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j : 14 January 2006 • 10:00AM -0500

Re: JabiruEngine-List: Jabiru alternator noise
by Ray Montagne


--> JabiruEngine-List message posted by: Ray Montagne <r_montagne@mac....>

On 1/13/06 5:42 PM, "Tom Tomlin" <ThomasTomlin@comc...> wrote:

Hi Tom,

My comments below:

> --> JabiruEngine-List message posted by: "Tom Tomlin"
> <ThomasTomlin@comc...>
> Okay, I'll start a thread here.
> I also elected to not be on the yahoo group so here goes.
> I have a Kitfox Speedster with the Jabiru 3300 and a new Sensenich ground
> adjustable prop.
> Currently I am hearing what appears to be alternator noise coming through my
> headset and
> it is also being transmitted when I key the mike.
> I have a capacitor on the output from the alternator and recently went to a
> newer one from
> the Aeroelectrics supplier.
> The noise is a high pitched whine at cruise rpm and the pitch changes up or
> down
> with the throttle setting.  It is also only heard when pressing the PTT
> switch.

A high pitch wine that tracks RPM is indicative of alternator/generator
noise.  When you key the transmitter you are increasing the load on the
electrical system.

A simple capacitor may not be adequate.  An 'L' network filter may be
required.  The 'L' is constructed by having an inductor between the
electrical source (alternator/generator) and the load (radios/avionics) in
combination with the capacitor between ground and the load end of the
inductor.  The purpose of the inductor is to provide a high impedance (i.e.
Resistance) to high frequencies (i.e. The A.C. Noise being carried on your
D.C. System in this instance) while the capacitor presents a low impedance
to the high frequency noise.  In essence this is a voltage divider where the
inductor has very low resistance at D.C. While the capacitor has high
(infinite) resistance at D.C. While the noise sees the high resistance of
the inductor at A.C. And low resistance of the capacitor.  The result is
that the D.C. Passes through the 'L' network with little or now attenuation
while the A.C. Passes through the 'L' network with a great deal of

If you only have a capacitor installed then the inductor is presenting very
low impedance/resistance at both D.C. And A.C.  This results in very little
attenuation of the A.C. Component.

'L' networks are sold for a variety of applications ranging from car stereos
to avionics/amateur radio equipment, etc.  The trick is getting an 'L'
network that is appropriately designed for the electrical load as you don't
want the inductor to generate a voltage drop under D.C. Load (which may
create a whole different set of issues).

I'd recommend installation of an 'L' network and checking that the ground
strapping is of good quality and sufficiently large for the task.

Best regards, Ray Montagne
Cupertino, CA

> I have an alternator cut off switch and can turn off the alternator which
> eliminates the noise
> when transmitting.
> My headphone, PTT, and battery cables run parallel for about 3 feed, as my
> battery and headphone
> jacks are behind me
> I was hoping for an easier fix than rerouting the headphone/PTT wiring from
> behind me, and back up
> to the panel.
> Any thoughts?
> Tom Tomlin
> Kitfox IV
> Greeley Colorado

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