**3.1. Cable effects on measuring monopoles with small ground planes**

Nowadays, the design of antennas is usually done by using computer simulation. In simulation, normally the antenna is directly fed from a signal source and no feeding cable is used. However, when the antenna is fabricated and measured in a practical situation, a feeding cable is always used to connect the antenna to the measurement system and the signal is fed through the feeding cable to the antenna. In such arrangement, the cable could affect the measured results in two possible ways [14] as illustrated in Figure 3.

Since the feeding cable is quite near to the radiator and so is in the near field region of the antenna, the radiated EM fields incident on the cable will be scattered and reflected as shown in Figure 3(a). The feeding cable becomes a parasitic element [15]. Due to the small size of the cable, this cable effect on the measurement results is relatively small.

If the antenna is a planar monopole with a small ground plane, some EM fields will not be reflected as in the case of having an infinite ground plane shown in Figure 1(b). Instead, the EM fields arriving at the edges of the small ground plane will be diffracted. This induces surface currents to flow back on the outer surface of the feeding cable, resulting in secondary radiation as shown in Figure 3(b). This effect on measurements could be quite significant, depending on the electrical size of the ground plane. Computer simulation is carried out to study the effects of using large and small ground planes of a thin-wire monopole antenna fed by a coaxial cable on the Electric fields (E-fields). The large and small ground planes have a circular shape with the radii of 2.06λ and 0.41λ, respectively, and a thickness of 0.0008λ, where λ is the wavelength at the resonant frequency. The length of feeding cable is 1.64λ. Figure 4(a) shows a snap-shot of the E-fields radiated from the monopole antenna using the small ground plane. It can be seen that the E-fields arriving at the edges of the ground plane are quite strong. The ground-plane edges diffract the strong incident E-fields in all directions [16, 17]. After diffraction, some of the E-fields go to the upper free space and others go to the lower free space with respect to the ground plane. A significant portion of the E-fields diffracts onto the bottom surface of the ground plane, which will induce surface currents. The surface currents will flow towards the feeding cable at the center of the ground plane and onto the outer conductor-surface of the cable. Figure 4(a) shows that a standing wave is formed on the feeding cable and this will result in "secondary radiation" and affect the measured results. When the large ground plane with a radius of 2.06λ is used, Figure 4(b) shows a snap-shot of the E-fields radiating from the monopole antenna. It can be seen that the E-fields arriving at the edge of the ground plane are quite weak. As a result, the induced currents on the bottom surface of the ground plane and hence the currents flowing back onto the outer conductor-surface of the feeding cable are very weak. In this case, secondary radiation is much less.

The simulated 3D-radiation patterns for the two cases are shown in Figures 4(c) and (d). It can be seen that both radiation patterns have peak-radiation at elevation from the horizontal ground plane, typical for monopoles with finite ground planes. For the antenna with the large ground, peak-radiation is stronger and at a smaller elevation angle than those for the antenna with the small ground plane. Peak-radiation in the lower hemisphere of the radiation pattern is much weaker for the antenna with the large ground than for the antenna with the small ground plane. For the antenna with the small ground plane, Figure 4(c) shows that ripples occur in both the upper and lower hemispheres of the pattern. However, for the antenna with the large ground plane, Figures 4(d) shows no ripple in the upper hemisphere of the pattern, but many ripples with much smaller magnitudes in the lower hemisphere.

276 Ultra Wideband – Current Status and Future Trends

loss has been optimized using computer simulation, the measured performance of the prototyped antenna does not agree with the simulated performance. Large discrepancies usually occur at lower frequencies. This creates uncertainties and doubts in the design of the antenna. As will be shown in the following sections, the discrepancies at low frequencies are mainly caused by the feeding cable used to connect the antenna to the measurement system.

