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Clive J
Joined: 03 Nov 2007
Posts: 340
Location: UK
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 Posted: Tue Mar 31, 2009 1:42 pm Post subject: Radio Interference |
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Some comments from a guy on the Yahoo engines list who's been helping
folk out.
He's done extensive testing around the RFI that the Jabiru engine emits.
If I find the rest I'll post the link on here.
I did extra screening on my Esqual system and the radio still isn't as
good as my Jabiru.
Both have tail dipoles which should be a far better antenna than a
quarter wave fuselage one. However, I have been told of problems with
structure and even paint possibly masking the antenna in a tail.
Dave, what's a vhf-51?
Regards, Clive
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Yes the bosch leads you found look to be the same as I used. They have a
number marking on them something like 365 however some leads had a
different number. look at the picture in the files, you can read the
markings there. The Bosch premade leads cost $6 each here
New Jabiru engines, on the website, appear to have similar leads fitted,
not the old red carbon ones.
Screening the coil leads can be by braid or spiral winding copper tape
over them. But I dont like the idea as its easy to get breakdown
especially the 3300 where moisture could track via the joint. the same
level of improvement is obtained with 2 ferrite beads to each lead. 3
beads is more than needed, if you want to make sure of it. The best
beads are hard to find now, the ones I got were ideal.
Ralph
--- On Tue, 31/3/09, James, Clive R <clive.james(at)uk.bp.com> wrote:
From: James, Clive R <clive.james(at)uk.bp.com>
Subject: RE: More Radio Noise
To: sanson.r(at)xtra.co.nz
Received: Tuesday, 31 March, 2009, 12:33 AM
Thanks very much for the detailed feedback, I must have missed the
earlier explanation you made I'll have a dig back in Yahoo.
I've just replaced my plug leads on the 2200 plane as it's done 1060 hrs
and I was getting some breakdown. Not expensive as I made them up, but
the price of auto plug leads isn't great so I'll look into changing them
on my 3300 as that is the one with the worst radio.
When you say Bosch ferrite core helical resistance wire are these
standard leads or something different?
I've had a surf and come up with this page
http://www.bosch.com.au/content/language1/html/2253.htm
Doesn't figure on the UK website though.
Screening the coil leads would be by fitting a grounded braid?
Sounds like you've done some great work which it would be really good if
Jabiru picked up on.....
Regards, Clive
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From: Ralph S [mailto:sanson.r(at)xtra.co.nz]
Sent: 29 March 2009 22:51
To: James, Clive R
Subject: Re: More Radio Noise
Hi Clive,
Yes your idea of grounding the shields at the cockpit end does make
sense and I have tried it that way and with both ends grounded. But the
reason I say to only do it at the engine is mainly to avoid any
circulating currents getting to the electronics in the coils when for
instance you are cranking the engine with mags off or flying with one
disabled (because a rotor slipped and causes misfiring on that mag,
whatever. You then have a situation where the electronics in the coil is
subject to the voltage difference between the cabin ground and the
engine ground, with 200 amps starting current that may be 2 volts. Its
just not good practice. I flew my C150 for a couple of years before
discovering it had no earth straps fitted across the rubber engine
mounts (there should be 2) meaning the 400 amps starter current return
to the battery was soley via the throttle, mixture and various scat
hoses wires that connect to the fuselage! No wonder it was a slow
starter.
The jabiru charging regulator does put out some spike noise which can
make a pitter patter on the audio systems, but in my case its not a
problem and I dont have a large capacitor like the Rotax recommend. I do
have a large battery and its direct connected to the regulator both
sense and charge wires no switch contacts to cause trouble.
For eliminating the ignition noise, for my engine the most dominant
noise by a large amount, The best step was to replace the carbon film
leads (cars stopped using these after the 70's) with some Bosch ferrite
core helical resistance wire ones off the rack at our local car parts
store. I have no opportunity to test other brands, but assume Bosch
supply this lead to many european cars so its a matter of buying 8 leads
long enough to cut off the straight plug connector and crimp the angled
connectos on.
2nd step was to bead or shield the coil lead. This is a copper wire
lead. Some have said it can be replaced at the coil but if that was easy
Jabiru would already do it instead of soldering on an extension for the
3300. terminating these ferrite core leads is also harder to do.
putting the r plugs in last change cut my noise downn a little further,
but my static tests suggest the HT leads radiate the most noise. Avoid
adding more than the leads + the R plugs as its already over the normal
system resistance by then.
So other than trying the wiper to ground the flywheel, like someone in
Europe did, I dont think there is much more to do.
I have seen reports of
shielding the cowl with mesh or conductive paint
magnecore HT leads
ferrite beads on HT leads
filters on radio etc.
all not having any useful effect.
I suspect that some of the problem is that any one of those things
alone might not be notcable to the user, without the benefit or testing
in a repeatable environment. I used test equipment to find the best HT
leads and r sparkplug combination, and various radio frequencies with
known reference signals in flight, comparing the unmodified mag noise
with the modified side. The L mag is always slightly noisier than R, so
that has to be taken to account.
If you study the graphs of my files, and flick between them or print
them and overlay the traces, you can see the improvement made by each
type of lead, and the fact that the improvements are different depending
on the frequency being looked at. I have plots over much wider
frequencies, and the inductive leads are best overall, even better than
standard full shield aircraft leads (which are helical wire core, by the
way)
Its my belief that the coils will never be able to be totally
suppressed, and to do better would mean shielding the distributors as
well. but its more important that the engine is reliable. If the antenna
is already in the tail theres no where else to go.
Good luck.
Ralph
--- On Sun, 29/3/09, James, Clive R <clive.james(at)uk.bp...com> wrote:
From: James, Clive R <clive.james(at)uk.bp.com>
Subject: More Radio Noise
To: sanson.r(at)xtra.co.nz
Received: Sunday, 29 March, 2009, 10:25 PM
Hi Ralph, I was reading your post on radio noise with interest.
You suggest grounding the P lead shield at the engine end. I did it the
other way, grounded on the panel earth as I felt a cleaner shield would
be better and bringing the engine ground 'signal' into the plane would
be a backward step.
I did no end of shielding and have big cables on my ground circuits and
still have a ropey radio.
What's your thinking about grounding the P lead shields at the engine
end?
I'm open to any ideas to improve things and I suppose I could be taking
the interference that is in the P lead into the panel.....
BTW, the Antenna is a dipole in the tail which works very well in my
other aircraft (Jabiru).
Regards, Clive
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davemcc
Joined: 28 Feb 2007
Posts: 74
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| Posted: Wed Apr 01, 2009 3:03 pm Post subject: Radio Interference |
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Advanced Aircraft VHF-51 is a no ground flat bondable antenna that I believe
Nick is still using
Dave
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