La Ca

–Ca Ca

Figure 7.246 Negative capacitance replacing a large inductance in the resonance circuit of a small antenna.

consideration. The operating frequency is 2.45 GHz. Measured results show a resonant frequency of 2.35 GHz with return loss of 23 dB and the –10 dB bandwidth of 4.5%. From the radiation patterns, the gain is evaluated to be 0.16 dBi.

The proposed antenna as fabricated by using small patches and slots could offer fairly good performance with a compact size of λ0/11 square that is very small as compared with a conventional λ0/2 antenna (λ0: free space wavelength).

7.2.6Active circuit applications to impedance matching

An antenna always needs impedance matching in practical use. As was mentioned frequently, matching at the feed terminals of an electrically small antenna is a crucial problem, because of the need to compensate its highly reactive impedance with low loss, and to transform its low resistive impedance to the load impedance (50 ). To overcome this difficulty, two methods have recently been disclosed; one is the use of active circuits and another is matching in the near field of a radiator, instead of matching at the feed terminals. The latter has been described in 6.2.5. Meanwhile, active circuits are applied, for instance, to cancel the large capacitance Ca of a short dipole. In practice, as Figure 7.246 shows, a negative reactance –Ca is used instead of a large inductance La corresponding to the Ca of a short dipole. In the figure, ra denotes the radiation resistance of the dipole. Since La must be large enough to cancel Ca, it may have a large

ZL

–ZL

(b)

NIC

–ZL

Figure 7.247 Negative impedance converter (NIC) and example of practical implementation: (a) grounded type and (b) floating type.

loss that reduces the radiation efficiency, and hence, –Ca substitutes for it. The negative capacitance –Ca can be achieved by a negative reactance circuit, which is constituted of transistors. The negative reactance circuits can be realized by NIC as is illustrated in Figure 7.247, which shows two types of NIC; (a) grounded NIC and (b) floating NIC [126–134]. Examples of transistor circuits are shown on the right-hand side in the figure, where Cin (–C) and –ZL, respectively, are the capacitance converted from C by the grounded NIC and the impedance converted from ZL by the floating NIC.

On the recent advent of NIC application to antennas, antenna engineers have acquired a pragmatic and effective design technique to develop a novel electrically small antenna having high efficiency and possibly wide bandwidth.

Active circuits, which do not follow Foster Reactance Theorem, are referred to as Non-Foster circuits. Another way to obtain proper matching in small antennas is the use of metamaterials (MM), which can represent negative reactance, instead of NIC. An MNG introduced in 7.2.5.1 is a typical example.

It must be noted, when negative reactance circuits are used, that the circuit should maintain stable condition, noise be kept as low as possible, and the linearity be kept as high as possible.

Regarding stability, an NIC needs inevitably to be open-circuit stable (OCS) at one port and short-circuit stable (SCS) at the other port. OCS means that the network is stable for any passive load on one side with the other port open, while SCS means that the network is stable for any passive load on one port with the other port short. The inherent conditional stability of an NIC constrains the magnitude of the impedances that can be connected to the OCS port and to the SCS port. This can be interpreted by the requirements |ZL1| >|Zin1| and |ZL2| < |Zin2|. Figure 7.248(c) illustrates an NIC that has impedances ZL1 and ZL2, respectively, at each load terminal and impedances Zin1 and Zin2 at the input terminals of each side. In designing an NIC circuit, these requirements must always be met.

In addition, there is inconvenience in using the active circuit, as the transistor circuits are generally unidirectional, whereas an antenna is bi-directional, and hence the application is limited to either transmitter or receiver. In addition, transistor circuits need the bias supply, for which additional circuits are required within the antenna system.

Figure 7.248 NIC circuit and stability requirements: (a) shows OCS state, (b) gives SCS state, and

(c) illustrates impedances that concern stability of an NIC; ZL1 ZL2 at the load and Zin1, Zin2 at the input terminals each side.

L

Figure 7.249 Impedance transformer between a low resistance and the load (characteristic impedance Zo).

Instead, an NMG material, if it could be available, is greatly advantageous, since the material enables bi-directional application feasibility that is favorable in antenna applications.

Nevertheless, there are distinct advantages of using negative circuits in antenna design, as they can overcome matching difficulty and thus assist in enhancing gain and efficiency in transmitter applications, even when the size of antenna is electrically small, while they may facilitate extension of bandwidth and improvement of signal-to-noise ratio in receiver applications.

NIC may also be used as a circuit that converts low radiation resistance of small antennas to the load (or source) resistance (50 ) after the reactance is compensated. For this purpose, a T-shaped inductance circuit, for example, may be used. Figure 7.249 depicts the circuit, which is formed of a shunt L and two series –Ls. Here, the –L can be produced by an NIC. The matching circuit is then designed to employ NICs, which represent –Ca and –Lm as shown in Figure 7.250.

Figure 7.250 Matching circuit and impedance transformer using negative reactance.

7.2.6.1Antenna matching in transmitter/receiver

NICs are applied to matching circuits of antennas in the transmitter or the receiver [132, 133]. An example of antenna matching in the transmitter is introduced in [132], which describes a 15–30 MHz band transmitter, for which a two-foot monopole antenna is used. The matching circuit is illustrated in Figure 7.251, where two cases are shown:

(a) passive matching and (b) active matching. In these circuits, an inductance L = 1.8 µH is used for tuning at 20 MHz with the antenna capacitance Ca = 33 pF and a serial L-C circuit, which derives 50 of the load resistance RL from the low resistance 4 at the output stage of the tuning circuit, is used. This 4 at the output of the tuning circuit is attributed to the loss of the inductance L, whereas the radiation resistance of the antenna at 20 MHz is 1 .

In order to transform 1 to the source 50 , a serial L-C network is used as shown in Figure 7.251, which is a different type of circuit to that shown in Figure 7.249.

Meanwhile, in the active matching, a floating NIC, which uses two transistors, each being a pair of npn-pnp, is used to cancel L and Ca, to which the impedance transforming circuit is connected. Then the overall matching circuit will be that as shown in Figure 7.251(b).

The matching characteristics depend on the biasing of the NIC transistors. In [132], the matching characteristics are evaluated by |S21|2, which provides the ratio of the power delivered to the 1 load to the power from the source (50 ), and is compared with that of the passive matching. In the active matching, two biasing cases, that is, conventional class A and class B biasing, are compared.