Nowadays, the design of antennas is usually done by using computer simulation. In simulation, normally the antenna is directly fed from a signal source and no feeding cable is used. However, when the antenna is fabricated and measured in a practical situation, a feeding cable is always used to connect the antenna to the measurement system and the signal is fed through the feeding cable to the antenna. In such arrangement, the cable could

Since the feeding cable is quite near to the radiator and so is in the near field region of the antenna, the radiated EM fields incident on the cable will be scattered and reflected as shown in Figure 3(a). The feeding cable becomes a parasitic element [15]. Due to the small

If the antenna is a planar monopole with a small ground plane, some EM fields will not be reflected as in the case of having an infinite ground plane shown in Figure 1(b). Instead, the EM fields arriving at the edges of the small ground plane will be diffracted. This induces surface currents to flow back on the outer surface of the feeding cable, resulting in secondary radiation as shown in Figure 3(b). This effect on measurements could be quite significant, depending on the electrical size of the ground plane. Computer simulation is carried out to study the effects of using large and small ground planes of a thin-wire monopole antenna fed by a coaxial cable on the Electric fields (E-fields). The large and small ground planes have a circular shape with the radii of 2.06λ and 0.41λ, respectively, and a thickness of 0.0008λ, where λ is the wavelength at the resonant frequency. The length of feeding cable is 1.64λ. Figure 4(a) shows a snap-shot of the E-fields radiated from the monopole antenna using the small ground plane. It can be seen that the E-fields arriving at the edges of the ground plane are quite strong. The ground-plane edges diffract the strong incident E-fields in all directions [16, 17]. After diffraction, some of the E-fields go to the upper free space and others go to the lower free space with respect to the ground plane. A significant portion of the E-fields diffracts onto the bottom surface of the ground plane, which will induce surface currents. The surface currents will flow towards the feeding cable at the center of the ground plane and onto the outer conductor-surface of the cable. Figure 4(a) shows that a standing wave is formed on the feeding cable and this will result in "secondary radiation" and affect the measured results. When the large ground plane with a radius of 2.06λ is used, Figure 4(b) shows a snap-shot of the E-fields radiating from the monopole antenna. It can be seen that the E-fields arriving at the edge of the ground plane are quite weak. As a result, the induced currents on the bottom surface of the ground plane and hence the currents flowing back onto the outer conductor-surface of the feeding cable

**3.1. Cable effects on measuring monopoles with small ground planes** 

affect the measured results in two possible ways [14] as illustrated in Figure 3.

size of the cable, this cable effect on the measurement results is relatively small.

are very weak. In this case, secondary radiation is much less.

**Figure 3.** Illustration of two possible ways affecting measured results of antenna: (a) reflections of EM fields from antenna and (b) currents flowing back to feeding cable

Ripples on a 3D-radiation pattern are the results of EM fields with different phases adding together constructively and destructively in different spatial directions. To study the causes of these ripples on the 3D-radiation patterns of Figures 4(c) and (d), computer simulation is carried out on the same antenna with the same large and small ground planes but without using the feeding cable. Results at resonant frequency are shown in Figure 5. For the antenna with the small ground plane, Figure 5(a) shows that the ripples disappear. Thus the ripples in Figure 4(c) are mainly caused by the feeding cable. In fact, this agrees with Figure 4(a) which shows that, at the resonant frequency, a standing wave is developed on the feeding cable which gives out EM radiation. The EM fields are added together constructively and destructively in different spatial directions, producing ripples in the 3Dradiation pattern in Figure 4(c). For the antenna with the large ground plane and without using the feeding cable, Figure 5(b) shows that the 3D-radiation pattern is about the same as that in Figure 4(d) using the feeding cable. This agrees with Figure 4(b) which shows no standing wave developed on the feeding cable and so no radiation from the cable. Thus the feeding cable has no effect on the radiation pattern. However, Figure 5(b) shows that the

ripples still occur in the lower hemisphere of the pattern. This indicates that the ripples are mainly caused by diffraction of EM fields at the edges of the ground plane. The reason can be explained as follows. The EM fields radiated from the monopole are diffracted at the edge of the ground plane into free space below the ground plane. Since the ground plane has a diameter of 4.12λ*,* the diffracted EM fields are added together constructively and destructively in different spatial directions in the lower hemisphere, forming many ripples in the radiation pattern.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 279

**Figure 5.** Radiation patterns of thin-wire monopole antenna without using feeding cable (a) small

(a) (b)

**3.2. Studies of UWB monopole antennas with different ground-plane sizes** 

To investigate the effects of ground-plane size on measurements of small UWB monopole antennas, a group of nine antennas, Ants 1, 2, …, 9, are used. These antennas have an identical elliptical-shaped radiator printed on one side of the substrate but a ground plane with different sizes on the other side of the substrate [18]. They are designed on the Rogers substrate, RO4350, with a relative dielectric constant of 3.48, a thickness of 0.762 mm and a

(a) (b)

ground plane, and (b) large ground plane

loss tangent of 0.0037, as shown in Figure 6.

