Antenna Modelling – A Tale of Two Small Antennas

Two recent assignments in regard of ISM band antennas needed very different solutions, despite there being an important common aspect of being confined within a small mechanical envelope. They illustrate the need for a flexible and pragmatic approach when working on the incorporation of antennas in small products.

For one assignment, the client already had a promising partial solution for operation within the mechanical constraints of their product, but with which we were able to see a problem owing to the lack of an adequate counterpoise. We proposed a simple resolution to improve performance by modifying the way the partial solution was used. Then, by developing an electromagnetic model for the arrangement, appropriate tuning and matching components and values were determined for this new configuration at each of the operating frequencies of interest.

For the other assignment, the constraints and freedoms were different, including the lack of any suitably-sized cavity to accommodate such as a chip antenna, but there was an additional fact concerning the operational specification that turned out to be useful.

After considering options for where an antenna may be incorporated, options were modelled and refined, yielding an efficient, balanced 2D form for an outer surface that was tolerant of other (particularly metal) structural elements in its immediate neighbourhood.

By virtue of its efficiency, this structure presented the best impedance match for the RF circuit, but the match was still relatively poor, and a matching network to produce what would normally be considered a good match would only work for a narrow frequency band. Further, the actual centre frequency of this band would be prone to variation, due to various component and dimensional tolerances and temperature variations.

However, given the fact that the required range for transmission and reception was quite short, the relatively poor match, with only moderate further adjustment, would still permit adequate power to be radiated, even when using the intended RF circuit within the intended overall power budget. Also, crucially, this would be obtained with a sufficiently broad frequency-response peak for the antenna, to give robust antenna performance with stability and repeatability in output despite the aforementioned sources of variability. Hence, what otherwise may have been a problem, was in fact a solution that was good enough in the particular application.

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