Spiral antennas are traveling wave structures and are well-known for their wideband performance.  A bandwidth of 5:1 or 10:1 is easily obtained and a stable input impedance is achieved through a self-complementary geometry.  This wideband characteristic of the spiral antenna makes it an attractive choice where a single antenna is required to send / receive over multiple channels.  In [1] the application of spiral antennas in mobile communication and navigation is investigated.

The antenna under consideration is a two-arm self-complementary archimedean spiral antenna and has a diameter of 250mm.  Figure 1 shows the simulation model and the current distribution on the structure at 250MHz.  The antenna is fed in the center where the two arms connect.

Figure 1:  Current distribution on spiral antenna at 250MHz

Figure 2(a) shows the input impedance of the antenna across a wide frequency band and Figure 2(b) the reflection coefficient of the antenna relative to a system impedance of 200Ohm.  The impedance bandwidth for the antenna is observed to be greater than 8:1.

Figure 2:  Impedance characteristics of spiral antenna
(a) Input impedance  (b) Reflection relative to 200Ohm

Another consideration is the wideband behaviour of the antenna gain and radiation pattern.  Figure 3(a) shows the axial gain of the spiral antenna across a wide frequency band and Figure 3(b) the radiation pattern at a few selected frequency points within this band.  Antenna gain and radiation patters seem stable across a bandwidth of about 4:1.

Figure 3:  Radiation characteristics of spiral antenna
(a) Axial gain (theta = 0)  (b) Radiation pattern

References