**Figure 6.** Structure of UWB antennas: (a) top view and (b) side view

*3.2.1. Structure of the antennas* 

**Figure 4.** E-field radiation of thin-wire monopole antenna fed using coaxial cable for (a) small ground plane and (b) large ground plane, and corresponding radiation patterns (c) and (d).

**Figure 5.** Radiation patterns of thin-wire monopole antenna without using feeding cable (a) small ground plane, and (b) large ground plane

#### **3.2. Studies of UWB monopole antennas with different ground-plane sizes**

#### *3.2.1. Structure of the antennas*

278 Ultra Wideband – Current Status and Future Trends

λ

has a diameter of 4.12

in the radiation pattern.

ripples still occur in the lower hemisphere of the pattern. This indicates that the ripples are mainly caused by diffraction of EM fields at the edges of the ground plane. The reason can be explained as follows. The EM fields radiated from the monopole are diffracted at the edge of the ground plane into free space below the ground plane. Since the ground plane

destructively in different spatial directions in the lower hemisphere, forming many ripples

**Figure 4.** E-field radiation of thin-wire monopole antenna fed using coaxial cable for (a) small ground

(c) Small ground with cable (d) Large ground with cable

plane and (b) large ground plane, and corresponding radiation patterns (c) and (d).

(a) (b)

*,* the diffracted EM fields are added together constructively and

To investigate the effects of ground-plane size on measurements of small UWB monopole antennas, a group of nine antennas, Ants 1, 2, …, 9, are used. These antennas have an identical elliptical-shaped radiator printed on one side of the substrate but a ground plane with different sizes on the other side of the substrate [18]. They are designed on the Rogers substrate, RO4350, with a relative dielectric constant of 3.48, a thickness of 0.762 mm and a loss tangent of 0.0037, as shown in Figure 6.

**Figure 6.** Structure of UWB antennas: (a) top view and (b) side view

The tapered microstrip feed line and the gap between the radiator and the ground plane are important factors for impedance matching and so are optimized for maximum impedance bandwidth using computer simulation. The optimized dimensions of these nine antennas are listed in Table 1.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 281

efficiency, the measured results are always lower than the simulated results. The discrepancies are more obvious for antennas with smaller ground planes and at lower

To examine the effects of ground-plane size on the discrepancy of efficiency, we divide the whole frequency band from 2 to 12 GHz into three sub-bands, i.e. 2-4 GHz, 4-6 GHz, and 6- 12 GHz, and compute the average discrepancy in the whole band and in each of the subbands. Results are listed in Table 2, where each row has the same ground-plane width and increasing length and each column has the same ground-plane length and increasing width.

In Figures 9(a), (b) and (c), the ground planes have the same length of 15 mm but different widths. The lower cut-off frequencies (S11=-10 dB) of the antennas are all at about 2.8 GHz. This phenomenon is also observed in Figures 9(d), (e) and (f), and in Figures 9(g), (h) and (i). In Figures 9(a), (d) and (g), the ground planes have the same width. As the ground-plane length increases from 15 to 30 and 50 mm, the lower cut-off frequency decreases from 2.76 to 2.38 to 2.21, respectively. This phenomenon is also observed in Figures 9(b), (e) and (h), and Figures 9(c), (f) and (i). Thus the lower cut-off frequency reduces with increasing ground-

For dipole antenna, the lower cut-off frequency is inversely proportional to the length of the radiator. The results in Figure 9 show that the monopole antennas with small ground planes behave like asymmetric dipole antennas [19] and the lower cut-off frequency is inversely

