# Intermod Calculator

• What is Intermodulation Interference?

Intermodulation interference is the undesired combining of several signals in a nonlinear device, producing new, unwanted frequencies, which can cause interference in adjacent receivers located at repeater sites. Not all interference is a result of intermodulation distortion. It can come from co-channel interference, atmospheric conditions as well as man-made noise generated by medical, welding and heating equipment.

Most intermodulation occurs in a transmitter's nonlinear power amplifier (PA). The next most common mixing point is in the front end of a receiver. Usually it occurs in the unprotected first mixer of older model radios or in some cases an overdriven RF front-end amp. Intermodulation can also be produced in rusty or corroded tower joints, guy wires, turnbuckles and anchor rods or any nearby metallic object, which can act as a nonlinear "mixer/rectifier" device.

In the case of "transmitter" intermodulation, interference can only occur when both transmitters are on the air simultaneously. Suppose that transmitter "A" and transmitter "B" located nearby each other were on the air at the same instant on 150.00 MHz. and 151.00 MHz., respectively. If their antennas are close together it would be possible for the signal from transmitter "A" to be received by the antenna of transmitter "B" and coupled into it's final PA if no isolator, hybrid combiner or cavity filter protection devices are installed. The opposite case can also occur where B radiates into A. In either case mixing can occur in one or both of the transmitter's PA's generating several new frequencies at intervals of 1.0 MHz. (the difference between transmitter "A" and "B", 150.00 - 151.00). Spurious signals will occur at frequency intervals of ± 1.0 MHz. away from each transmit frequency. Examples are: 146.00 MHz., 147.00 MHz. 148.00 MHz., 149.00 MHz., 150.00 MHz., 151.00 MHz., 152.00 MHz., 153.00 MHz., 154.00 MHz., 155.00 MHz. The carrier frequencies are 150.00 MHz. and 151.00 MHz. The third order products are 149.00 MHz. and 152.00 MHz. The fifth order products are 148.00 MHz. and 153.00 MHz. The seventh orders are 147.00 MHz. and 154.00 MHz. The ninth orders are 146.00 MHz. and 155.00 MHz. Please notice how the signal level diminishes as the odd order increases. Generally the seventh and ninth orders are too weak to cause intermodulation interference. See the spectrum analyzer drawing below.

Receiver intermodulation occurs as the results of two or more high-level off-channel signals overloading the receiver's RF amplifier causing it to operate in it's nonlinear region thus acting as a mixer. Older radios may not employ an RF amp and go straight into the first mixer.

It may be difficult to determine where intermodulation is occurring because the new frequencies being generated are precisely the same whether they are generated in the receiver front end, transmitter PA or some other non-linearity. Inserting a 6 db. attenuator in the receiver front-end will attenuate "transmitter" intermod by 6 db. "Receiver" intermod will be attenuated more rapidly because both signals A & B are simultaneously reduced by 6 db each.

Intermodulation products are classified by their order (2nd, 3rd, 4th, 5th, ..., Nth). The frequencies which give rise to the products are designated by capital letters (A, B, C, etc.). These terms may include harmonics which can be treated in the following manner: the 2nd harmonic of frequency A is designated "2A", and, therefore, contributes 2 terms; the 3rd harmonic of frequency B is designated "3B" and contributes 3 terms, and so on. The general equation for the intermodulation product is written:

I.M. = n1A ± n2B ± n3C ± ...

Where A, B, C, etc. are the mixing frequencies and n1, n2, n3, etc. are the harmonics, or multiples, of the mixing frequencies. The order of the intermodulation product is equal to the sum of the harmonics (n1, n2, n3, etc.). For instance, A + 2B + 2C generates a 5th order intermodulation term (1 + 2 + 2).

Additional examples of product order: the frequency obtained by adding or subtracting the fundamental frequencies A and B, that is, A + B or A - B, are even 2nd order products; likewise, 2A - B and A + B - C are odd 3rd order products while 3A - 2B is a odd 5th order product while, A + B + C + D − E - F - G generates a odd 7th order intermodulation term (1 + 1 + 1 + 1 + 1 + 1 + 1).

There is no limit to the number of products, but only a few are of any practical concern. Even order products (2nd, 4th, 6th, etc.) can generally, but not always, be dismissed since they usually fall outside the frequency "band" of interest. The InterMod Calculator provides a checkbox to enable "Odd Order Only" calculations. By not checking the checkbox, both odd and even orders are tested which increases the number of calculations and time required for processing.

The "Low Order" and "High Order" entry fields are normally set to "Low Order" of 3 and the "High Order" of 5. If nothing is found, try incrementing the "High Order" to 7 or 9 or higher if necessary.

