STK Antenna Models

Helix Antenna Pattern

STK Communications provides standard models for helix antennas.

1Johnson, Richard C, Antenna Engineering Handbook, 3rd ed., New York: McGraw-Hill (1993), chapter 13

2Kraus, John D., Antennas, 2nd ed., New York: McGraw-Hill (1988), chapter 7.

These models are valid under the following conditions:

  • Only the axial mode of radiation is supported.
  • The helix has fixed turn spacing.
  • The helix has uniform diameter (tapered helices are not supported).
  • The ratio of the circumference of the helix to wavelength is between 0.75 and 1.33.
  • The turnspace distance is approximately equal to one-quarter of the wavelength.
  • The pitch turn angle (calculated by STK) is between 12 and 15 degrees.

This antenna uses a polar coordinate system.

The following options are available for Helix antennas:

Field Description
Design Frequency

The frequency of the antenna.

For an embedded antenna on a receiver, transmitter or radar, this value corresponds to the Frequency entered on the Model Specs tab for the transmitter or receiver and the Transmitter Specs tab for the Radar.

For an antenna linked to a receiver, transmitter, or radar, the Antenna Design Frequency value gets overridden by the Frequency value set on the Model Specs tab for the transmitter or receiver, and the on the Transmitter Specs tab for the Radar. However, when computing the antenna's gain matrix (or a link budget, etc.) from one of the objects linking to the antenna, that other object's frequency settings override the antenna's Design Frequency setting.
Diameter The diameter of the helix.
Efficiency Specifies the efficiency factor of the antenna, varying from 0 to 100 percent. Since no antenna is perfect, the efficiency factor allows you to specify the degradation in performance. Typical values fall in the range of 70 to 90 percent.
Turn Spacing The center-to-center distance between the turns of the helix.
Number of Turns The total number of turns of the helix.
Back-Lobe Gain Back-lobe gain can be used two different ways:
  1. A constant gain value from 90 to 270 degrees in elevation from the boresight of the antenna.
  2. Modeled as a fraction of the Main-lobe Gain, but in the opposite direction. For example, boresight gain is computed at a 0 degree elevation angle, the back-lobe attenuation factor is applied and the result is used as the gain at the 180 degree elevation angle. The backlobe gain is a mirror image of the forward gain, but with the backlobe gain applied as an attenuation factor. If “Use as main-lobe attenuation” is selected, the attenuation factor is applied on the main-lobe gain and the result is used in the back-lobe direction.

If you use the default frequency of 14.5 GHz (instead of a frequency in the usual range for helix operation), make sure to adjust the above parameters accordingly, e.g. 0.005 m for Diameter, 0.005 m for Turn Spacing and 10-25 as the Number of Turns.

The following illustration shows a helix antenna pattern:

Helix antenna pattern

The radiation gain pattern is assumed to be around the axis of the Helix. Currently, it is calculated up to 90 degrees from the axis. Radiation gain toward the back direction (i.e. more than 90 degrees from the axis) is not calculated.

The default helix antenna values are good for S-band frequencies (1.0-2.5 GHz), the normal operating band for helix antennas. For other frequencies proper helix sizes must be entered.

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