Crazy Radio Issue, Help Needed.

[Mike-Bracknell]

If it's a specific rpm-band, and changed when items on the engine were refurbed, could it be emanating from the ECU or something controlled by the ECU that only gets activated at a specific RPM - e.g. manifold bypass or something.

(sorry for the vagueness, I come from the world of cars rather than planes)

[mike-wsm]

Originally Posted by Mike-Bracknell

I come from the world of cars

Er, Bracknell, cars, sounds rather interesting. Looking forward to 13 March I guess? Or 27th?

I think the point about ignition improvements relates to vibration. By improving the ignition and making the engine run smoother the onset of propellor vibration is at a higher rpm. Prop vibration is torsional in nature and is caused by aero engines having no flywheel so producing torque variation as the cylinders take it in turns to go bang. In this airplane the prop vibration band is 2250rpm to 2550rpm. Vibration is affected by prop diameter (mustn't be reduced below 72.5 inches during repairs) and compression ratio (nominally 8.5:1).
.

[good spark]

had a very similar problem to this once on a seneca 3, turned out to be a radio noise filter that was in the wireharness behind the centre panel, btw it illustrates just how directional the shape of the airframe can be, perhaps heli peoples can add a few points here
going back to the seneca we found it after doing a 360 while listening to the noise and at one position hey presto the sound reappeared but inflight it was there all the time, let us know what you find



gs

[FullOppositeRudder]

I'm not sure if I should be doing this, but is this the one we are working with? If so, there is another photo on the WWW which shows the two antennas more clearly. It may help in getting a clearer picture of how the fuselage might actually shield a weaker signal. Two days ago I discounted this possibility. Now I'm not so sure.

Was the aircraft exposed to a nearby lightning event prior to the problem presenting?

Fascinating thread. We need to to solve this one - we really do. Very sweet looking machine BTW

FOR

[hangflyer]

I'll add an update and answer some more questions.

Firstly FullOppositeRudder has correctly identified the aircraft, well done! That photo is a few months old and she is now sporting a new front antenna which is one of the large angled fibreglass units, mounted with 4 bolts, and perfectly bonded.

As far as I know the aircraft was never exposed to any lightening. This problem started for no apparent reason on an ordinary day and due to it's intermittent nature is extremely hard to correlate with anything. We have investigated the possibility of a magnetised crankshaft or something else, but everybody seems to agree that this would always have the same (or similar) effect. It could not start and stop.

During the last few days the problem has been present for about 50% of the flights. On one day it was there from cold first thing in the morning and then mid afternoon it stopped during the peak heat of the day (37'C)
On another day it was not there in the morning and started in the afternoon. The following day it was not there all day. It absolutely does not coincide with variations in temperature or humidity, it seems completely random.

Each time I have had time to troubleshoot the issue it seems to stop and so I have not been able to disconnect the coax to the main antenna to see what happens.

When I listen to the audio on the VOR when I am having the issue it seems to be clean, however the VOR antenna is mounted high on the tail and as I am unsure as to the directionality of the VOR antenna or the source of the transmitting station I cannot be sure it is not affected.

The noise is the same whether listening through any headset point and when bypassing the intercom, there is no speaker fitted.

When the problem is there breaking the squelch makes no difference. The noise is there on all VHF channels when receiving a transmission, if pointed straight at the transmitting station.. I have not tried tuning to an unused frequency and breaking the squelch to see what happens, I will try that next time I fly.

I'm leaning towards the dodgy diode/capacitor/connection somewhere in combination with vibration being the cause, but I am running out of places to look!

[mike-wsm]

Canis Loopus

Aaah, lovely photos, I want the 45 minute Scenic Flight plus aerobatics, 4g loop and Immelman, please.

Aerobatic Flights from Jandakot Airport



PS Here's the pic mentioned by FOR above:
Fuji FA-200 VH-FJI - Recreational Pilots Australian Aviation Image Gallery
.

