Fall 2008 Issue

ANTENNAS

Ultra Wide Band (UWB)

By Kent Britain, WA5VJB

Photo A. Ultra-wide-band transmitter and antenna.

We certainly are seeing a lot of articles these days on Ultra Wide Band, or UWB. The FCC has set aside 3.1 GHz to 10.7 GHz for UWB use. There is a lot of bandwidth and there are a lot of challenges for both transmitters and the antennas (photo A).

There are three main types of UWB signals being used at this time. The first type is simple FM. If I take my 5-GHz walkie-talkie and crank the FM deviation up to 500 MHz, this meets the FCC definition of UWB. No, that is not a typo. I didnít mean 5 kHz, but 500 MHz. Even the old C-band TVRO only used 30 MHz wide FM video. However, the idea is the signal is spread so thin that there isnít enough signal in any one part of the band to cause much interference. This is legal according to the FCC, but not commonly used.

The next UWB modulation is Orthogonal Frequency Division Multiplexing (OFDM). OFDM can be thought of as hundreds or even thousands of carriers each being separately modulated. It is kind of like one-thousand 9600-kb modems running in parallel. Thousands of these signals can result in data rates of over 250 Megabits/second. Demodulation of all these carriers is somewhat math intensive. However, with such little power in each carrier, again the interference potential is low. Also, to keep the FCC happy, the signals must be spread out over at least 500 MHz, and there are some complex formulas on how evenly the energy is spread out.

Impulse or Pulse Position Modulation was the original UWB modulation. The transmitter in photo A puts out a 1-watt pulse for 1-billionth of a second. This fast pulse isnít done with super-fast digital circuits, but rather with clever oscillator design. As the oscillator is turned on, the oscillator puts out five or six sine waves centered at 6 GHz and then shuts down as all the DC energy is used from the capacitors in the circuit. In many ways this is very similar to the self-squelching oscillators used in super-regenerative circuits for the last 90 years.

We had to use a Tektronix 11801 scope with a 26-GHz bandwidth to look at these fast pulses. The timing between pulses is used to send data. While the transmitter is putting out 1 watt, it has to transmit one-billion pulses to use up just one watt-second from the battery. That lithium coil cell will run the transmitter for over a year.

UWB Antennas

There are two big engineering problems with UWB antennas. The first is bandwidth; the antenna has to work over several GHz of bandwidth. The next problem is the Q of the antenna.

The typical resonant antenna is a high-Q structure. One way of looking at a high Q is to think of it as being similar to a flywheel that is spinning and spinning, thereby storing energy.

As shown in figure 1, on average, an electron must go back and forth on a dipole about 30 times before it leaves as an electromagnetic wave. Furthermore, it is only after the energy has built up on the antenna that it starts to look like 50 ohms. Back on 40 meters, this means that for the first 1/7,000,000 second your inverted-V looks like a dead short to your transmitter. Then, 1/7,000,000 second later the antenna has an impedance of a few ohms. Also, only after a few dozen waves have gone into the antenna does the voltage start to build up and it begins to approach the typical 50-ohm load. Of course, the average ham isnít all that worried that it takes few millionths of a second for the impedance of the antenna to stabilize.

However, for the designers of high-speed data networks and high-resolution RADAR systems, the transmitter impedance and the antenna impedance may be a lot different for these short pulses than it is for a CW signal. Also, the time it takes for the voltage to build up delays the pulse. Now my nice short pulse has been delayed by the ringing currents in the antenna, lengthening and delaying the data.

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