Figure 9 shows that the discrepancy is larger at lower frequencies and smaller at higher frequencies. This phenomenon can also be observed in Table 2 which shows the discrepancy is always smallest in the higher sub-band and largest in the lower sub-band. This is because

3. Discrepancy reduces with increasing ground-plane width and ground-plane length.

4. The width of ground plane has more effect on the efficiency than the length.

Table 2 shows the discrepancy reduces with increasing ground-plane width and ground-

Each row in Table 2 represents the ground planes of the same widths but different lengths. Table 2 shows the average discrepancy through 2-12 GHz decreases significantly with increasing ground-plane length. Each column in Table 2 represents the ground planes of the same length but different widths and the results show the discrepancy does not change much. This can also be seen in Figure 9 by comparing the simulated and measured efficiencies of the corresponding antennas. These results show that increasing the groundplane width has more effect on reducing the discrepancy than increasing the ground-plane

From Figure 9 and Table 2, we can observe the following phenomena:

1. The lower cut-off frequency reduces with increasing ground-plane length (gl).

frequencies.

plane length (*gl*).

plane length.

proportional to the length of the ground plane.

2. Discrepancy reduces with increasing frequency

at higher frequencies, the ground plane becomes electrically larger.



**Figure 7.** Prototypes of nine planar monopole antennas with different ground-plane sizes: (a) top view and (b) bottom view.

#### *3.2.2. Results and discussions*

The performances of the nine antennas, in terms of S11 and efficiency, are studied by computer simulation. In simulation, no feeding cable is used and the antennas are fed directly by the signal source. Using the optimized dimensions in Table 1, the nine antennas are also prototyped using the Rogers substrate, RO4350, as shown in Figure 7, and measured using the antenna measurement system, Satimo Starlab, shown in Figure 8. In measurements, of course, a feeding cable with an SMA connector (provided by Satimo) is used to connect the antennas to the Starlab system. The cable is enclosed by an EMI suppressant tube to absorb EM radiation. The simulated and measured S11 and efficiencies of the antennas are shown in Figure 9. It can be seen that the simulated and measured impedance bandwidths (S11<-10 dB) for all antennas show good agreements. However, for efficiency, the measured results are always lower than the simulated results. The discrepancies are more obvious for antennas with smaller ground planes and at lower frequencies.

To examine the effects of ground-plane size on the discrepancy of efficiency, we divide the whole frequency band from 2 to 12 GHz into three sub-bands, i.e. 2-4 GHz, 4-6 GHz, and 6- 12 GHz, and compute the average discrepancy in the whole band and in each of the subbands. Results are listed in Table 2, where each row has the same ground-plane width and increasing length and each column has the same ground-plane length and increasing width. From Figure 9 and Table 2, we can observe the following phenomena:

1. The lower cut-off frequency reduces with increasing ground-plane length (gl).

In Figures 9(a), (b) and (c), the ground planes have the same length of 15 mm but different widths. The lower cut-off frequencies (S11=-10 dB) of the antennas are all at about 2.8 GHz. This phenomenon is also observed in Figures 9(d), (e) and (f), and in Figures 9(g), (h) and (i).

In Figures 9(a), (d) and (g), the ground planes have the same width. As the ground-plane length increases from 15 to 30 and 50 mm, the lower cut-off frequency decreases from 2.76 to 2.38 to 2.21, respectively. This phenomenon is also observed in Figures 9(b), (e) and (h), and Figures 9(c), (f) and (i). Thus the lower cut-off frequency reduces with increasing groundplane length (*gl*).

For dipole antenna, the lower cut-off frequency is inversely proportional to the length of the radiator. The results in Figure 9 show that the monopole antennas with small ground planes behave like asymmetric dipole antennas [19] and the lower cut-off frequency is inversely proportional to the length of the ground plane.

2. Discrepancy reduces with increasing frequency

280 Ultra Wideband – Current Status and Future Trends

*Ant 5 30×50 1.73 0.83 23 0.3 12 11* 

**Table 1.** Ground-plane dimensions of antennas (unit: mm)

are listed in Table 1.

and (b) bottom view.

