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Reflect Array

Latest Results

The fact that this is an ongoing project and progress can be sporadic due to other commitments, I have decided to introduce this page to record any additional information that maybe useful. At some point I am hoping to compile a more formal report to document my investigation in more detail.

Latest Results

After some further experimentation with the finite groundplane model, as described on page3. I found that whilst having an effect on the sidelobe profiles, it was not as significant as I expected, so I revisited my measurement setup.

My test range is indoor and not being blessed with thousands of pounds worth of Emerson and Cuming’s finest, it does suffer from multi-path reflections. For circularly polarised co-polar measurements this is not normally too great a problem. This is because polarisation reversal of the reflected signals causes them to appear as cross-polar at the range receive antenna (assuming it is circularly polarised).

For previous measurements I had used a commercial broadband log spiral antenna as the range receive antenna. However, with the reflect array I was attempting to look at lower sidelobe levels coupled with a lot more directive gain. The strong cross-polar multi-path signals highlighted the poor axial ratio performance of the log spiral. Basically I needed a more directional range receive antenna with better axial ratio performance.

Using a 10-turn LHCP helix as the range receive antenna (similar to the feed antenna for the reflect array) the following patterns were measured.

Patterns at 2425 Mhz

Patterns at 2307 MHz

Measured and calculated patterns for the 2366Mhz Reflect Array (Using the new range receive antenna)

The improved measurements now give a clearer indication of any trends in the results, such as frequency shift. These new patterns, together with others, suggest that the actual array is operating slightly lower in frequency compared to the model. This may well be due to dielectric loading by the ‘Styrodur’ (Er=1.03 Approx).

It is probably worth noting the absence of cross polar measurements in my results. This is because all the advantages of making the co-polar measurements become disadvantages in the cross-polar case. To begin with, the overall levels are going to be low, not high, assuming the antenna under test has good axial ratio. The next problem  is the powerful reflections from the main lobe will be polarisation reversed, as before. However, this time they will be co-polar as far as the range receive antenna is concerned. Basically any cross-polar signal from the antenna under test is likely to be swamped by reflections of the main lobe. Spin-linear type measurements will suffer from a similar fate.

For the reasons above I do not believe that I am currently able to make sufficiently accurate cross-polar measurements to be able to glean any useful information. Note that even expensive commercial test ranges have their limitations, always keep in mind what is going where during the measurement.

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Reflect Array

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