Vacuum and Mica Capacitor Inductor Ratings – Application Note 11

Component Design Factors in Medium Wave Broadcasting

Technical personnel need to know appropriate design factors to apply in selecting components for broadcast RF systems. There are no standards in this area, but LBA has determined a set of internal guidance principles that condense known industry practice and have worked reliably in our products for years.

All our antenna system component computations are based on RMS Carrier voltage and current. These computations are adjusted for modulation peaks and related to the specific labeling system used on mica and vacuum capacitors, which are different! A mica capacitor (292, G3, etc.) with a voltage rating of 20 KV is roughly equal to a rating of 35 KV in a vacuum capacitor. Current ratings do not require such an adjustment.

Bear in mind that these ratings are based on safety factors appropriate to a design based on "solid" parameters. In the absence of design limits, if parameters (i.e. - antenna impedance, etc.) are estimated or uncertain, higher design factors must be applied, which are a matter of engineering judgment. On the other hand, lower design factors may be justified where special considerations of component cooling, placement or operation prevail.

Our preferred practice is to use the above factors at the worst-case design operating values representing the limits of the frequencies and impedances for which the equipment is designed. This practice usually results in even more conservative design factors near the center of the designed range. However, in simple low power systems, increasing the above values 10 – 15% and applying that to the estimated antenna impedance may be adequate to accommodate variations encountered in the field

When systems are designed based upon estimated data, it is critical to verify the adequacy of component selection by measurement or computations based on measured parameters before applying rated system power!

Bear in mind that components shared with multiple frequencies, such as in filters for diplexers or triplexers, have a special set of rules. Basically, voltages add vectorially, and currents add on an RSS basis. Evaluation of these circuits is best left to an experienced RF design engineer.