This information is provided as a guide
for shielding your aircraft for the successful installation and operation
of any AM radio system.
Engines used in ultralight aircraft are
traditionally two stroke 20 to 75 horse-power units which use a magneto to
generate electric current in order to produce a high energy spark for
ignition. This type of ignition is reliable and effective and used on most
aircraft utilizing gasoline-fueled engines.
However, this set-up interferes with radio
broadcasting. The problem with high energy spark ignition is its inherent
characteristic of broadcasting a complete spectrum of radio signals in the
form of static, especially on the AM band.
Years ago, when aircraft communication was
initiated, the only radios available were of the amplitude modulation (AM
type). Since that time, no major changes in aircraft radio design have
Engine ignition noise (static) has always been a
problem for aircraft communication systems, and it remains so today. With
the advent of the all-metal airplane, the problem has been reduced for a
number of reasons.
Most important of these is the complete shrouding
of the engine in a metal cowl with a metal firewall between the engine and
radio installation. But even shrouding the engine in metal does not remove
all of the interference. By shielding the actual source, most unwanted
signals can be suppressed.
High tension wires should be shielded with wire
braid, spark plugs jacketed in metal, and the magnetos housed in a metal
case with all of these grounded to the engine block. As a final measure,
the antenna for the radio should be placed some distance from the engine
and connected to the radio with a shielded coaxial cable which has its
outer braid grounded at each end.