For example, an even second order product of 155.00 MHz. and 154.00 MHz. when mixed produces a difference frequency of 1.00 MHz. and a summation frequency of 309.00 MHz., both of which are far removed from the 150.00 MHz. "band" of interest. However, if there was a 309.00 MHz. military channel near this site it could be affected by a mix occurring in a rusty tower joint or the receivers front-end. "Transmitter" intermod at 309.00 MHz. is unlikely because it would be attenuated by the low pass filter in either the 155.00 MHz. or 154.00 MHz. transmitters.

Intermodulation interference most often concerns odd-order products. The most common interference are the 3rd and 5th orders. Higher order products are potential sources of intermod which cannot be ignored, because they also fall on in-band frequencies. However, their practical importance is minimized due to the fact that they are formed at lower power levels than the 3rd and 5th order products and are usually too weak to cause interference. The number of calculations increase exponentially as the order number goes up.

Even if an intermodulation product falls outside the receiver passband, the deviation (especially of the higher order products) may swing across the receiver passband, causing interference. The deviation of the intermodulation product is equal to the sum of each individual's signal deviation, multiplied by the coefficient for that signal indicated in the above formulas. If signal "A" has a ± 5 KHz. swing and signal "B" has a ± 5 KHz. swing, the fifth-order intermodulation product represented by "2A + 3B" will have a peak deviation of (2 x 5) + (3 x 5) = ± 25 KHz.

For example, two transmitter frequencies: 155.00 MHz. and 154.00 MHz. gives us a 5th order (3A − 2B) = (465 − 308) = 157.00 MHz. Suppose your receiver frequency that is being interfered with is 157.025 MHz. It would appear that this calculation would not produce a hit on your receiver. However, if tone signaling or voice modulation is present on each channel at the legal maximum of 5 KHz, then because of the 5th order calculation we would experience a peak deviation of (2*5) + (3*5) = 25 KHz. which will now sweep across the channel causing interference and possible falsing of CTCSS/DCS/Burst Tone/Touch Tone/Two Tone/Five Tone decoders. The "Near Hit Window" feature of the InterMod Calculator allows you to check for this type of problem by selecting how many ± KHz. off of the direct-hit frequency you wish to test for.

The Receive First High IF, on rare occasions, may be susceptible to strong signals and thus contribute to the potential of Intermodulation. This is why the "Receiver (IF)" field has been included but is rarely if ever used with modern day receivers. Furthermore, any "transmitter" intermod on a receiver's high IF would again be attenuated by the transmitter's PA final tank circuit.

One way to test for Intermodulation on rusty tower joints, rusty or corroded guy wire turnbuckles is to use an ultrasonic sniffer. You can actually hear the interference. Try installing a heavy copper gauge wire around the turnbuckle with clamps on each side. The most important thing of course is to utilize good site maintenance procedures such as isolators, combiners, cavity filters and vertical/horizontal antenna separation. There are several companies that can help you with these products such as EMR Corporation, Telewave, Decibel Products, TX RX Systems, Remec/Waycom.

For good reading checkout, "InterMod Control" by William F. Lieske, President of EMR Corporation.

• Setting Up Midian's InterMod Calculator Sites & Locations

The number of sites, locations, user/frequencies in this program are limited to the resources available in your computer. The database structure is configured such that long integers are used for record pointers. The size of a long integer is 32 bits signed. This gives each record a plus value of over 2,000,000,000 records for user/frequencies per up to 2,000,000,000 locations per up to 2,000,000,000 sites.

The program has been setup with some "dummy" user defaults and site frequencies. Take a look at them to gain some ideas on how the InterMod Calculator can be used. Play with the order windows and perform calculations to see how the product performs with the "dummy" user/frequencies. You may then add some new sites with locations and users and then run those. You may also want to merge your sites with the "dummy" sites and run the calculations again. Otherwise feel free to delete the "dummy" site, locations, user/frequencies and start entering your own site related data into the program.

An example of a site would be the following:

A mountain top site with 3 building locations: Pima Government Building, Federal Building and Amateur Radio Building.

Another example would be:

A Bank Building site with a Penthouse and 2 floors/walls for locations.

A second Bank Building on the opposite street corner might have the following screen-shot as below:

An example of a merged site could be two Bank Buildings on opposite street corners that are close enough to each other to create interference with the occupants of either site. See the above screen-shots of the two bank buildings and see the screen-shot further below the "Site Merge Selection" for an example screen-shot of the "Merged Bank Buildings" site.

Once you have selected the sites to merge and typed in the New Merged Site Name, click on the button labeled "Create Merged Site" and you will then see a merged site similar to the screen-shot below.

• Using the InterMod Calculator

To get to the color configuration, go to the main menu entry labeled "Options" and click on the menu entry labeled "Color Configuration." This will bring up the window labeled "Options" as shown below.

