How can I include the effect of side-mounting my omni-directional antenna on a tower?

Omni Antenna Distortion

From the TAP Editors | Antennas menu, select the Omni Antenna Distortion option:

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This program computes the distortion of an omni-directional antenna when the antenna is side-mounted on a vertical tower.

The program allows you to specify the antenna frequency, the tower (triangular or rectangular) and size of the tower face, the antenna mounting method and mounting distance from the tower. You can save the pattern to a file for use with other TAP propagation software for computing field strength values.

The pattern distortion is computed on a model developed empirically by The Antenna Specialists Company in Cleveland, Ohio. Antenna Specialists developed the model over several years based on over 3000 measured patterns of various antennas in different mounting configurations. The model assumes a single, vertically polarized antenna with no other nearby passive radiators installed for directivity. The model treats the tower legs as passive radiators that affect the pattern. Note that other features on the tower, such as a ladder or transmission lines, or other antennas, may also act as additional passive radiators that affect the pattern, but are not treated by this model.

You should consult the manufacturer of the antenna(s) you plan to use for any particular characteristics of the omni-directional antenna you plan to use. While this program is a useful tool for predicting pattern distortion, this is never a substitute for a measured pattern from the antenna manufacturer. Note that no vertical pattern is computed for the antenna. However, vertical (or “elevation”) pattern information can be entered manually using the TAP Antenna Editor program.

The Omni Antenna Pattern Distortion form is displayed by the program. This form allows you to enter the parameters for computing the distortion:

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Enter a description of the antenna and mounting configuration. This description will be used later when you save and lookup the antenna from a library file. If you check the “Auto” box next to the description, the program will generate an automatic description including the mounting parameters, frequency, etc. This may be useful to document the parameters used to produce a particular pattern when you want to look up the pattern later.

Enter the operating frequency of the antenna. All mounting and antenna spacing values are considered in terms of the wavelength of the operating frequency.

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The Antenna Specialists model was developed using frequencies from 25 MHz to 1300 MHz. The effectiveness of the model has not been thoroughly tested outside of these limits.

Note that pattern distortion calculations can be performed using frequencies outside the tested range by using a frequency in the range of 25-1300 MHz and appropriately scaling the tower face and mounting dimensions. For example, to compute the effect of mounting a 1500 MHz antenna 1 foot from a tower with a 1 foot face, specify a tower face of 2 feet, a mounting distance of 2 feet, and a frequency of 750 MHz (doubling the tower face and mounting distance, and halving the frequency) so that the relationship between the tower and mounting dimensions and the wavelength is maintained. The calculation using scaled values should only be used if the tower face dimensions are in the tested range noted below.

Select the gain and units to use for the antenna gain information. The gain should be the manufacturer’s specified or measured gain value for the antenna without mounting distortion.

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Enter the dimension of the tower face:

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For self-supporting towers, or other structures of non-uniform cross section, you should enter the width of the tower at the mounting location of the antenna.

The Antenna Specialists model was developed for calculations involving tower faces from one foot to eight feet. The effectiveness of the model has not been thoroughly tested outside of these limits.

Note that pattern distortion calculations can be performed on towers outside the recommended range of tower faces by using a tower face in the range of 1-8 feet and appropriately scaling the operating frequency and mounting dimension. For example, to compute the effect of mounting a 450 MHz antenna 2 feet from a tower with a 12 foot face, specify a tower face of 6 feet, a mounting distance of 1 foot, and a frequency of 900 MHz (halving the tower face and mounting distance, and doubling the frequency) so that the relationship between the tower and mounting dimensions and the wavelength is maintained. The calculation using scaled values should only be used if the scaled frequency is in the specified range noted above.

Enter the mounting distance of the antenna from the tower face:

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The Antenna Specialists model was developed using antenna mounting spacing from 0.5 feet (6 inches) to 7 feet. The effectiveness of the model has not been thoroughly tested outside of these limits. It is recommended that no antenna mounting spacing less than 0.5 feet (6 inches) be used to avoid effects of capacitive coupling between the tower and antenna.

Select the mounting configuration of the antenna by clicking on the appropriate diagram:

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You can select any of the eight antenna mounting configurations shown.

The types are:
  • Type 1: Triangular tower; antenna in-line with right 2 legs.
  • Type 2: Triangular tower; antenna off leg of tower.
  • Type 3: Triangular tower; antenna mounted perpendicular to face of tower.
  • Type 4: Triangular tower; antenna in-line with left 2 legs.
  • Type 5: Square tower; antenna in-line with right 2 legs at corner.
  • Type 6: Square tower; antenna in-line with left 2 legs at corner.
  • Type 7: Square tower; antenna mounted on diagonal leg at corner.
  • Type 8: Square tower; antenna at center of face of tower.

Select the reference for the pattern gain calculations. The gain reference selection determines what reference point is used for the gain values computed by this program. The computed gain values can be referenced to either the maximum pattern value of the computed pattern, or to the average value:

  • MAX: The computed variations in antenna gain on different azimuths are computed relative to the maximum value. This results in all pattern values expressed as suppression below the undistorted pattern. It should be noted that this is the method normally used by The Antenna Specialists Co. who developed the mounting model. This method disregards any anticipated gain above omni-directional performance.
  • AVG: The computed variations in antenna gain on different azimuths are computed relative to the arithmetic mean value. This results in an effective gain above the omni-directional gain specified on some azimuths.

When all of the parameters have been set, click the “Compute” button to start the calculation.

When the calculation is finished, a label is displayed in the “Pattern Calculation” section of the form, showing the minimum and maximum values of the computed pattern. The difference between these values provides a measure of the distortion introduced. (A perfectly omni-directional pattern would have a difference of zero.) Note also, if the “Auto” description box is checked, the description of the antenna reflects the current parameters.

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After the antenna distortion has been computed, the “Plot Azimuth” button is enabled. Click the “Plot Azimuth Pattern” button to display the computed antenna pattern.

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