Roof-mounted vehicle single-arm spiral antenna without a balun circuit

The deterioration in the antenna characteristics that are observed when a single-arm spiral antenna is located in front of a shallow conducting cavity can be mitigated by using an absorbing strip (ABS), which is placed behind the outer arm filaments. The use of lumped resistor elements connected to the arm end results in wideband antenna characteristics similar to those of the spiral with the ABS. A comparison between the spiral with the ABS and with the resistors shows that the former provides antenna characteristics in the lower frequency region that are more stable than the characteristics of the latter.