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A Tech’s Guide to Component Design Factors in AM RF Gear

Lawrence Behr offers ideas that have worked reliably in transmitter and antenna systems for years

One in a series of occasional articles to help AM radio engineers and owners enhance their operations.

Broadcast RF technicians should know some approximate design factors in selecting components for AM RF systems. This is helpful when selecting a component to replace one blasted by lightning, or to assess why one is running “hot.”

Lawrence Behr

There are no standards in this area, but here are some general thoughts that condense known industry practice and have worked reliably in our transmitter and antenna systems for years.

Actual equipment designs may use safety margins different from those I set out here. An RF design engineer must take into consideration many factors of intended use, installation and environment. As they say, “Results may vary!”

RMS carrier

To many techs, AM RF component labels may be a mystery.

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.

As a general matter, RF capacitors in mica, be they “G series” ceramic case, or “29x series” plastic molded, are interchangeable, G-1 for 291, etc. Similarly, vacuum capacitors with the same rating are interchangeable in glass or ceramic, but may be very different physically.

See the accompanying charts, at bottom.

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 are often applied, as a matter of engineering judgment. On the other hand, lower safety factors may be justified by a designer where special considerations of component cooling, placement, or operation prevail.

If you are uncertain about parameters, increasing the above values 10–15% 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. This is important to remember if you are commissioning new RF equipment and are uncertain of the design information.

More AM stations are sharing towers these days. 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.

While there is little difference between component ratings in IBOC and straight AM systems, this is not the case for DRM digital systems. While not a factor in the United States at this time, readers elsewhere are recommended to get good engineering advice before modifying such systems, or any high-power AM.

Lawrence Behr is founder of broadcast consulting firm Lawrence Behr Associates and RF equipment manufacturer LBA Technology, headquartered in Greenville, N.C. A former radio and TV station owner, Behr was a founding member of the Society of Broadcast Engineers; he is a Radio Club of America Fellow and an active ham radio operator, K4JRZ. Reach him at [email protected].