An Introduction to UWB Antennas
What is UWB?
UWB stands for ultra wideband or ultraband frequencies. The FCC set aside the frequencies 3.1 GHz to 10.7 GHz for ultrawide band (UWB) use in the United States. Frequencies above 10.7 GHz are mostly satellite TV and military radar.
Uses of UWB Antennas
The UWB range is being used for simple FM signals when the signal is spread across 500 MHz. The signal is spread so thin on that range that that there isn’t much interference created by it. However, this is a rare use of UWB.
Orthogonal Frequency Division Multiplexing or OFDM is another use of UWB. It lets you run thousands of signals at data rates of over 250 megabits per second. Demodulating the signals is computationally intensive. But by sending out the signals across at least 500 MHz, there is little interference with each of these signals. This broad signal is essential for the lower UWB band so that UWB antennas don’t interfere with the GPS and satellite communications using the spectrum below 6 GHz.
Impulse or Pulse Position Modulation is the original signal modulation type for Vivaldi antennas. The fast pulsing signal is generated by an oscillator, producing several sine waves centered on a frequency like 6 GHz. This technology is close to the squelching oscillators used in super-regenerative circuits since the early 1900s.
There are two engineering challenges with ultrawide band antennas. The first problem is bandwidth. The antenna has to work over several gigahertz of bandwidth. The second problem is the Q, the bandwidth of the antenna relative to the center frequency of the bandwidth. Resonant antennas are usually high-Q. For UWB Vivaldi antennas, it takes a fraction of a second for the antenna impedance to stabilize. This doesn’t matter if you’re sending amateur radio signals, but this affects the signal when the UWB antenna is used for a high speed data network or high resolution radar system. The fraction of a second it takes to warm up to the full impedance means the transmitter and antenna impedance is a lot different for the short signals than it would for a CW signal. This type of quick modulating signal also requires voltage to build up, which delays the signal being sent. This both lengthens and delays the data being sent often in applications where response time is essential.
Some of the early uses of UWB antennas were UHF TV, cell phones and GPS. GPS shifted off ultra wide band antennas early on because it interfered with the navigation systems recommended by the FAA. Sensor data collection and location tracking applications sometimes use the UWB frequency range today.
Types of UWB Antennas
The simplest antenna is a dipole antenna. As the elements in the antenna get longer, the gain goes up.
The most common ultra-wideband (UWB) antennas are Vivaldi antennas. Vivaldi antennas are also called exponential antennas. According to Wikipedia, a Vivaldi antenna is a coplanar (two sided) broadband antenna made from a dielectric plate metalized on both sides. A Vivaldi antenna is similar to a transmission line antenna, providing very wide bandwidth and modest gain, but there’s very low circulating current.
One common way of making Vivaldi antennas is using printed circuit boards, which gives them a low cost per unit. Vivaldi antennas can be used for a wide range of frequencies but are often used as UWB antennas. One of the benefits of using Vivaldis as UWB antennas is balancing the input impedance to output impedance (impedance matching) to maximize power transfer or minimize signal reflection.
Planar UWB Antenna
Log periodic antennas consist of an array of dipoles. This design causes the signal to resonate and it will stretch out a sharp pulse. There are smaller log periodic antennas that cover the UWB frequency range like the 2 to 11 GHz log periodic antenna shown here.
The scimitar antenna is a talon shaped curve resembling a scimitar sword, with the inner and outer curves having a different radius. Scimitar antennas have an inner radius that sets the high frequency value of the antenna and an outer radius that sets the low frequency range. A scimitar antenna with a 400-1500 MHz bandwidth will have a natural input impedance of around 20 ohms.
Fractal antennas use a fractal pattern or self-similar pattern to maximize the length or perimeter of the antenna that can send or receive a signal. The most common type of UWB fractal antennas uses a triangular fractal pattern.There is a tremendous amount of hype about Fractal antennas.That I know of no commercial product actually using one on a large scale says a lot about how well they work in the real world.
The 2-26 GHz planar disk antenna shown here and other planar disk antennas also cover the ultraband frequency range.