The field patterns, input impedance for each dipole, overall input impedance
and VSWR are computed.
The array is fed at the high frequency end and loaded at the low frequency end.
The LPMA specified by the tau and sigma provided is displayed to ensure a realistic design was specified.
The LPMA is oriented such that the smallest element is at the top and largest element is at the bottom with respect to the E and H field pattern plots.
The picture below depicts a LPMA over a ground plane:
This schematic view of the LPMA (below) shows how the image generated by the ground plane causes the LPMA to appear much like a Log Periodic Dipole Array (LPDA) with the exception of its feed structure. Tau and sigma are the standard defining parameters for log periodic antennas and are defined by the equations given in the figure. These are the same tau and sigma that are given in the LPDA simulation for convenience (they are based on Lamba/4 lengths as opposed to Lambda/2 lengths).
The primary difference between a LPDA and a LPMA is in the feed network. Unlike the LPDA, there is not a single standard feed that is used. Our experimental work has been done with high-frequency (K-band) array and microstrip feeds. A convenient feed for this application is the modulated feed line structure. It consists of cells that have two different impedances separated by a step-discontinuity. The characteristic impedance (Zo) section is generally the same as the source and load (Zs and Zl). The modulated impedance (Zm) is normally higher and is connected to the elements. By varying:
we can attempt to match the LPMA and the source for a given LPMA physical structure. After matching, the characteristic impedance sections can be varied in length by the phase factor to change the pattern itself with little impact on the VSWR. The feed network is illustrated below:
To simulate a LPMA you need to specify the following:
|This script was developed at the Center for Computational Electromagnetics (CCEM) in the University of Illinois at Urbana with funding from the Sloan Center for Asynchronous Learning Environments(SCALE).|
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