Bob,
I'm unsure of the legality, or even wisdom of this, but what I did on my
microlight was to sheath the HT leads in copper braid, (stripped from coax)
and terminate this - at the spark plug end on the (still available) metal
caps Rotax supplied at the time, and at the other end on the die cast case
housing the electronic ignition system and coils. This should be possible
no matter what the wiring, of the Jabiru ignition system is. It's important
to make a good seal and to ground the metal case to the engine.
The right course of action is to try to identify the noise at source, rather
than supress it's reception. Does the noise frequency have any component
which goes up and down with engine speed? If so then it's likely to be the
alternator or ignition system. If it's independent, try elswhere - GPS for
example.
BTW Tony's telling porkies - I'm not an expert. Your best bet may be to
contact a radio ham. Have you checked the antenna's good? If it isn't, as
has already been said, you can expect bad signal to noise ratio (lots of
interference) on receive.
In answer to your question "Is there a one hit place". Often no. Your
interference could be coming down a line or being radiated. since you've
attacked the supply line, next best bet is the antenna and its lead I'd say.
Dave
----- Original Message -----
From: Bob Harrison <ptag.dev@ukonline.co.uk>
Subject: Re: Bundling of coax cables
> Hi! Dave.
> I guess you were expecting a message from me!
> Bearing in mind I'm "out of my depth" ! I have installed Magnacore HT
leads
> but of course they can't be "grounded" unless I get an aircraft engine
> grounded spark plug system, which due to the
> Jabiru Ignition Coil system would be impossible.
> It seems to me that I need to establish what the damn noise is from first
> ..... how? Bill Bell has heard it but can't identify it , although he did
> say it varied as he walked round the aircraft with the engine running,
> that's where I started applying nickle based paint.
> I have tried a large automotive suppressor in the com power supply line,
to
> no effect, other than increasing my MTOW !
> If I can't identify the noise does the logic below need applying to all
> circuits in turn until
> the noise stops OR is there a single application point anywhere possible
in
> connection with, say, the bus bars?(Not that I understand your epistle!)
> Regards
> Bob H G-PTAG
>
> -----Original Message-----
> From: forum-owner@europaclub.org.uk
> Subject: Re: Bundling of coax cables
>
>
> Tony,
>
> You clearly know what you're talking about and I can't fault your logic
and
> practical advice. I also suspect that most builders are concerned about
the
> right problem ie. radiated RF noise. However there is another problem
which
> you've left out, possibly to keep things simple, that is impedance.
>
> Some consumers draw current interrupted at high frequencies (commutating
DC
> motors, transponders, switches, electronic ignition systems) and these
> generate significant levels of RF in their supplies' return lines. The
> RESISTANCE of the line back to ground may be low, but the IMPEDANCE of
only
> a few microhenries at umpteen MHz, becomes large - and it's easy to have
> lots of microhenries in even a short line in the plane. This gives rise
to
> RF noise on the consumer's local ground, although the grounded point at
the
> busbar is quiet. When the consumer's local ground is also used as a
> reference ground for, say, a microphone input, then this noise is
introduced
> on to the signal (we call this series mode noise). On most good quality
> avionics, the designer will connect two types of capacitor across the
supply
> at the consumer's power input - one large electrolytic, whose size is
> limited only by the surge current capabilities of the supply's fuse, and a
> second, say 0.1 microfarad ceramic or similar, with good RF performance.
> (this is because large electrolytic capacitors usually possess quite high
> self inductances making them less effective for RF smoothing for the same
> reasons I've tried to explain above). This normally does the trick.
However
> all avionics are not good quality and in some cases (eg. the old Narco
> handhelds I know for certain), don't have this feature. There may be
other
> handhelds suffering from the same deficiency since they are all short of
> space inside.
>
> The solution is fortunately quite simple - connect a 50 microfarad
capacitor
> the right way round, in parallel with a 0.1 microfarad capacitor, to the
> consumer's supply, right at its power terminals.
>
> I had a similar interference problem on a two stroke Rotax powered
> microlight some years ago - couldn't receive anything without the squelch
> turned down, then it was unintelligible. I tried all sorts, including
> antenna re-wiring and re-siting but to no avail. In the end I screened
the
> HT leads, screen grounded at BOTH ends, and enclosed the ignition coils in
a
> die cast box electrically bonded to the crankcase. That got the &$%&
*.
> After that there was no difference between the squelch setting engine off
> and full chat.
>
> And there's another real problem with RF energy - it radiates from leads
> coming out of the box, not just the box itself. I have a wonderful
Lowrance
> Airmap hand held GPS; wonderful that is except for it radiating
interference
> from the antenna lead and antenna as well as the unit. I could screen the
> lead of course, but I can't screen the antenna.
>
> Isn't life a bitch sometimes.
>
> Dave Simpson
>
> ----- Original Message -----
> From: Tony S. Krzyzewski <tonyk@kaon.co.nz>
> Subject: Re: Bundling of coax cables
>
>
> >
> >
> > So a "negative" bus bar, with about a dozen connections, should be
> > thrown and replaced with a single point connection for all the "circuit"
> > return wires ,as well as a single point connection for the "grounding "
> > wires? (Just convince me please that circuit negative wires are the same
> > or different to "ground" wires?) Regards Bob
> >
> > A negative bus bar is a single point ground if all of your
> > ground/negative wires terminate on the bar. If all your wiring
> > terminates on this single bus bar then you have the best form of ground
> > return other than terminating all of the wires at the battery.
> >
> > The key is to have all the circuit return wires terminate at as few
> > (preferably one) point on the aircraft and to have minimal resistance
> > (read big fat wire)to the current source return (battery). This single
> > point can be a bus bar, a bolt or a plate with connectors. The point
> > here is that the resistance between the individual return line
> > termination points should be very, very low and a block of brass bus bar
> > does this very nicely.
> >
> > Circuit negative wires are the same as ground wires which are the same
> > as return wires.
> >
> > On a tin can aircraft the skin and structure effectively can act as a
> > very large single point ground with the return current flowing over the
> > entire skin. As it's a massive area electrically the current flow per sq
> > cm is miniscule. Composite aircraft don't have the luxury of having this
> > cheap return path so we have to build an individual return path for each
> > and every circuit. Unfortunately electricity has the habit of finding
> > the easiest path to ground and, if the returns form multiple paths to
> > grounds and there is a variation in resistance of those return lines,
> > then voltage differences can be induced between the ground points as the
> > current flows between them. Bringing all return lines to a single point
> > and ensuring that the single point has a low resistance to the aircraft
> > true ground point removes the problem.
> >
> > I have built several large computer controlled model railway systems
> > over the years which have amazing parallels to wiring the aircraft.
> > Failure to bring the grounds back to a single point in those systems can
> > induce so much electrical noise that the microelectronics simply fails
> > to work.
> >
> > Tony
> >
>
>
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