Task 4.3: Prototype construction and measurement

• Deliverables: D11 T4.3 Final Task Report
• Outputs: Prototypes and prototype measurements.

A few prototypes will be constructed and measured in order to validate the simulations and check the technology limitations. It is foreseen to construct and validate at least the following:

a) Miniature antenna. A prototype of miniature antenna that in accordance to the outputs of tasks 2.1, 4.1, and 4.2 approaches the fundamental limit for small antennas.

Miniature resonator. A prototype of miniature resonator in planar technology as potential building block for filter design will be verified. In addition, other prototypes will be constructed thorough the project if necessary for experimental verification of the computer model.

Measurement of radiation efficiency and quality factor of fractal antennas: the Wheeler cap method

Several methods used to characterize the performance of electrically small antennas have been reviewed, in order to select the one that best fits to the needs of the project: accuracy and repeatability for the measurement of small pre-fractal antennas radiation efficiency and quality factor. The radiation pattern measurement is of secondary importance because pattern differences among small antennas are negligible.

The following techniques to measure radiation efficiency and Q factor of antennas has been reviewed by UPC partner:

• the Wheeler Cap method;
• the Directivity/Gain method (also called Pattern Integration);
• the Radiometric method;
• the Q method;
• the Random Field Measurement Technique.

Figure 56. Scheme of the Wheeler cap method and some of the Wheeler caps used in the measurements.

The Wheeler Cap method (Fig. 56 and 57) has been implemented for the characterization of small pre-fractal antennas and conventional antennas. To assess the quality of the selected method in terms of accuracy, several experiments on well-known antennas have been carried out (Fig. 58).

Through the development of the task:

• Experience has been gained on the design of small pre-fractal wire monopoles and its manufacturing using conventional printed circuits technology.
• Radiation shields as Wheeler caps have been designed for the most accurate measurement of radiation efficiency and quality factor.
• A post-processing technique has been developed in order to improve the accuracy of the measurement results.

Fig. 57 Spherical, rectangular and cylindrical radiation shields used in the Wheeler cap method

Genetically designed planar monopoles

Genetically optimised planar monopoles have been designed in Task 4.1 to assess if fractal shapes are the best alternative for the design of efficient antennas with minimum resonant frequency. Koch-like, meander-line and zigzag monopoles have been analysed in the frame of this work. It was observed that Koch-like designs offer worst radiation performances than optimised meander-line and zigzag designs. Only in a few cases the pre-fractal designs attain similar performances than the Euclidean ones.

An example of these particular cases is shown in this section. A Koch-like optimum design was manufactured with the standard technique used for the fabrication of printed circuit boards, and compared with optimum meander-line and zigzag monopoles that with the same wire length yield have similar performance in the numerical simulations (Fig. 58).

All these antennas were experimentally characterized using the Wheeler cap method. The experimental measurement of prototypes lead to the same conclusions as the numerical simulations made in Task 4.1.

Figure 58. Prototypes of genetically designed monopoles and conventional antennas.

The 3D Hilbert monopole: an example of 3D pre-fractal small antenna

The effective use of the radiansphere that encloses an antenna is supposed to reduce its quality factor approaching its value closer to the fundamental limit. Three-dimensional Hilbert pre-fractal monopole prototypes have been built as potential candidates to attain minimum Q factors for a given electrical size (Fig. 59). However, the measurements reveal that, due to their large wires and their intricate topology, the increase in the ohmic resistance and the intense coupling reduce drastically the expected figures of the Q factor.

The experimental measurement of 3-D pre-fractal prototypes lead to the following conclusions:

• The space-filling property of Hilbert curves is highlighted in a three dimensional design, attaining higher miniaturization ratios (smaller electrical sizes) than with a planar configuration.
• The shape of the Hilbert pre-fractal provokes a strong cancellation of radiation, resulting in high Q values and small radiation resistances.
• The small radiation resistance along with the significant loss resistance of 3D Hilbert pre-fractal antennas makes them unpractical designs for a radiating system.

Figure 59: Some of the 3D Hilbert monopole prototypes. The 10 eurocents coin is used as a reference for their size.

Prototypes and measurements at EPFL

Several prototypes of pre-fractal antennas and printed line fractal devices have been built and measured by the EPFL partner. The kind of measurements of these devices can be separated into three groups:

• The measurements of the reflection coefficient.
• The measurement of the radiation characteristics (radiation pattern, efficiency).
• Near field measurements for diagnosis purposes.

