Fig. 2 Measured return loss for the proposed antenna
ing line (Le) and the height of the matching line (He) are chosen to be 1.5 mm, 2.5 mm for good impedance match, respectively.
III. EXPERIMENTAL RESULTS
Fig. 2 shows the measured return loss. A very wide impedance bandwidth of about 700 MHz centered at about 1.575 GHz is observed, which is due to the wide bandwidth characteristic of the feed network.
In QHA, reducing the size of the antenna causes a decrement of the input impedance of each helix since the mutual coupling between helices is increased. If the input impedance is decreased, the radiation efficiency is decreased and the impedance mismatch is generated between each helix and feed network. Hence, the folded inverted-F antenna as a helix is adopted to increase the input impedance. When the shorted line is added to the helix, the inductance of the shorted line is connected to the original impedance of the helix in parallel. Therefore, the input impedance is increased when the length of the shorted line is increased. In Fig. 3(a), we present the real part of the measured input impedance and the corresponding imaginary part in Fig. 3(b). Without the shorted
line, the When the
input impedance height (He) of the
shorted line is increased, both real
and imaginary part of input shown in Fig. 3. The corresponding to the value of
is also increased as
He is given
. For good
impedance matching between the input output of a feed network, we choose the line as 2.5 mm in the proposed antenna.
of each helix and the height (He) of shorted
The measured peak gain is about -2.5 dBic with a radiation efficiency of 30% at center frequency of 1.575 GHz as shown in Fig. 4. Also, since the frequency band of GPS L1 (1563.42~1587.42) is located in 3-dB gain bandwidth, the proposed antenna provides stable gain properties. The 3-dB gain bandwidth is about 100 MHz.
Fig. 3 Measured input impedance of each helix (a) real part (b) imaginary part
TABLE MEASURED INPUT IMPEDANCE
Measured input impedance (Ω)
0 1.5 2.5 3.5 4.5
9.27 – j37.33 73.91 + j5.90 93.59 + j8.54 208.04 + j70.12 327.03 + j126.74
The measured axial ratio is shown in Fig. 4. The proposed antenna has a good CP performance because the axial ratio is under 0.5 dB at all frequency in GPS L1 band. The axial ratio radiation pattern is also measured at 1.575 GHz in Fig. 5. Because the proposed antenna radiates a good CP waves through forward direction and 3-dB axial ratio beamwidth is about 140°, the antenna rejects unwanted cross polarized and multipath signals effectively.