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Schadler: “A Power Increase to -10 dBc IBOC on Multichannel Systems Has Substantial Impact”

Schadler will discuss how a power increase to -10 dBc IBOC on multi-channel systems has a substantial impact on the design of every component

You�re speaking at NAB about -10 dB IBOC at combined transmission sites. What�s the headline, what is the main conclusion of your paper or why is this topic so important to radio engineers?

A power increase to -10 dBc IBOC on multi-channel systems has a substantial impact on the design of every component, making certain they can handle the instantaneous voltage peaks. It is important to know �how voltage peaks for combined stations are calculated and how RF component and antenna voltage and power handling is derived. During the design phase, practical safety factors necessary to ensure reliable service must be taken into account for the RF system and transmission line as well as the antenna.

According to the summary of your paper, the potential power peaks from multiple FM stations running -10 dB can co-phase and create voltages large enough to cause voltage breakdowns. Why are the implications of that and what can radio engineers do about it?

If the combined voltage applied to a component is high enough, an RF breakdown or arc may result. An RF arc has the potential to severely damage RF components leading to costly down time and repairs for the broadcaster.

Briefly, what kind of safety factors can be employed for antennas and combiners?

Safety factors must be placed on the voltage breakdown and rated power level to ensure that environmental stresses and tolerances cannot stack up and cause failure. Not all components in the RF transmission system require the same safety factors. Since large portions of the radiating elements of antennas are typically exposed to weather, they require the highest safety factors whereas transmission lines and RF systems will only require a subset of the safety factors. Safety factors can be grouped into four categories; pressure, medium, geometrical configuration and unknown effects. After assigning appropriate safety factors within each group that are applicable to the component in question, a composite resultant safety factor (the multiplication of each factor from each group) is what needs to be used at the design stage. In addition to safety factors, correction factors for VSWR need also be applied to the breakdown calculations.

Obviously many of the details in your presentation are beyond the scope of this interview. But are there other main points or findings that readers should be aware of?

Currently, voltage breakdown is one of the major limitations in high power multi-channel FM-IBOC transmission system design. Due to the fact that the pulse lengths of FM-IBOC are much greater than the critical pulse length defining the CW condition, the OFDM carriers must be treated as CW. In doing so, the probability of co-phased voltage addition of multiple stations can be calculated as well as the number of probable events for a given PAPR. This statistical approach has one major drawback. It does not guarantee that an over voltage will not occur. If 100 years is specified for the average occurrence, there is no guarantee that the breakdown event will not occur in the first 5 minutes or two or three events will occur within 100 years. The odds are very small, but they are simply that, odds.

Can you supply a sample graphic from your presentation with a caption?

An increase -10 dB IBOC provides a challenge not only to the transmitter manufactures but to the RF component and antenna manufactures as well.

Anything else engineering readers should know?

The method of calculating peak powers for multi-station FM installations is not standardized among broadcast equipment manufactures. The method I recommend in determining the peak voltage and power rating of RF systems, transmission lines and antennas is to perform a DC Hi-Pot (High Potential) test on each component and relate the breakdown level to an RF condition mathematically.