The Evolution of Master FM (multi-channel) Designs at Dielectric

What makes Dielectric unique when designing a Master FM system? 

Traditionally, most manufacturers design master FM antennas and the associated RF system separately. The RF combiner and FM Antenna are designed independently by their respective proposal teams. When taking this silo approach, you miss the opportunity to fully maximize the system and take advantage of new developments in technology into account, which yields higher safety margins, reliability, and longevity.

Technology has also changed on the broadcaster side of things with the introduction of IBOC (In-band on-channel) over a decade ago and then more recently the increase in IBOC levels. IBOC has forced a more rigorous design process due to peak powers. It is inherent when designing the system that the manufacturer understands the limitations of each component and takes care not undersize any component.

When taking the Team approach to the design and using new technology, noted further in this paper, Dielectric’s System Design enhances all components. In addition to the Team approach at the proposal stage, we find it equally important for the design team to be in sync with the manufacturing team to ensure an optimized final product. Dielectric has over 1,000 years of experience between engineering and manufacturing, allowing for the maximization of long term reliability when we are ready to ship the Master FM system.  One example of our design approach is demonstrated in the following master FM antenna case study of a 2018 installation in Peru.

The end user requested an eight-station master FM system. They had numerous antennas and towers and wanted to consolidate all stations on top of a mountain in Lima. 

When the request came in, the engineering teams sat down and started to brainstorm.  The traditional RF System would have been an eight-station CIF combiner chain. The issue with this approach is the stations furthest away from the antenna on the chain suffer in many characteristics (higher losses, group delay, etc…) that are critical. In addition, when doing an eight-station combiner chain, the components in the combiner chain towards the antenna see substantial peak powers, forcing the components to be oversized to account for the peak powers.

Finally, taking the traditional design approach for the antenna yields peak power and average power limitations in the feed system.

Our goal was to achieve maximum voltage safety margins and not run to the limit on average power. The output for the design was unique. Dielectric split the combiner into two four-station manifold designs. By doing so, all stations in the chain did not suffer the traditional loss characteristics and group delay characteristics that the CIF combiner would have resulted in. No matter where the station is in the chain, it’s equal to all other stations.

Dielectric was able to achieve this because the Master FM antenna had dual input, one radiating right hand circular and the other left hand circular. The RF design requires two line runs up the tower and two feed systems on the antenna. One can see quickly that this approach maximizes the peak and average powers in the system, reducing the load and increasing the long-term reliability. The eight stations are combined using a hybrid at the antenna elements. This waveguide hybrid is designed for high peak and average power levels and is the only component in the system that sees all eight stations. The traditional approach to this solution would have been a challenge as all the peak and average powers would have been in each component, driving up costs and complexity.

For more information, go to