**4. Effects of feeding cable on small UWB monopoles measurements**

As explained before, the feeding cable used in measurements will scatter, reflect and radiate EM fields, causing interference to the measured results of antennas. Here the effects of the feeding cable are investigated by using simulation and measurement. In the antenna measurement system, Starlab, the feeding cable is enclosed by an EMI suppressant material which is highly lossy. The EM fields incident on it and radiated from it will be absorbed. This significantly reduces unwanted interference to the measured radiation patterns. However, absorbing the EM radiation leads to reduced efficiency. That is why the measured radiation efficiencies are always lower than the simulated results.

#### **4.1. Modeling of feeding cables**

Here, we describe the simulation models for two types of feeding cables, denoted here as cables A and B as shown in Figures 10(a) and (b), respectively [20], and use the models in our simulation to study their effects on the measurements of the antenna performances. Cable A is just an ordinary coaxial cable, having a center conductor with a radius of 0.45 mm, and an outer conductor with inner and outer radii of 1.5 and 1.8 mm, respectively. Both cables have a length of about 250 mm. Figure 11(a) shows the cross section of cable A. The space between the center and outer conductors is filled with a dielectric Teflon having a permittivity of 2.08. For cable B shown in Figure 10(b), it is a coaxial cable provided by Satimo for use with the antenna measurement system, Starlab. The cross section of the cable is shown in Figure 11(b) which is identical to cable A, except that the cable has an EMI suppressant tube with a thickness of 1.25 mm on the surface. The property of the tubing material is quite complicated and hard to express precisely. Simulation results show that, by setting both the permittivity and the permeability to 5, and the electric and magnetic loss tangents to 0.004 and 0.5, respectively, the discrepancies between the simulated and measured S11 and efficiency are much reduced, thus these parameters are used in our simulation model for cable B.

**Figure 10.** Two types of feeding cables used for studies.

284 Ultra Wideband – Current Status and Future Trends

plane)

**(Ant) GP dimension (1) 15 (***gl***) × 30 (***gw***)=450 (2) 15 (***gl***) × 50 (***gw***)=750 (3) 15 (***gl***) × 80 (***gw***)=1200 Avg. 2-12 GHz** 0.110 0.074 0.040 **2-4 GHz, 4-6GHz, 6-12 GHz** 0.244 0.121 0.060 0.172 0.026 0.055 0.050 0.051 0.034 **(Ant) GP dimension (4) 30 (***gl***) × 30 (***gw***)=900 (5) 30 (***gl***) × 50 (***gw***)=1500 (6) 30 (***gl***) × 80 (***gw***)=2400 Avg. 2-12 GHz** 0.102 0.081 0.049 **2-4 GHz, 4-6GHz, 6-12 GHz** 0.225 0.114 0.055 0.180 0.056 0.054 0.050 0.052 0.038 **(Ant) GP dimension (7) 50 (***gl***) × 30 (***gw***)=1500 (8) 50 (***gl***) × 50 (***gw***)=2500 (9) 50 (***gl***) × 80 (***gw***)=4000 Avg. 2-12 GHz** 0.151 0.072 0.061 **2-4 GHz, 4-6GHz, 6-12 GHz** 0.186 0.116 0.058 0.130 0.057 0.056 0.069 0.058 0.061

**Table 2.** Average discrepancy of efficiency between simulated and measured results (GP: Ground

**4. Effects of feeding cable on small UWB monopoles measurements** 

radiation efficiencies are always lower than the simulated results.

**4.1. Modeling of feeding cables** 

simulation model for cable B.

As explained before, the feeding cable used in measurements will scatter, reflect and radiate EM fields, causing interference to the measured results of antennas. Here the effects of the feeding cable are investigated by using simulation and measurement. In the antenna measurement system, Starlab, the feeding cable is enclosed by an EMI suppressant material which is highly lossy. The EM fields incident on it and radiated from it will be absorbed. This significantly reduces unwanted interference to the measured radiation patterns. However, absorbing the EM radiation leads to reduced efficiency. That is why the measured

Here, we describe the simulation models for two types of feeding cables, denoted here as cables A and B as shown in Figures 10(a) and (b), respectively [20], and use the models in our simulation to study their effects on the measurements of the antenna performances. Cable A is just an ordinary coaxial cable, having a center conductor with a radius of 0.45 mm, and an outer conductor with inner and outer radii of 1.5 and 1.8 mm, respectively. Both cables have a length of about 250 mm. Figure 11(a) shows the cross section of cable A. The space between the center and outer conductors is filled with a dielectric Teflon having a permittivity of 2.08. For cable B shown in Figure 10(b), it is a coaxial cable provided by Satimo for use with the antenna measurement system, Starlab. The cross section of the cable is shown in Figure 11(b) which is identical to cable A, except that the cable has an EMI suppressant tube with a thickness of 1.25 mm on the surface. The property of the tubing material is quite complicated and hard to express precisely. Simulation results show that, by setting both the permittivity and the permeability to 5, and the electric and magnetic loss tangents to 0.004 and 0.5, respectively, the discrepancies between the simulated and measured S11 and efficiency are much reduced, thus these parameters are used in our

