intermod program. You may want to add a bit of extra protection by specifying a guard band of +/- 7 kHz, but specifying a 20 kHz guard band will result in program output showing intermod products that should easily be rejected by the normal receiver operation. Specifying a reasonable guard band for each receiver will eliminate unnecessary output from the intermod program.

In the TAP intermod program you can focus the program to investigate the potential intermod effect of a particular transmitter.

If you select (A)ll combinations, every transmit frequency in the file will be used in combination with every other transmit frequency to check for intermodulation products.

If you select (O)nly the frequency under study, the FIRST frequency in the transmit file will be used in combination with every other transmit frequency to check for intermodulation products. This option is typically used when you want to check the possible impact of adding a new frequency to a site and ignore all existing intermod products. Intermod products which include the new frequency will be examined, but any products resulting only from existing frequencies (and not including the new frequency) are ignored. This substantially reduces the execution time and quantity of printed output. However, note that an intermod product generated by a combination including the new frequency might not be harmful if a high level of intermod already exists on that frequency.

Combinations of up to four (4) frequencies can be examined. If you enter one (1), only the harmonics of each individual transmit frequency will be examined. No combinations between transmitters will be evaluated.

As before, while it is possible to generate an intermod product based on the interaction of four different transmitters, the most likely practical problems will result from the more probable circumstance of two or possibly three frequencies combining and generating a harmful intermod product. Limiting the number of combinations to two will produce the most likely problem combinations.

The "order" of the product is the sum of the harmonic values for each of the transmit frequencies. For example, an intermod product generated from the second harmonic of transmitter frequency A and the fundamental frequencies of B and C (2xA + B + C) is a fourth order product (2+1+1).

You can select the range of orders to print during this session. For example, if you specified the up to the 9th harmonic for several transmitters and combinations up to four transmitters, a 36th order product is mathematically possible (9xA + 9xB + 9xC + 9xD). However, such a product is generally unlikely to be of practical significance. By filtering out higher order products, the quantity of the output from this program can be reduced. For example, if you specify orders 1 through 5, all higher order products will be ignored. If you later want to check higher order products, you can specify orders 6 through 36.