A Tech’s Guide to Component Design Factors in AM RF Gear
One in a
series of occasional articles to help AM radio engineers and owners enhance
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.”
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.
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!”
techs, AM RF component labels may be a mystery.
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.
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.
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
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.
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