Measurement of small antennas radiation parameters

The EPFL partner has setup an unique facility for the measurement of small antenna parameters, in particular, loss efficiency and gain. Electrically small antennas are difficult to measure properly because they are neither purely symmetrical nor asymmetrical due to the limited size of ground planes or feeding baluns. Therefore, when the antenna is connected to a measuring device, a current flows in the outer conductor of the cable connecting the antenna, creating spurious radiation that completely masks the characteristics of the antenna under test. In order to overcome this problems, an original solution based on a random positioner has been developed (see Figure 60). Small antennas gain and loss efficiency can be accurately measured with the proposed solution.

Figure 60: Random positioner for the measurement of small antennas at EPFL.

Near filed measurements

The EPFL has also used in this project a set up for measurement of the near field in the proximity of the antenna (Fig. 61). This set-up uses a very small probe sensible to the three components of the electric field and is very useful for diagnostic purposes.

Figure 61: Picture of the near-field measurement setup and near field measurements of the iteration 3 of a printed Sierpinski antenna.

Pre-fractal antenna prototypes

In this report three different families of microwave fractal-shaped devices are studied. The first two are based on well-known fractals, one is a surface fractal, the Sierpinski gasket, and one is a line fractal, the Koch curve, while the third is a new structure based on capillary filters.

The previously existing literature studies the Sierpinski as a monopole, as well as a patch. In the current contribution the measurements of a Sierpinski printed antenna are presented. The gaskets have been printed on a copper-berilium layer (fig. 62a). To end the construction of these antennas the gaskets are going to be glued to a substrate of ε_r=1.07 and h=2mm and to a ground plane. The sticking process is going to be done with the help of a thin film of glue in order not to affect the permittivity of the substrate. The ground plane, the substrate, the film of glue and the gaskets are stacked (Fig. 62b) and kept together with the help of two aluminum plates screwed tight.

Figure 62: a) Copper-berilium built Sierpinski gaskets, b) printed Sierpinski antennas.

The Koch shaped devices studied in the existing literature are mainly based on the Koch monopole, but in the present report the study is done on different printed Koch-shaped structures (Fig. 63). The antennas have been printed on epoxy of thickness 0.1 mm and following the standard procedure for printed circuits. Once the Koch lines are printed on the epoxy, a brass ground plane, a substrate ε_r=1.07 of 3 mm of thickness and the epoxy with the printed lines are glued together in a hot glue process, as explained for the Sierpinski antennas.

Figure 63: Printed Koch antenna prototypes and measurement set-up for the H-plane radiation pattern.

Finally, different line fractal shaped capillary filters belonging to the tree family have been built and measured (Fig. 64).

Figure 64: 5^th order square capillar filter and measurement set-up.

Task conclusions

Conclusions from this task are:

• The Wheeler cap method is an accurate, fast, and easy to implement technique for the measurement of radiation efficiency and quality factor of electrically small antennas.
• The Directivity/Gain method (in the near-field) can be also used to assess the measured results obtained with the Wheeler method because a measurement facility is accessible and measurement times are reasonable.
• A random positioner allows accurate measurement of antenna efficiency and maximum gain.
• Near field measurements show the field concentration at small parts of the antenna, typical from pre-fractal structures.
• Standard techniques for the fabrication of printed circuit boards are also useful for the manufacture of pre-fractal designs (technique used at UPC).
• Pre-fractal printed antennas can also be constructed by masking the shape on a copper-berilium layer and stacking the ground plane, the substrate, a thin film of glue and the gaskets (technique used at EPFL).
• For a given antenna size and length, non-fractal designs usually have lower measured resonance frequencies and wider bandwidths than pre-fractal monopoles, resulting in lower Q factor and higher efficiencies. Genetically designed pre-fractal antennas attain, in few cases, similar performances than conventional antennas.
• Three-dimensional pre-fractal design does not provide further improvements than planar design in the radiation performance of monopoles. In spite of the smaller electrical sizes attainable thanks to their increased space-filling capability, they have a more intricate topology and larger wires than their planar counterparts. Consequently, efficiency and Q factor for these 3D pre-fractals have unpractical values to real-world applications.