Click on one of the available buttons to bring up the color palette (shown above on right) to change the color of the associated frequency or name you desire.

This enables you to colorize your reports for visually separating the transmitter frequencies from the receiver frequencies and distinguishing the hit frequencies from the near hit frequencies.

Before creating any sites, you should enter frequencies into the Frequency Database. This is located under the menu heading of Utilities > Frequency Database.

The screen-shot below is the first window and the Main Window of the InterMod Calculator that you will see when creating a New Site.

To create a new site, right click in the "Site Selection" area of the Main Screen window. The "Selected Site Current Parameters" will be initialized to default values for a "New Site". Now tab down past the "Site Selection" field to the "Selected Site Current Parameters" section to the field labeled "Site Name" and enter in the desired name for this site.

Tab down to the "Low Order" field and enter a "3" to start with a 3rd order study. Tab over to the "High Order" field and enter a "5" to end with a 5th order study. If no results are achieved here, then increase your "High Order" to a "7", "9" or "11" order.

Tab down to the "Near Hit Window" and enter how many KHz. you will accept as a near-hit. 25 to 30 KHz. is probably good enough for a 3rd and 5th order study. 35 to 50 KHz. is good for a 7th to 9th order study. Entering in a 25 will automatically force the calculation into KHz.

Tab over to the "Odd Orders Only" checkbox and check it if you are interested in odd order only studies. If you leave it unchecked then both odd and even order studies will be calculated. For example, if you put a Low Order of "3" and a High Order of "5", you will wind up with a 3rd odd order study, a 4th even order study and a 5th odd order study.

Tab down to the "In Band Only" checkbox and check it if you are NOT interested in calculations that produce out-of-band products.

Tab down to the "Save Changes to Selected Site" button and click on it to save all the changes you have just made to the site.

Tab over to the right side of the Main Screen to the "Site Locations" and right click with the mouse in that sectional area of the screen. Select "Create New Location" from the pop up menu that appears. After the selection has been made you should see a similar screen as shown below:

Below is a screen-shot example of a location that has been filled in:

The title bar of the above window now shows the site name that was entered in the previous step. You can now enter in building or floor names as locations at this site. You may now tab down to the description window and enter in descriptive information regarding the location that you are creating. Some examples of this type of information are Latitude and Longitude coordinates, Street Address, Point of contact, description of the area. Press OK or Cancel to accept or disregard changes that may have been made.

Now tab down to the button labeled "Create New User/Frequency for this Location" for the named site and press the "ENTER" key.

Once back at the main screen, you can right click with the mouse in the "Location Frequencies" section of the Main Screen and a pop up menu should appear allowing you to "Add a Frequency" to this site location. This frequency that you are going to add should have originally been added to the database by way of the main menu, as described above.

Below is a screen-shot of a filled in User/Frequency example.

The example above shows the screen-shot labeled "Add New User/Frequency to Location: Federal Building" where "Federal Building" is the name of the Location at the "Tumamoc Hill Site". You can then install customers into this location starting with the labeled field "User/Frequency Name".

The "User/Frequency Name" field is used to give a name (customer or descriptive) to a frequency pair or you may decide to split the frequency pair apart and make two separate entries (1 entry for transmit and 1 entry for receive).

Tab down to the TX and RX fields and enter the assigned radio frequencies. Once you assign a Receive Frequency the Receiver (IF) field will become enabled and you can then enter in the Receiver (IF) if applicable. Otherwise leave the Receiver (IF) field set to the 0.000 value.

PLEASE NOTE: The Receiver (IF) field is rarely used. Refer to the manufacturer service manual for the particular model of radio to determine the high IF frequency.

Tab down to the button labeled "Add this User/Frequency" and press "ENTER". This will bring you back to the previous window (Location Details For the Selected Site) and you will see the new customer you have just added to the location show up in the grid labeled "View the User/Freq. At this Location". To add more customers just repeat the same steps.

The Location List (customers) can be edited or removed with the buttons located in the group box labeled "Location List Changes".

The last step is to return to the Main Screen and click on the button labeled "Perform Calculations". Depending on the number of locations, number of frequencies and especially the value of the higher order window will determine the length of time needed to perform the calculations. The screen shot below has an indicator bar that depicts the remaining orders that are needed to finish the study. The calculations performed are shown as well.

• Printing the Results of an InterMod Study

Below is a sample report for the Tumamoc Hill Site using the odd 3rd and 5th order product study in a "Rich Text Format" which will allow you to print the report to a color printer or if you print to a laser printer you should observe dithered grays for the colors. Upon completion of the study the report is generated and automatically saved for later viewing.