[Aerials]

Hangflyer, thank you for the update at post #59. Going back over some of the posts the problem is definitely related to engine revs so I wonder if it is possible to temporarily borrow another alternator? I know you've had the engine run with alternator field disconnected and I don't know enough about alternators to be sure that there isn't some residual magnetism keeping the field alive perhaps. Not enough to be able to draw much current but enough to be rectified in the diodes.

What is immediately 'downstream' of the alternator? Is there a voltage regulator that could be temporarily swapped?

Like many others, I feel that antenna shielding, weak received signal (as a consequence of shielding) and the rpm are major factors in this. I'm sure many of us are pleased that the noise is not apparent on ADF or Nav frequencies, this indicates that the noise is of somewhat narrow bandwidth and this can be a sign of a voltage regulation circuit causing the problem.

Keep at it - we will get to the bottom of it in the end!

[Self Loading Freight]

You say the noise is there when you're receiving a transmission and actually reduces the signal strength - I'm assuming you do mean signal strength, and not just readability, and that the noise isn't there when there's no transmission.

This sounds like modulation noise. That happens when the radio signal is getting into some part of the electronics and being modified before getting into the radio's signal processing chain. In domestic receivers, it can sound like a loud hum that only appears when a station's tuned in -- in such cases, the mains rectifiers are getting the signal from the mains lead, modulating it with the mains frequency, and then propagating the resultant mess back into the radio. You can also get similar effects when a completely independent circuit has a fault that makes it marginally unstable, and the radio signal either tips it over into instability or combines with an oscillation or noise source.

Diagnostic characteristics include the problem getting worse (or only appearing) at strong signal strengths and often going away completely for no apparent reason. It needs quite an obscure set of circumstances to kick off.

I think the chances of it being connected with power regulation are high, as such circuits do have a good ability to become wideband, powerful noise sources combined with lots of non-linear junctions that make good modulators and plenty of long wiring runs that act as antennas. Plus, although it's a rare beast, you do seem to have eliminated everything else!

[forget]

Have we been told what model/type radios they are; and serial numbers?

[Gillespie Field]

Comparative Testing with a Handheld

Sorry if this has been already suggested (I don't have the time to read all the posts).

When the handheld was used was its own rubber antenna used or was a coax link used to hook up to the aux antenna output?
If the Aux Antenna socket wasn't used then could you either, do this or if you don't have a link cable (coax with BNCs professionally fitted on both ends) remove the BNC connector from the back of the radio, bring it into the cabin and connect directly to the handheld. If the noise is still absent from the handheld at all times then your Antenna installation is not causing the problem. If the noise is present as originally described then I would (initially for testing purposes) run a temporary brand new coax cable from the handheld (again with a professionally fitted BNC plug on the radio end) to the antenna connection (removing the original coax temporarily) making sure that both the centre core and the braid are not bridged by loose braid hairs using a magnifying glass and very bright torch to be 100% certain.
If as a result of this new temporary installation the radio test is clear of all interference then you existing Coax cable is causing the problem. Replace it with a brand new cable and professionally fitting a new BNC plug on the radio end.

[Mr Optimistic]

Not an a/c man

but is there an electrical generator which gives an output dependent on input rpm such that at a certain rpm some voltage or current switched effect occurs eg current dump or voltage regulation ?

Just an ignorant guess.

[mike-wsm]

Another thing to try with a hand-held is to turn down the squelch until you hear the noise and then move it around looking for the noise source. Best to use a loop aerial for this, just a loop of wire soldered to a bnc connector, it should be directional, max noise when its axis is toward the source and zero when at right-angles.

[cone zone]

I don't believe that it is alternator or ignition noise as almost invariably this would come through in your ADF.


Try a new battery, this is the biggest capacitor in the aircraft and may just arrest that noise.

good luck

[GarageYears]

Any more news - while not quite "keeping me up at night", is certainly a mystery I would like to follow to the bitter-end.

There are some points we should not loose - the noise is not evident on the nav aids (were those ever explicitly identified - ADF, ???) and not on the hand-held, but on both aircraft radios.

So far we have not heard what happens if the hand-held is connected to the aircraft antenna...? I believe the aircraft antenna WAS updated at some recent point?