**Figure 11.** Cross sections of cables used for studies

#### **4.2. Antennas used for studies**

Three of the nine UWB antennas, Ants 4, 5 and 9 with ground-plane sizes (*gl* × *gw* ) of 30 × 30 mm2, 30 × 50 mm2, and 50 × 80 mm2, respectively, shown in Figure 7 are selected for investigation of the cable effects using computer simulation. The simulated models developed for cables A and B are used in simulation to feed the signal to the antenna as shown in Figures 12(a) and (b), respectively. In the Starlab system, there is a system cable with EMI suppressant tubing (similar to that used in cable B) used to connect the feeding cable (cable A or cable B) to the network analyzer of the system. The system cable has a length of about 3-4 m long. However, to reduce the simulation time, we only use a total length of 400 mm in our simulation models. Thus when cable A with a length of 250 mm is used, we append a 150-mm cable B to it, making it a total length of 400 mm, as shown in Fig. 12(a). When cable B with a length of 250 mm is used, we actually use a total length of 400 mm, instead of 250 mm. In Figure 12, a metal brick with a size of 6.5 × 6.5 × 13.5 mm3 is used to model the SMA connector.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 287

**Figure 13.** Simulated and measured S11 and efficiencies using cable A with different ground-plane

(c) Ant 9: 50 × 80 mm2

(a) Ant 4: 30 × 30 mm2 (b) Ant 5: 30 × 50 mm2

pattern becomes very similar to the corresponding radiation pattern in Figure 14(a) without using the cable. This can also be seen in Figure 15(b) which shows the simulated surfacecurrent on the feeding cable at 11 GHz. The standing wave on the feeding cable becomes insignificant. The measured 3D-radiation patterns using cable A at 3, 7 and 11 GHz are shown in Figure 14(c), indicating very good agreements with the corresponding simulated radiation patterns in Figure 14(a). These results verify the validity of our simulation mode

sizes.

for the cable.

**Figure 12.** Simulation models of antenna connected to (a) cable A, and (b) cable B

#### **4.3. Results and discussions**

#### *4.3.1. Effects of ordinary coaxial cable (cable A)*

With the use of cable A as the feeding cable, the simulated and measured S11 and efficiencies of the three antennas are shown in Figure 13. For comparison, the simulation results without a feeding cable are also shown in the same figure. It can be seen in Figure 13 that at high frequencies the simulated efficiencies of the antenna with and without using the cable model are about the same. This is because at high frequencies the ground planes are electrically large, leading to little cable effects on measurements. As the frequency reduces, the ground planes become smaller and discrepancies occur. For impedance bandwidth (S11 = -10 dB), all results agree well. This seems to indicate that the cable does not have much effect on the measurements, which as shown later is not true. In fact, at low frequencies, the current flows back from the small ground plane to the surface of the feeding cable, as described previously, giving rise to EM radiation, and then get measured by the system. The measured and simulated results on 3D-radiation patterns reveal that the feeding cable has serious effects on measurements.

The simulated and measured 3D-radiation patterns of the antenna with the ground-plane size of 30 × 30 mm2 at the frequencies of 3, 7 and 11 GHz, are shown in Figure 14. Without using the feeding cable, the simulated result in Figure 14(a) shows that the antenna has an "apple-shape" radiation pattern at the frequency of 3 GHz which is typical for monopole antennas. At higher frequencies of 7 and 11 GHz, the radiation patterns become slightly directional due to operating in the higher modes. However, when cable A is used, the simulated radiation patterns in Figure 14(b) show many ripples, particularly serious at the lower frequency of 3 GHz. This is because, at 3 GHz, the ground-plane size of 30 × 30 mm2 (only about half wavelength) is too small to serve as an infinite ground for the monopole. As showed previously, with a small ground plane, the EM fields radiated from antenna are diffracted at the edges and induce currents to flow back to the feeding cable. This can be seen in Figure 15(a) which shows a snap-shot of the simulated surface-current on the feeding cable at 3 GHz. A standing wave is developed along the feeding cable. This produces secondary EM radiation and causes the ripples on the 3D-radiation patterns of Figure 14(b). At 11 GHz, the ground plane is electrically larger and so the 3D-radiation

**4.3. Results and discussions** 

serious effects on measurements.

