Mr. Ta-wei Hee

BEng(Hons)

Biography:

David T.W. Hee graduated with a Diploma In Electronics, Communications And Computer Engineering (Micro-electronics option) from Singapore Polytechnic, Singapore, in 1995. After completion of his National Service, Singapore, he furthered his studies in the University of Birmingham, United Kingdom. He went in as a direct entry student, and he graduated in 1997 with a BEng(Hons) In Electronics And Communications Engineering. Upon his first degree graduation, he continued with his post-graduate (PhD.) studies in the same university, and is scheduled to graduate by April 2004. Recently, he is employed as a Research Staff member of the university to work on an antenna design research contract. His research interests are in passive antenna design, electromagnetic wave propagation, computational electromagnetics and system integration. 

There is a current need for wide bandwidth array antennas that have a low profile, and are capable of being conformally mounted. The bandwidth of such arrays should be as wide as possible, and a 3 to 1 range is desirable. 

The design of such an array, is technically challenging on several levels. Firstly, the array element must have the desired bandwidth, have well-controlled input impedance and radiation patterns. Secondly, the element must be electrically small at the lowest frequency of operation, to allow a  very small element spacing to be used. This allows the array to have well-controlled patterns, and in a particular no grating lobes at the highest frequency. Finally, the array feeding structure should be wideband and low profile. 

Preliminary Results

Stacked Monopoles on Flat Ground Plane  

It is known that a single fractal monopole element gives multi-band response, and that the fractal elements must be stacked in order to achieve wide-band response [1]. Other elements have been examined as follows.

·        Stacked plain squares are similar to the stacked Sierpinski carpet antenna but without the etched gasket.  They are made from plain copper clad board.

·        Stacked three ring monopole is again similar to the stacked Sierpinski carpet but the gasket is replaced by three etched concentric rings, as shown in Fig 19a.  It is based on the multi-circular loop monopole antenna, proposed by C.T.P. Song et al [2]. In the non-stacked version the loop diameter determine the resonances

·        Diamond carpet is a stacked Sierpinski carpet antenna with its side vertexes removed, as shown in Fig 1b.

                                              (a)                                                                                            (b)

Figure 1:  Stacked Monopole Configurations.

(a) Three ring monopole,  (b) Diamond carpet – only first iteration of carpet shown for simplicity

 The input return loss of the various stacked elements has been measured and shown in Figure 2 below.

 Figure 2: Various Stacked Elements Input Return Losses.

 From figure 2, it can be seen that

 (a) the stacked plain squares antenna is well matched, to –10dB and below, at 2GHz and above, and is similar to the stacked Sierpinski carpets.

 (b) the stacked diamond carpet antenna is very well matched, at a frequency band of 1.2GHz to 5.6GHz.  The return losses only go above –10dB from 5.65GHz to 6.95GHz.

 (c) the stacked circular 3-ring antenna is matched, at a frequency band of 1.95GHz to 3GHz and 5GHz to 7.4GHz.

 The results of Figure 2 suggest that two antennas show potential as cavity feeds.  The stacked circular ring has a resonance just above 1GHz.  The stacked diamond antenna has a better performance between 1 and 2 GHz than the Sierpinski.  Measurements as cavity feeds are currently being taken.

Stacked Plain Antenna in Cavity

The stacked plain carpet element is similar to the Sierpinski element but does not have the pattern etched on the printed circuit substrates. Figure 3 shows a photograph of the antenna structure.

 Figure 3: Photograph Of Stacked Plain Carpet Antenna In Cavity.

Figure 4 shows the measured return loss. It shows worse performance than an isolated plain element which, is assumed to be due to the effect of the cavity.  The peak around 5 GHz is still present although performance above 5GHz is good.

Its measured gain has been compared to the stacked Sierpinski carpet antenna in cavity. In general, the result is similar, although there is a gain reduction in the frequency of the feature above 3GHz.  The features (of both stacked feed elements) below 3GHz occur at the same frequency.  Thus, it may well be that these two sets of features are caused by separate effects.  For example the low frequency effects could be due to the cavity alone, whilst those at higher frequencies by the interaction of the antenna and the cavity. 

The radiation patterns of this antenna has also been done. Similar asymmetries are noted in the E-plane, to those in the patterns of the stacked Sierpinski Carpet antenna. Its cross-polarisation levels are also similar.

Figure 4: Measured S11 (return loss) Of Plain Element In Cavity.

Stacked Stacked Sierpinski Carpet Antenna in Cavity

The stacked Sierpinski carpet element has the 2nd-iteration Sierpinski Carpet pattern etched on the printed circuit substrates. Figure 5 shows a photograph of the antenna structure.

Figure 5: Photo of Stacked Siepinski Carpet Antenna In Cavity.

The measured input return loss of the stacked Sierpinski carpet element with cavity and flange is shown in Figure 6.  Cut-off is around 1Ghz, as noted for the isolated carpet element, although the return loss from 1 to 3 GHz has worsened. It is presumed, to be due to the action of the cavity.  However a peak above –5dB occur at 5 GHz, it is again assumed to be due to the action of the cavity.

Figure 6: Measured Input Return Loss Of Antenna Of Figure 5. 

(Cavity depth, black – 60mm, grey – 30mm)

The measured gain of the Stacked Sierpinski Carpet in the cavity, has also been done. Measurements above 8GHz were not possible due to limitations with our standard gain antenna.We are currently trying to widen our measurement capability.  It can be seen though, that the gain has an average value around 5dBi from 1 to 3GHz. Above this, significant gain variation is noted with a narrow band where the gain drops to about –10dBi. There are two forms of gain variation with frequency. From 1 to 3GHz a rather fast variation is noted. At higher frequencies the variation is somewhat slower. If this variation is due to the interaction of the element with the bottom of the cavity, a faster variation would be expected at higher frequencies. We are currently examining our gain measurement set-up to make sure that none of the artifacts in the measurements are due to, for instance, poor return loss in the system components or other effects.  However, we believe that the measurements shown here will allow useful conclusions to be made about the potential of the antenna.

The radiation patterns for this antenna (with cavity depth of 60mm) have been plotted. The E-plane patterns show good symmetry in the wanted polarisation, with some asymmetry together with high levels in the cross polarisation. H-plane patterns show asymmetry in the wanted polarisation, and high cross polarisation also.  The asymmetries in the wanted polarisations, will lead to significant gain variations when this element is used in a scanning array. The good E-plane co-polarised plots, suggest that well controlled E-plane scanning should be possible. 

References

1. C.T.P. Song, P.S. Hall, H. Ghafouri-Shiraz and D. Wake : ‘Fractal stacked   

    monopole with very wide bandwidth’, Electron. Lett., 10^th June 1999, Vol. 35,

    No. 12, pp. 945-946

 2. C.T.P. Song, P.S. Hall, H. Ghafouri-Shiraz and D. Wake : ‘Multi-circular loop

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