Model VL-2901-D496 is a small Amateur Dual Band, VHF/UHF Antenna (145-148 MHz, 2m and 420-450 MHz, 70cm Amateur Radio Bands), designed for mobile platforms (vehicles) with height restrictions. Measuring only approximately 4" (10 cm) in diameter and 6" (15.3 cm) tall, this antenna is a very efficient Low Profile radiator. When installed properly on the metal chassis of a vehicle, the VL-2901-D496 will perform comparably with a full size 1/4 wave monopole antenna. For optimum operation, install near the center of a flat surface, such as the roof of a truck's cab or van with minimum approximately flat dimensions of 40" x 40" (1m x 1m). We do not recommend the use of a magnetic mount or a trunk bracket mount.
The VL-2901-D496 Amateur VHF Antenna features a DC shunt connection to the chassis to help protect its attached radio from limited Electro-Static Discharge (you will measure a short circuit when measuring across its terminals with a multimeter).
An electrically conductive chassis/ground plane is required for the proper operation of the VL-2901-D496 Antenna. If the intended mounting surface is non-conductive, such as a fiberglass roof, line the underside of the non-conducting surface with a thin foil of any convenient metal (Aluminum is widely used). Ensure that the NMO mount contacts the chassis/ground plane/foil securely.