# A New Observation in Spiral and Curl Antenna Configurations above the Ground Plane
## 3 Curl Antenna
### Electromagnetic Coupling Feed  
Impedance matching is performed with an electromagnetic coupling feed system. Figure 6 shows a curl antenna electromagnetically coupled to an inverted \(L\)-wire of radius \(\rho\). The vertical and horizontal lengths of the \(L\)-wire are designated as \(L_{\text{V}}\) and \(L_{\text{H}}\), respectively. The lower end of the vertical part, point \(o\), is excited by a coaxial line. The horizontal part of the \(L\)-wire is just under the horizontal part (\(o^{\prime}\)-\(a\)) of the curl antenna, without physical contact. Figure 5: Curl antenna with a direct feed: (**a**) perspective and top views; (**b**) side view. We realize impedance matching for a low-profile curl antenna, holding the antenna height at \(h=0.10\lambda_{0}\) with \(C\), \(d\), \(L_{\text{f}}=1.09\lambda_{0}\), \(0.007\lambda_{0}\), \(0.25\lambda_{0}\), respectively, revealed in Figure 2. The lengths (\(L_{\text{V}}\), \(L_{\text{H}}\)) are optimized at a test frequency \(f_{0}\) so that the VSWR is as small as possible. The VSWR relative to a 50-\(\Omega\) coaxial line is evaluated to be 1.7 for, respectively, \(L_{\text{V}}\), \(L_{\text{H}}=0.067\lambda_{0}\), \(0.134\lambda_{0}\). So far, we have revealed the input impedance at a test frequency of \(f_{0}\). We next investigate the frequency responses of the antenna characteristics. The solid line in Figure 7a shows the VSWR relative to a 50-\(\Omega\) coaxial line. The VSWR for the curl with the direct feed, discussed in Section 3.1, is also presented with a dotted line. The VSWR for the electromagnetic coupling feed shows a remarkable improvement. The experimental results confirm this. For the experiment, we fabricate an antenna at \(f_{0}=3\) GHz using a large conducting plane of \(10\lambda_{0}\times 10\lambda_{0}\) to approximate the theoretical ground plane of an infinite extent. The prototype's photographs are shown in Figure 8. The VSWR is measured using a vector network analyzer (Anritsu MS46322A, Advanced Test Equipment Corporation, San Diego, CA, USA). The solid lines in Figure 7b show the frequency responses of the axial ratio and gain. The axial ratios are less than 3 dB in a frequency range of \(0.980f_{0}\) to \(1.007f_{0}\) (3% CP wave bandwidth), within which the gain is almost constant (9 dBi). This CP wave bandwidth is the same as that for the direct feed (see the dotted line for the direct feed). The experimental results are also presented in this figure. Good agreement is seen to exist between the  
Figure 6: Curl antenna with an electromagnetic coupling feed: (**a**) perspective and top views; (**b**) side view. theoretical and experimental results. The axial ratio and gain are measured in an anechoic chamber using broadband (Q-par Angus) and standard gain horn (Narda 644) antennas. We use the vector network analyzer as a transmitter and receiver to determine the radiation phase for evaluating the axial ratio. Note that the antenna does not have a conventional lossy dielectric substrate. Therefore, the radiation efficiency is almost 100% (up to 12 GHz with negligible conductor losses [18]). Figure 9 shows the radiation patterns when the axial ratio becomes minimal at \(f=0.993f_{0}\). The radiation field is decomposed into right- and left-handed CP waves with  
Figure 8: Photographs of a prototype: (**a**) perspective view; (**b**) side view. Figure 7: Frequency responses of a curl antenna: (**a**) VSWR; (**b**) axial ratio and gain. \(E_{\text{R}}\) and \(E_{\text{L}}\) intensities. It is found that a broad CP beam is radiated in the direction normal to the antenna plane. The half-power beamwidths are 71\({}^{\circ}\) and 70\({}^{\circ}\) in the \(\phi\) = 0\({}^{\circ}\) and 90\({}^{\circ}\) planes, respectively. The axial ratios in the half-power beamwidths are less than 2.5 dB. This figure also shows the experimental results, which agree with the theoretical results. The radiation patterns are measured in the anechoic chamber using the broadband horn antenna at a distance of 3 m from the fabricated antenna.