Simulation optimises design of antenna array

A circular polarised patch antenna has recently been simulated using software from Vector Fields, and the simulation considerably reduced the development time.

The increase in demand for RF power sources at microwave frequencies, particularly low-cost spatial power, in personal communications has been considerable.

There are also a considerable number of similar applications in radar systems.

Some work has been done on coupled active antenna oscillator arrays in power combining at microwave and millimetre-wave frequencies.

These can be injection locked to form electronic beam scanning arrays without phase shifters.

A circular polarised patch antenna was simulated using software by Vector Fields, of Kidlington, Oxford, the simulation considerably reducing development times.

The optimised design was then prototyped and measured by Queens University Belfast using an HP 8510B network analyser to measure the full port scattering parameters of the antenna array.

Such a 2 x 2 array can be used as a building block in larger arrays reducing complex feed requirements.

Each antenna was constructed on a substrate of FR4 with a thickness of 1.6mm and with relative permittivity 4.55, and designed to be a 4.4GHz circular patch antenna of about one guided wavelength in circumference.

It was matched using a CPW structure with an inset microstrip via-hole feed of 50ohm.

The patch feed-through slot width and matching microstrip length were carefully chosen so that mutual couplings through surface wave propagation on the substrate, and free space coupling between patches are both reduced.

As a result of this, each patch element could be independently driven at a different phase by external excitation.

Vector Field's Concerto uses the well established FDTD (finite difference time domain) technique, including the use of conforming elements to allow accurate handling of inclined and curved boundaries.

The mesh sizes in the x, y and z directions are specified, with local refinement in order to concentrate on particular regions of interest.

Particular care was been taken to ensure mesh conformity near the matching slots in the patches and microstrip regions.

In the simulations, each antenna had a progressive 90-degree phase delay added to replicate the circular polarisation and radiation pattern.

The measured return loss when compared with the simulated results showed remarkable correlation.

The simulated results included cases where the cavity walls and patch elements were treated as PEC and when conductivity of 50MS/m was used.

In the experiments a VSWR of 1.6 at each antenna port was achieved at 4.4GHz with a bandwidth of 250MHz.

In addition the simulated antenna gain was 5.6dB with an antenna directivity of 6.0.

Concerto with its recently upgraded user interface has an enviable record for speed and accuracy in high frequency analysis in the field of RF and microwave design.

Working closely with client user groups Vector Fields is continuously developing simulation packages in this and other fields such as magnetic and thermodynamic systems.

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