Reflect Array 1

Antennas and Microwaves

Reflect Array

Overview

This design came about during an investigation into antenna options for 1420Mhz Hydrogen line reception. I was looking for a low cost alternative to a parabolic reflector, to give a ‘pencil beam’ type coverage. I was pleasantly surprised by how easily a practical implementation of the array came together. Initial work has been carried out using the NEC2 (Numerical Electromagnetic Code) coupled with the 4Nec2 front end by Arie Voors. Although it is early days in terms of my investigation, I have built a scaled prototype at 2366Mhz (see page 2) whose performance, agrees well with calculated data. Since the design appears to scale easily I thought it may be of interest to others.

To determine whether it is of interest, it is probably easiest to give a brief description and summary of performance, before delving into details. The performance figures given are my best estimate of what should be achievable, based on a combination of modelled and measured results.

DESCRIPTION

The antenna is an axial-mode-helix fed, circularly polarised reflect array. The resonant elements are wire ring resonators spaced one quarter wavelength from the main circular ground plane (infinite in the NEC model). There are 43 resonators, a centre element and 3 rings comprising 6, 12 and 24 elements, arranged concentrically around the bore sight axis of the antenna. The Right Hand Circularly Polarised (RHCP) feed helix is nominally 6 turns, launched from a 0.7 wavelength diameter circular ground plane.

PERFORMANCE

Polarisation............... LHCP ( ! Feed polarisation is reversed off the reflector)

Axial Ratio ................. 0.8 (over the 3dB beamwidth)

Impedance ................ 50 Ohm (A match to 75 Ohms would not be difficult)

VSWR......................... 1.3:1 (or -17.7dB Rloss, see also Note1)

Gain............................ 20dBi (Verified by antenna substitution measurement)

Sidelobes ................. -15dB

Beamwidth 3dB......... 14 Deg

Beamwidth 10dB ...... 23 Deg

Bandwidth ................. 5% (see Note2)

Note 1 : The VSWR figure is based on practical measurement (using a VNA), it is easier to tune the actual helix, rather than spend time tuning the NEC model for impedance. Due to the way NEC calculates pattern and gain data, a good match is not essential.

Note 2 : The bandwidth of 5% is the range over which the other data is valid. The principal degradation out of band is rapidly increasing sidelobe levels.

4Nec2 Output for 1420Mhz Array LHCP Co-pol

Use mouse roll-over to see RHCP Cx-pol

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