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High speed pulse radiation from switched electrically small antennas

By taking a time domain approach, Wheeler's limit can be understood as the following. As the antenna becomes smaller, the ratio of energy required to be stored in the near field to the energy radiated per period becomes larger. Therefore, the time it takes to replenish the near-field active energy before the antenna can radiate efficiently increases. When the rate of signal becomes higher than the replenishing speed, the radiation cannot follow this signal variation. This causes the signal to be distorted, or radiation efficiency lowered if compensation is applied to decrease the Q. However, with a switch, it is possible to pre-charge the antenna and allow the stored energy turn to radiation immediately. Similar concept has been applied to increase the radiation bandwidth of resonant antennas. The pre-charge process are shown to improve the efficiency on wideband pulse radiation for electrically small antennas. Analytical studies also show that the bandwidth of the radiated pulse, which is controlled by switching, can reach to 100%. However, in practical case, the loss of the switch has an adverse effect on efficiency. To minimize this loss, it is desirable to have low on-resistance, high off-resistance, and fast switching speed. It is expected that the semiconductor technology advancement can lead to practical implementation of the proposed technique in the near future. To validate the concept, a lambda/10 switched dipole antenna operating at 100 MHz is analyzed and simulated with both circuit and full-wave simulators. With a typical set of switch parameters, this technique achieves an efficiency-bandwidth product of 0.105, compared to 0.0013 from traditional design