There was also some discussion regarding selection of squelch on/off - was that ever explored?

- GY

[FullOppositeRudder]

Yes, I've been wrestling with all the possibilities for this one also in my spare moments. It continues to elude a simple 'can't miss' diagnosis, and is made all the more frustrating because of its intermittent nature. The points raised in previous posts are worth checking.

I find it hard to latch onto the fact that it occurs only in a narrow RPM range, and that as 'improvements' were made to the ignition systems, the RPM trigger point came up 300 rpm from the previous one. Can this be linked only to vibration, or is there some other factor which causes the noise to appear at that specific figure? Are there other instruments or monitor systems which come into play at that number?

Do the antennas have some sort of static grounding system at the feed point which has reduced the overall RF sensitivity so seriously as to make the direction of the signal source so very narrow and specific? An SWR check would normally betray this, but lets not forget that a 50 ohm resistor at the other end of the coax will quite probably return an excellent VSWR measurement. Is the noise equivalent to an open squelch level and character or is it 'louder' than that?

There's one other factor which the rest of us need to keep in mind. While it hasn't been spelled as such, it seems likely that HF is working at this by himself as both fix-it man and pilot. If that's the case, it adds additional complications into the whole process because flying the aircraft is always the absolute priority, and trying our suggestions will - of necessity - come much further down the list in any given exercise. We all need to be patient as I'm sure he is, but we all share in the frustration which this problem is causing, There has to be a fix somewhere!

FOR

[mike-wsm]

Originally Posted by FullOppositeRudder

I find it hard to latch onto the fact that it occurs only in a narrow RPM range, and that as 'improvements' were made to the ignition systems, the RPM trigger point came up 300 rpm from the previous one.

The vibration occurs over a wide rpm range, but only above a certain figure. The onset figure was increased by making the engine run smoother. Most likely this is propeller vibration. The normal prop vibration band for this aircraft/engine/propeller combination is 2250rpm to 2550rpm. With the engine running rough the onset would be at a lower rpm.

[FullOppositeRudder]

Thanks for that Mike, I haven't had much to do with the subtleties of aircraft engines. Then might we suspect something in the airframe resonating with this vibration band which is then causing some sort of mechanical partial 'break' or other disturbance to the 'normal' installation?

What sort of coax cable is normally specified in these installations? Elsewhere it would probably be one of the RG-58 species, but whether that's used in aviation systems or some more exotic teflon covered variety might be expected needs checking. It's not impossible that if the coax were routed through some close fitting areas of the structure that it may in time be subject to abrasion or other factors which could wear through the outer coating and then the shield.

What problems might result from this - assuming that only the outer braid might be touching the airframe both of which can be considered 'grounded' anyway - and why it affects both radios remains something of a mystery.

But a mystery is what we've had all along anyway.

FOR
(thinking aloud before the second cup of coffee kicks in ...)

[Phalconphixer]

FOR...
RG.58 cable was indeed an aviation industry standard for many years and may well be present on this aircraft depending on its age. It's use was phased out about 10 years ago when it was realised that the dielectric (pvc) would give off noxious fumes in the event of a fire. It's also a bitch to work with for soldered connectors.

In my last job we removed all RG58 from our fleet and replaced it with either RG.142 or RG.400 depending on the application. Generally speaking both RG142 and RG400 can use the same type of BNC, TNC, N, and C connectors as the old RG58.

As I recall RG.400 can be used for most applications up 1030/1090 Mhz.

Beyond this RG393 is the cable of choice. But RG.393 is a replacement for RG213 / RG214 which was outlawed at the same time and for the same reasons as RG.58.

RG.400 is double screened and proved to be very good at system noise reduction and inter system interference where RF cables for different systems are run close together. Insertion loss is also substantially less than an equal length of RG.58

pp

[FullOppositeRudder]

Thanks for that information PP. Helpful indeed - I just don't see these alternative coax types in normal land based equipment I work with..

FOR

[Graybeard]

Has the prop spinner been removed for a test?