Time Analysis of UWB Antennas
Ultra-wideband (UWB) antennas are often used for short range, indoor transmission due to low FCC emission requirements and where short pulse lengths enable transmission of high data rate signals. Typical applications include high precision through-wall radar imaging and PC-peripherals like wireless printers. In such cases, additional time domain analysis is necessary to characterize the antenna’s overall performance for pulsed signal transmission. Parameters (e.g. pulse distortion, antenna fidelity) need to be analysed together with standard antenna performance parameters for reliable design. In this white-paper, a printed elliptical UWB monopole antenna is used to illustrate these concepts.
Synthesis with Antenna Magus
The initial antenna synthesis was performed with Antenna Magus using the microstrip fed planar elliptical monopole antenna model with a design goal of fmin = 2 GHz. The initial design was tweaked to improve the operational bandwidth (BW) by optimizing the feed gap spacing. The final design is exported for fullwave simulation in FEKO.
Antenna geometry and Antenna Magus synthesis results
Fullwave Simulation with FEKO
FEKO Time Analysis and Fidelity Calculation
A modulated Gaussian pulse is defined mathematically in FEKO for the excitation signal that will be used for the Time Analysis:
The voltage signal (fmod = 10 GHz) used to excite the antenna and the normalized radiated pulses at theta, phi = (0, 90) and (90, 0) respectively are shown in the figure below. It is clear that the radiated pulses are neither identical to the excitation signal nor to one another. This implies that the distortion of the signal is a function of the radiation direction, which is essentially the motivation to investigate the fidelity for the antenna.
Excitation voltage and radiated signal waveforms for the antenna.
The antenna fidelity can be defined as 
where a(t) is the excitation signal and the response r(t) is the radiated pulse for a given theta and phi. The fidelity is in essence a measure of how similarly a pulse is radiated compared to the excitation signal. When the 2 signals are identical the fidelity reaches its maximum value 1. The equation uses the normalized signals to factor out any scaling effects and a time delay which allows for phase shifts that may occur. A Python script was written to calculate the convolution operation in the fidelity equation. The results are plotted in POSTFEKO and the data is exported as a text file, which is used as an input for the script.
Comparison of the calculated fidelity values for the 2 pulses with published results
|theta, phi||modulated 6.5 GHz||published ||modulated 10 GHz|
The table shows the calculated fidelity values which agree well with the results published in . The dependency of the fidelity value on the radiation direction is highlighted again in the table and can also be visualized in the previous figure: the distortions for the (0, 90) signal (fidelity = 0.975) are smaller than the (90, 0) signal (fidelity = 0.871). In general the fidelity values for the antenna are high which means that it is suitable for UWB applications where pulse radiation characteristics are important.