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R/C 6-Element Phased Array

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Having designed a low cost servo-operated phase shifter it seemed rude not to build a phased array with some of them. The objective was to build a simple array that would be capable of  demonstrating the main features of a phased array. Namely, beam scanning and beam shaping under electronic control.

To keep the design simple a fixed amplitude distribution was chosen. This was afforded by a 6-way asymmetric Wilkinson splitter, designed to give a modified Taylor distribution (to give sidelobe suppression). The patches were also kept simple using a single probe feed and ‘air’ spaced. Giving the patch some ‘height’ increases the bandwidth considerably, hopefully avoiding potential mismatch problems. The following are links to

some dev work : Single Isolated Patch  and its Return Loss  and associated Smith Chart

The patches were orientated such that inter-patch coupling would be in the H-plane, which is significantly lower than that in the E-plane for the same separation. This was to minimise mutual coupling problems.

The element spacing was a trade off between a useful scan range and mutual coupling. The closer the element spacing, the larger the scan angle that is possible without grating lobes. Unfortunately, the closer spacing also increases inter-element coupling and its associated problems.  A spacing of 0.55 lambda was a reasonable compromise.

The array (phase shifters) was controlled using a standard R/C receiver (Cirrus 8-ch FM 35Mhz). The Pulse Position (PPM) modulation signal was generated directly using a Win98 laptop and transmitted using a synthesised source.

My thanks to Steve Webb Models for their excellent service and advice regarding the  Radio Control aspects of the project.  Note that licensed R/C bands will vary depending on your local regulations. The 35Mhz band is normally for Aero modelling in France but my Tx power is  -10dBm  (1/10th mW) and we are half way up a mountain, so not likely to disturb anyone.  

A brief specification for the array is as follows :

Array Details

Frequency :                     2.45 GHz

Number of elements :    6

Element Separation :     0.55 lambda

Element :                         Rectangular Patch

Polarisation :                   Linear

Scanning Plane :            H-plane  

Scan Range :                  Approx -45 to +45 deg  (Grating lobe started to appear)     

Ground Plane :               110mm x 450mm

Amplitude Taper :           Fixed (Modified Taylor for -20dB 1st sidelobe)

Directivity :                      15.1 dBi  (0deg Scan Angle)    

Element Details           

Feed :                              Single probe

Probe Compensation :  Series Capacitance

Patch Height :                 6mm (low  density foam) +carrier substrate

Substrate :                       Rogers RO4350 0.76mm  Er=3.48

Overall I didn’t think the performance was too shabby, considering the distinctly low cost construction methods. When I have some more time I hope to do a proper write up and include some downloads for the control software. In the meantime the ArrayCalc files for the scanned beams and wide beams can be downloaded from here : RCarray.zip

Front view of the array. The small circles round the probe feed points give the series capacitance to compensate out the inductive probe feed. Click on image to enlarge.

Top view of the array, showing the 6-way asymmetric Wilkinson splitter and 1m distribution cables (standard rf cables from RS components).  Click on image to enlarge.

Underside view of the array, showing the servo-operated phase shifters and connections to the Cirrus R/C receiver unit. Click on image to enlarge.  

The proof of the pudding was how  the array performed. Measurement of the array patterns was actually quite straight forward since the only signal that needed to pass through the rotary joint was the 2.45Ghz rf, marvellous!  There are 2 sets of plots, the first show the array scanning from -45 to +45 degrees in 15 deg steps. The second set shows some of the wide/multiple beam coverage. With only phase control and 6 elements the possibilities are limited but I think it shows some useful experiments can be done.

Mouse over the plots to see the calculated patterns (using ArrayCalc) for comparison.

Scanned Beams

All the patterns have been normalised to 0dB for ease of comparison. In reality the directivity and therefore Gain drops off with scan angle. This is due to broadening of the main lobe and increasing sidelobe levels.

Notice also that as the selected scan angle increases, the array falls short. This is due to the effect of the element (patch) pattern ‘distorting’ the main beam. In practice the demand value for the scan would have to be adjusted. The effect is quite normal and is visible in the calculated results.

All the patterns have been normalised to 0dB for ease of comparison. These ‘wide beam’ plots have large variations in the coverage area. This is due to the limited number of elements and lack of amplitude control.

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Wide Beams