Jabiru Aircraft
Pty Ltd
Installation Manual
Jabiru 3300 Aircraft Engine
REVISION
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Dated : 1
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Dec 2016
Page: 23 of 56
4.9
Radio Frequency (RF) Noise Reduction
RF noise is a common problem with aircraft. Symptoms include:
i.
Radio squelch setting needs to be high
ii.
Excess noise in the background during transmissions
iii.
Squeals or other feedback noises heard during transmission
iv.
Intermittent static or noise breaking through the squelch.
RF noise is a complex problem and is influenced by many different factors. The following points do not
contain everything there is to know about RF noise, but they are given as recommendations of general good
practice to minimise it’s effect.
Ensure all connections, particularly engine earths, are clean and un-corroded.
If the aircraft has a metallic firewall it can be used as a shield to block the majority of RF noise. To be most
effective any wire that passes through the firewall should be fitted with a Ferrite Bead (also known as a
Suppressor or RF Suppressor). Bundles of wires can have a single large Suppressor fitted rather than a
Suppressor for each wire. The wiring diagram in Figure 26 shows suppressors in schematic form. These
suppressors are readily available at local electronics stores.
A Noise Fil
ter can be fitted to the radio’s power supply. Again, these filters are readily available from local
electronics stores. The manufacturer’s instructions must be followed for installation.
Cables passing through the firewall (such as throttle cables, choke, carburettor heat and cabin heat cables)
can transmit RF noise back into the cabin. This can be minimised by earthing the cables at ONE end. On
the Jabiru Engine an earth wire (Shown in Figure 27) is provided connecting the carburettor to the rest of the
engine, so the throttle and choke cables are connected to earth through this wire.
It is normal & unavoidable that the
engine’s ignition system produces some RF noise. This can be
minimised by:
i.
Ensuring all spark plug gaps are set properly.
ii.
Ensure ignition coil gaps are set properly
iii.
Ensure all high-tension leads (Spark plug leads) are firmly fitted at both ends
– to the spark plug and
to the distributor. In addition, the lead from each ignition coil to the distributor must be firmly fitted to
the distributor.
iv.
Ensure Distributor caps and rotors are in good condition.
To counteract RF noise, Jabiru Aircraft run shielded wiring on all radio and intercom wiring. In our
experience, the “Earth Return” method of shielding (where the shield for the wire is also used to form the
earth connection) does not work as well as the “Faraday Cage” (where the shield is a shield only – it is not a
part of the circuit) method of shielding
“Earth Loops” – where a wire is connected to earth at both ends – can introduce RF noise into the system.
All shields should be connected to the aircraft’s earth system at one end only.
The cable used for the Antenna should be high quality, such as RG400 (Shown in Figure 25). This cable
has a double layer of shielding and better RF insulation than other cable types. Note that the coaxial cable
included in most antenna kits tends to have a single layer of insulation. BNC connectors are recommended
for most applications, and wherever possible crimped connectors which require a special crimper to
assemble should be used. Crimped connectors are much less prone to RF leakage or assembly issues
than other types (such as screw-together BNC connectors).
Wires and antenna cables must be routed carefully. Bending or coiling Co-axial cable (such as is used for
antennas)
sharply will significantly degrade the cable’s RF shielding and must be avoided wherever possible.
Coiling antenna cables or any wire carrying current (sensor wires carry very low current so are generally
exempt from this requirement) into loops can induce RF noise in other systems. GPS antennas in particular
are powered
– both the antenna and any excess antenna cable must be positioned carefully, as far away
from the radios, antennas and intercom as possible.
While not a part of the engine installation, strobes can produce significant RF noise. Most brands of strobes
require that the box containing the strobe head unit electronics is earthed, and this is essential to minimise
noise. The cables used for the strobe lights themselves must be shielded and the shield must be earthed
properly, at ONE end only. The Box containing the strobe electronics can also be installed on the engine
side of the fir
ewall to further reduce RF noise. The strobe unit’s manufacturer normally provides good
instructions for minimising their effect on radio noises.
