Reflect Array 3

Antennas and Microwaves

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Reflect Array

Results and Discussion

Testing the prototype antenna consisted of first matching the antenna, as described on the previous page. Then, co-polar (LHCP) patterns were measured in angles of Theta (+/-180deg) for Phi=0deg. Referring to the 3D patterns on Page1, this constitutes a pattern cut through the X-Z plane. Since the antenna is nominally symmetric around the axis of propagation (Z-axis), there is not likely to be much variation as a function of Phi, as can be seen from the 3D pattern. The following figure and links show measured and calculated patterns for the 2366Mhz array over a 5% bandwidth (2366 +/- 59 Mhz) .

Patterns at 2425 Mhz

Patterns at 2307 MHz

Measured and calculated patterns for the 2366Mhz Reflect Array

The greatest difference between the calculated and measured results is the sidelobe profile. This is probably attributable to the greatest difference between the model and the prototype I.e. the main groundplane. In the model I used an infinite, perfect ground in order to reduce computation time. This made optimization, which required many simulation runs, a realisitic option. It is worth noting however that maximum sidelobe levels are fairly representative. The NEC model linked at the end of this page has been further optimized and gives calculated sidelobe levels below -17dB over the full 5% bandwidth, hence my figure of -15dB in the specification. (Example @ 1420Mhz)

Also, I have since run a model using a radial grid type groundplane, as used for the helix feed. The grid was quite coarse and rather surprisingly produced a significant reduction in sidelobe levels, at the expense of some gain. While this is very interesting for development of the design I would still like to narrow the disparity between my model and measured results. As such, my next task will probably be to try using a finer grid or surface patches for the groundplane.This may include modifying the design to take computational advantage of the circular symmetry of the array, at the moment the helical feed destroys the model symmetry.

Another test I performed involved modelling the coaxial feed run betwwen the helix and the groundplane. If the coax follows the z-axis it has little effect on performance, however any deviation from the z-axis introduces components in the X-Y plane. This results in the coax cutting the propagating E-field, thereby inducing currents on the outer of the coax. That said, even modelling the coax as a shallow helix, diameter 20mm ( the diameter of the plastic support tube) the effects were minimal.

Assuming that the sidelobe issue can be resolved and I’m sure it can, an obvious question is, can the gain be increased further using additional rings of elements? My initial findings suggest that it becomes increasingly difficult to effectively optimize the design. This is due to the increasing number of independent variables and limitations on the amount of phase error that can be corrected by element size and spacing alone. My feeling at present is that the design lends itself more to use as part of a larger array or interferometer.

For anyone interested in trying out the design, the following ZIP-file contains the 4nec2 model, design spreadsheet and explanations of how to use them. Array3R.zip