*3.2.2. Results and discussions* 

The tapered microstrip feed line and the gap between the radiator and the ground plane are important factors for impedance matching and so are optimized for maximum impedance bandwidth using computer simulation. The optimized dimensions of these nine antennas

*Antenna gl×gw w1 w2 l1 gap r1 r2 Antenna gl×gw w1 w2 l1 gap r1 r2 Ant 1 15×30 1.73 0.9 8 0.3 12 11 Ant 6 30×80 1.73 0.83 23 0.3 12 11 Ant 2 15×50 1.73 0.76 5 0.2 12 11 Ant 7 50×30 1.73 0.94 44 0.3 12 11 Ant 3 15×80 1.73 0.72 3 0.3 12 11 Ant 8 50×50 1.73 0.69 43 0.3 12 11 Ant 4 30×30 1.73 0.9 23 0.3 12 11 Ant 9 50×80 1.73 0.72 38 0.3 12 11* 

**Figure 7.** Prototypes of nine planar monopole antennas with different ground-plane sizes: (a) top view

(a) (b)

The performances of the nine antennas, in terms of S11 and efficiency, are studied by computer simulation. In simulation, no feeding cable is used and the antennas are fed directly by the signal source. Using the optimized dimensions in Table 1, the nine antennas are also prototyped using the Rogers substrate, RO4350, as shown in Figure 7, and measured using the antenna measurement system, Satimo Starlab, shown in Figure 8. In measurements, of course, a feeding cable with an SMA connector (provided by Satimo) is used to connect the antennas to the Starlab system. The cable is enclosed by an EMI suppressant tube to absorb EM radiation. The simulated and measured S11 and efficiencies of the antennas are shown in Figure 9. It can be seen that the simulated and measured impedance bandwidths (S11<-10 dB) for all antennas show good agreements. However, for Figure 9 shows that the discrepancy is larger at lower frequencies and smaller at higher frequencies. This phenomenon can also be observed in Table 2 which shows the discrepancy is always smallest in the higher sub-band and largest in the lower sub-band. This is because at higher frequencies, the ground plane becomes electrically larger.

3. Discrepancy reduces with increasing ground-plane width and ground-plane length.

Table 2 shows the discrepancy reduces with increasing ground-plane width and groundplane length.

4. The width of ground plane has more effect on the efficiency than the length.

Each row in Table 2 represents the ground planes of the same widths but different lengths. Table 2 shows the average discrepancy through 2-12 GHz decreases significantly with increasing ground-plane length. Each column in Table 2 represents the ground planes of the same length but different widths and the results show the discrepancy does not change much. This can also be seen in Figure 9 by comparing the simulated and measured efficiencies of the corresponding antennas. These results show that increasing the groundplane width has more effect on reducing the discrepancy than increasing the ground-plane

length. This is because a small ground plane serves as a radiator and the width of the radiator improves the impedance bandwidth.

Cable Effects on Measuring Small Planar UWB Monopole Antennas 283

(f) Ant 6: 30×80 mm2

(h) Ant 8: 50×50 mm2

**Figure 9.** Simulated and measured S11 and efficiencies of nine antennas with different ground-plane

(i) Ant 9: 50×80 mm2

sizes *gl*×*gw*

(e) Ant 5: 30×50 mm2

(g) Ant 7: 50×30 mm2

**Figure 8.** Antenna in Starlab system for measurement

radiator improves the impedance bandwidth.

**Figure 8.** Antenna in Starlab system for measurement

length. This is because a small ground plane serves as a radiator and the width of the

(a) Ant 1: 15×30 mm2 (b) Ant 2: 15×50 mm2

(c) Ant 3: 15×80 mmc2 (d) Ant 4: 30×30 mm2

**Figure 9.** Simulated and measured S11 and efficiencies of nine antennas with different ground-plane sizes *gl*×*gw*


Cable Effects on Measuring Small Planar UWB Monopole Antennas 285

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

*Rin*

*Rout <sup>r</sup>*

**Figure 11.** Cross sections of cables 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

(a) Cable A (b) Cable B

(a) Cable A

(b) Cable B

*Rout*

Conductor

Dielectric

*Rin*

*d*

*r*

EMI Suppressant tubing

**4.2. Antennas used for studies** 

Conductor

Dielectric

to model the SMA connector.

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