Circularly Polarized FM Antenna for 88-108 MHz
Model: PAFM-SDCPB-L - Low Wind Load Circular FM Antenna
The PAFM-SDCPB-L antenna is circularly polarized, cross dipole antenna covering the 88-108 MHz spectrum. The model "L" of the PAFM-SDCPB family is a strong, robust antenna featuring smaller sized material for those locations requiring reduced wind load values. The antenna is composed of stainless steel and mounts to an offset mast which, in turn, is side-mounted to a tower (mast not included). The radiating dipoles are welded in place and only minor on-site assembly is required.
Many options exist for tuning the PAFM-SDCPB-L depending on the required specifications. The antenna can be tuned for a VSWR of less than 1.10:1 on the operating frequency; less than 1.10:1 for 3 MHz of band width; or less than 1.5:1 across the entire band from 88-108 MHz.
Once a circularly polarized cross dipole antenna is mounted to a rear vertical metallic support mast, the mast provides some signal reflection of the Vpol signal in the horizontal plane, resulting in modest directivity over the dipoles being in free space. Gain of the antenna system can be increased by adding additional antennas and mounting them vertically one on top of another, each layer being call a "bay". A stack of 4 PAFM-SDCPB-L's, for example, is a 4 bay array.
A critical component of the cross-dipole antenna is tuning because once adjacent metal is in close proximity to the antenna, such as a rear metallic support mast, the antenna tunes much different than without the mast due to the masts' passive influence on impedance. Tuning of the PAFM-SDCPB-L for optimum VSWR takes into consideration a rear mounting mast as the primary support structure otherwise final operation of the array would not be optimally tuned for impedance.