From the Whelen Install and Service Manual
R.F.I. and E.M.I. Problems (Radio Noise)
Whelen Engineering strobe light power supplies are designed with a
low pass filter built in to keep R.F.I. and E.M.I. down to a minimum,
however, sometimes there will be interference in the radios by the
strobe light system. Most always, this is an installation problem, not a
strobe light power supply problem.
The power supply should acquire its power from a low impedance
source, such as the alternator or generator end of the electrical buss.
Historically, the rotating beacon or strobe light circuit breaker is added
on the electrical buss at the opposite end, with the radio in between the
strobe breaker and the low impedance end of the electrical buss. Any
noise generated by the power supply will be transmitted into the radio
through the A+ input lead to the radio. Most of the new radio equipment
manufactured today has inadequate input filtering, and any noise
on the electrical buss is amplified in the radio and produced through
the speaker and/or head phones loud and clear.
Two things can be done to alleviate the problem:
1. Connect the strobe light circuit breaker to the low impedance end
of the electrical buss, using a 16 gauge jumper, as close to the
battery as possible.
2. Install additional filtering in the radio A+ line, or provide an isolated
A+ source for the radios by installing a filter choke in series
with the radio input power lead and a filter adapter to ground and
reference all radios to their filter. This will also improve the radio
system from other line noises. REF 2200 mfd@50V Capacitor.
Frequently the noise is not on the A+ lead but is conducted through the
ground circuit. Alternator, electrical motor, fuel pumps and strobe light
power supplies draw heavy current through the ground circuit of the
aircraft’s frame. Any voltage drop in the ground circuit between the
battery ground and the radio ground can look like a signal to the radios.
When the speaker, head phone and microphone use the aircraft’s
ground for return to the radios, one will always experience some interference.
The amount of interference depends upon how much potential
difference there is between the two ground points. Isolating the audio
grounds from the airplane ground at the speaker, head phone and
microphone junctions, and grounding the aforementioned with the
radio at one central ground point, will eliminate the majority of all
ground inducted radio noise.
Do not parallel any audio leads with any power lead supplying energy
to a noise generator; (i.e.) alternator, electric motor or DC choppers
such as inverters and strobe light power supplies.
The interconnecting cable between the power supply and the remote
strobe lighthead assembly radiate very little, for the output circuit of the
power supply is very low impedance. They can radiate RF like an
antenna if the shield is not terminated to ground. The radiation of RF
energy is reduced to a minimum by properly terminating the shield at
one end or the other, generally the power supply end, but which ever
proves the quietest ground. Do not terminate both ends.
When installing a strobe light system, provide a good ground and a low
impedance source to the strobe light power supply. Eliminate ground
loops in audio circuits by using a centrally located ground point for all
audio grounds.
Whelen Engineering has available RF shielded flash tubes and strobe
lighthead assemblies to suppress the trigger pulse or clicking sometimes
heard in the radios.
If noise problems persist, and the procedures described have not
cleared them up, please contact the Whelen Engineering Company for
assistance.
