Ultra-Wideband Antenna EE198B Fall 2004 Team Members: Ryan Clarke Roshini Karunaratne Chad Schrader Advisor: Dr. Ray KwokIntroduction Ultra-wideband (UWB) communication systems have the promise of very high bandwidth, reduced fading from multipath, and low power requirements [1]. For our project, we designed an UWB antenna for a handheld communications device with a bandwidth of 225 to 400 MHz, a voltage standing wave ratio (VSWR) of less than 1.5 to 1, and an efficiency of greater than 75 percent. The antenna had to be resistant to body effects, which means that if the communications unit is put up to the user’s head or put on a large metal surface, that the radiation pattern will not be greatly affected. Our antenna also had to be small enough to fit on the communication device, which was ten inches high, by three inches wide, by one inch thick. Theory The main concept behind UWB radio systems is that they transmit pulses of very short duration, as opposed to traditional communication schemes, which send sinusoidal waves. The role that UWB antennas play in all of this is that they have to be able to transmit these pulses as accurately and efficiently as possible. For this project, we had four main parameters that we had to satisfy. Those parameters were the bandwidth of the antenna, the VSWR of the antenna, the efficiency of the antenna, and the radiation pattern of the antenna. These parameters will help us understand if the antenna we are designing will be the optimal design for our application.The first parameter that we had to consider for our design is the bandwidth. The bandwidth is basically the frequency (or frequencies) that the antenna is designed to radiate. In many cases, i.e. narrowband systems, the bandwidth specified for an antenna is very small because there is just one frequency that the antenna is required to radiate. In our case, we had to be able to radiate signals with frequencies between 225 MHz and 400 MHz. This required us to limit the antenna designs that we considered to strictly broadband antennas. The second parameter that we had to take in to account for our design is the VSWR of the antenna. The VSWR is defined as [2] VSWR = minmaxVV = Γ−Γ+11. (1) The voltage reflection coefficient, , is defined as Γ = Γ+−ooVV = oLoLZZZZ+− , (2) where Z is the load impedance and Z is the characteristic impedance. This reflection coefficient is also equivalent to the scattering parameter s . The characteristic impedance is considered to be the impedance of the antenna for our purposes. The incident wave and the reflected wave V can also be related through the following equation for the total voltage on the line: L o11+oV−o V(z) = Voezj + V e = V [e + e ], (3) +β− −0zjβ+ozjβ−Γzjβwhere equals βλπ2. The VSWR is a way of calculating how well two transmission lines are matched. The number for the VSWR ranges from one to infinity, with one meaning that the two transmission lines are perfectly matched. In regards to antenna design, a VSWR that is as low as possible is desired because any reflections between the load andthe antenna will reduce the effectiveness of the antenna. The third parameter that we took into account for our antenna design is the efficiency of the antenna. The radiation efficiency of an antenna is defined as [2] e = INRADPP (4) where P is the power radiated by the antenna and P is the power supplied to the antenna. The efficiency of an antenna is a measure of how much power is lost in radiating a signal from the antenna. RAD INThe fourth parameter is the radiation pattern of the antenna. This parameter is highly dependent on the application of the antenna. In the case of the antenna our group designed, we had to have an omnidirectional radiation pattern. This means that the radiation pattern had to be spread evenly 360 degrees around the antenna. The reason for this is because since the location of the transmitter is not fixed, you want to spread the radiated signal out as far as possible so the receiver will be able to pick up the transmitted signal. One aspect of choosing a UWB antenna design that is important is ensuring that the design will not cause the pulse to spread when it is transmitted. Another aspect that is important is making sure that the antenna will be highly efficient in radiating electromagnetic energy. This is due to the fact that the transmit power used in UWB systems is very low (-41.3 dBm/MHz) [1]. Finally, the UWB antenna needs to be broadband enough to handle the bandwidth requirements for UWB (a fractional bandwidth greater than 20% [3]).Our first challenge for this project was trying to decide what antenna design to use for this project. We needed a design that would be able to meet our specifications and also be small enough to fit on the communication device. One of the first designs that our group considered was the planar diamond dipole antenna [4]. This antenna has the advantage of having a low profile and a large bandwidth. Unfortunately, for the frequency range that we are designing the antenna for, the diamond dipole would have ended up being to large for our application. This is because substrate-based antennas have to have a length roughly proportional to the width of the antenna [5]. If we were to design such an antenna with a length of 1/4 , with λ = λfc (5) With c = 2.998 10 m/s and taking f to equal 225 10 Hz, then ¼ would equal 13.1 inches. Coupled with the fact that we would have to use a thicker substrate to increase the efficiency of the antenna, our group decided this would not be a feasible antenna for our application. Another antenna design that we considered was the planar inverted-F antenna, or PIFA antenna. This is a very popular antenna design for mobile phones [6] because of its small size and its resistance to body effects, which is an antenna characteristic we were looking for. Unfortunately, this design also had many of the same drawbacks as the diamond dipole antenna design, such as large size and low efficiency. We finally decided on the dual-L monopole design. This design had the benefits of being easy to design, as well as being easy to construct. ×8×6λThe dual-L monopole antenna is a design based on the open-sleeve monopole antenna [7]. This antenna consists of two copper tubes with the ends bent at ninety degree angles. The purpose of having the ends of the elements bent is to