*4.3.1. Effects of ordinary coaxial cable (cable A)* 

**Figure 12.** Simulation models of antenna connected to (a) cable A, and (b) cable B

With the use of cable A as the feeding cable, the simulated and measured S11 and efficiencies of the three antennas are shown in Figure 13. For comparison, the simulation results without a feeding cable are also shown in the same figure. It can be seen in Figure 13 that at high frequencies the simulated efficiencies of the antenna with and without using the cable model are about the same. This is because at high frequencies the ground planes are electrically large, leading to little cable effects on measurements. As the frequency reduces, the ground planes become smaller and discrepancies occur. For impedance bandwidth (S11 = -10 dB), all results agree well. This seems to indicate that the cable does not have much effect on the measurements, which as shown later is not true. In fact, at low frequencies, the current flows back from the small ground plane to the surface of the feeding cable, as described previously, giving rise to EM radiation, and then get measured by the system. The measured and simulated results on 3D-radiation patterns reveal that the feeding cable has

(a)

(b)

The simulated and measured 3D-radiation patterns of the antenna with the ground-plane size of 30 × 30 mm2 at the frequencies of 3, 7 and 11 GHz, are shown in Figure 14. Without using the feeding cable, the simulated result in Figure 14(a) shows that the antenna has an "apple-shape" radiation pattern at the frequency of 3 GHz which is typical for monopole antennas. At higher frequencies of 7 and 11 GHz, the radiation patterns become slightly directional due to operating in the higher modes. However, when cable A is used, the simulated radiation patterns in Figure 14(b) show many ripples, particularly serious at the lower frequency of 3 GHz. This is because, at 3 GHz, the ground-plane size of 30 × 30 mm2 (only about half wavelength) is too small to serve as an infinite ground for the monopole. As showed previously, with a small ground plane, the EM fields radiated from antenna are diffracted at the edges and induce currents to flow back to the feeding cable. This can be seen in Figure 15(a) which shows a snap-shot of the simulated surface-current on the feeding cable at 3 GHz. A standing wave is developed along the feeding cable. This produces secondary EM radiation and causes the ripples on the 3D-radiation patterns of Figure 14(b). At 11 GHz, the ground plane is electrically larger and so the 3D-radiation

**Figure 13.** Simulated and measured S11 and efficiencies using cable A with different ground-plane sizes.

pattern becomes very similar to the corresponding radiation pattern in Figure 14(a) without using the cable. This can also be seen in Figure 15(b) which shows the simulated surfacecurrent on the feeding cable at 11 GHz. The standing wave on the feeding cable becomes insignificant. The measured 3D-radiation patterns using cable A at 3, 7 and 11 GHz are shown in Figure 14(c), indicating very good agreements with the corresponding simulated radiation patterns in Figure 14(a). These results verify the validity of our simulation mode for the cable.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 289

**Figure 15.** Surface current distributions of antenna using cable A at (a) 3 GHz, and (b) 11 GHz

Figure 16 shows the simulated and measured results for using cable B. It can be seen that, with the use of the simulation model for cable B, the measured and simulated S11 and efficiencies have very good agreements for the three antennas even at lower frequencies. These results confirm the accuracy of our simulation model for the feeding cable used in the antenna measurement system. The simulated results without using the feeding cable are also shown in the same figure for comparison. It can be seen that the simulated efficiency without using the feeding cable at low frequencies is much higher than the simulated or measured efficiencies using the feeding cable. This is because at low frequencies, the current flows back to the feeding cable and causes secondary radiation which is mostly absorbed by

(b)

(a)

The 3D-radiation patterns of the antenna with a ground-plane size of 30 × 30 mm2 at the frequencies of 3, 7 and 11 GHz are shown in Figure 17. With the use of cable B, Figures 17(b) and (c) show no ripple on the 3D-radiation patterns. The simulated 3D-radiation patterns with and without using cable B agree quite well, indicating the effectiveness of using EMI suppressant tubing for the feeding cable. The measured 3D-radiation patterns in Figure 17(c) are similar to the corresponding simulated radiation patterns shown in Figure 17(b). Figure 18 shows the simulated current distributions on the outer surface of the feeding cable at 3 and 7 GHz. Compared with those in Figure 15, it can be seen that the surface current is very small even at 3 GHz because of the EMI suppressant material. It should be noted that, at low frequencies, since the EM fields radiated from the feeding cable are mostly absorbed

by EMI suppressant tubing, the efficiency and hence the gain are much reduced.

*4.3.2. Effects of feeding cable with EMI suppressant tubing (cable B)* 

the EMI suppressant tube enclosing the cable.

(a)
