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)

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).
.

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

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

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!

Canis Loopus

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

Aerobatic Flights from Jandakot Airport

:D

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

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!

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!

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

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.

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.

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.

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

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

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

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.

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 ...)

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

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

FOR

Has the prop spinner been removed for a test?

Ok, another update.

Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.

If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.

If you change to an unused frequency and turn off the squelch there is none of the noise.

The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.

The 2 radios have seperate cables and antennaes and they both exibit the same behaviour, with the newer radio with the new antenna being worse ONLY since the antenna was replaced with a higher gain unit.

NAV1 is the second half of COM2 which is a KX175BTSO

All last week the issue was absent, the comms were perfect. On Saturday half the flights were perfect and the other half had a little noise. On Sunday it got worse and we stopped flying at mid day and cancelled the remaining flights as it was becoming a safety issue with stations in front of the aircraft becoming unreadable at times.

This morning the noise is there, it starts at 2450RPM and stops at 2550RPM on the ground, it is still directional only on recieve on both comms.

It is possible to park the aircraft close enough to the tower that there is clear line of sight between the antennae on the top of the aircraft and the tower aerial on the tower, even in this situation the behaviour is unchanged, clean audio until the nose if pointed at the tower then with the nose within 20 degrees of the tower the signal degrades to 2s and then back to 5s again in every other direction.

I'm still going crazy trying to find a solution! I appreciate the suggestions.

so the noise is coming in on a cable, either power or antenna feed ? (apologies if this is already known and I am being dumb). But if this is the case, why is it directional ?

Oh my God. He gets worse. :ugh: Mods, can you please do something about this buffoon?

I have been unable to correlate the issue with any of the following items:

Temperature
Humidity
Cloud Height
Day of Week
Moisture
Potential Static Build Up
Hard Landings
Time of Day
Aircraft Use

Solar Flares!
SpaceWeather.com -- News and information about meteor showers, solar flares, auroras, and near-Earth asteroids
Check for yourself!

Not RFI
 

A: The problem occurs over a wide range of recevied signal strengths. This rules out reduction of signal strength by shadowing.

B: Everything but the battery and one radio have been disabled and noise was still there.

C: No hint of it on the handheld.

D: Most telling: The noise is only there when receiving, not in background with squelch disabled.

My theory:
The prop is modulating the Rx signal. This would account for the extreme directional sensititivity, only happpens whe prop is between antenna and source.

I have no idea of the normal grounding(/bonding) path between prop blade
and airplane but suspect as the prop vibration sets in at ~2100 rpm one or more of the blades are intermitantly disconnected.

Given prop length and radio wavelength this could cause a a reflection/phase shift that would modulate the signal.
The impact would also be critically dependant on distance from prop to antenna, which would explain lack of noise on handheld.

One test, not conclusive, would be to measure resistance from each blade to chassis while applying force in every direction to each blade of prop (engine secured!!).

Few more things to try: Is there any hint of the noise when the base station is directly behind you?

Turn to the "edge" of the noise (~15degrees?) and see if problem goes away with airplane pitch. (Another way of moving prop in/out of line of sight of antenna.).

(Added)
Note the theory behind this is that the rapdidly fluctuating Rx signal strength is causing the receivers AGC (automatic gain control) to wig out resulting in incorrect levels at the detector.

More on why not RFI:

Simple shadowing would slowly change the Rx level, much less than flying from .5 miles to 20 miles.

If the inteference was external generated RFI it would be at a (relatively) constant level so a few Db change in Rx level would only matter at some fairly critical distance from base station, farther away it would be pressent all the time, close in would not be there at all.

This may be completely off beam, and I can see no reason for the directional implications, but here goes.

You mentioned that it was there all day, but then wasn't on a test. What changed, in terms of the radio equipment in use, By that, was it a case of 2 people, headsets etc in flight, and only one when testing? I'm thinking around things like headset connectors in the panel for the second person position, maybe an intercom related issue, are you using active noise cancelling headsets, could a socket connection be vibrating with engine speed, or even (perish the thought) is there an issue with the passenger seat bonding that's not the same empty or loaded.

There is also the implication that it's not just radios, there is also possibly an intercom unit/switching unit somwhere in the loop. Has that been eliminated from the possible suspects. Also the wiring to and from headset sockets to the intercom and on to the radios, any chance of a change here, especially related to ground loops in coax and the like, on microphone leads etc.

PTT wiring, any possibility of a chafe on them that is giving airframe contact at certain harmonic frequencies?

Appreciate that this is very much on the crazy end of speculative, but from what I can see, all the obvious and some of the very much not obvious have already been done to death.

Are there any engine instruments or other instruments which can't be switched off that have a voltage regulator as part of their construction.

Are there any maintenance or other access panels that were removed around the time of the start of this, not necessarily forward of the cabin?

what beacons and or strobes are there on the aircraft, and have they been eliminated from the possibilities?

Is there any possibility of harmonic resonance in something like a control cable run causing contact with an ajacent fuselage member?

I'm about out of ideas at this point, best of luck.

My vote is with Mig15.
I'd suggest that if you try what DERG suggests, then you'd be able to measure the SWR just before the front end gives up due to reflected power returning.
Just my humble opinion.

I agree with Murphy

The problem occurs when the received signal is coming through the prop and the effect is the same on strong and weak signals. This rules out many possibilities.

What may be happening is that the incoming signal is modulated by the prop, quite literally mechanically chopped. This will put strong amplitude modulation onto the carrier. With a two blade prop running at 2400rpm this is 80Hz am. The modulation will be a constant proportion of carrier and so bear a constant relationship to the received signal (also am). Presumably the receiver agc sees the chop and turns the gain down, reducing the amount of wanted signal.

The effect may be specific to a particular type of receiver or even to the mode of operation set on the receiver or the software version loaded. I don't have a schematic so can't even begin to guess what is going on inside the receiver.

I think we likely have a winner. I'm not sure if we ever got a result from @hangflyer for the experiment to connect the handheld to the aircraft antenna, but I'm not sure if that would be conclusive anyhow for exactly the reasons @mike-wsm just spelled out, namely that this kind of AM modulation might play havoc with only some AGC designs. @hangflyers other observations now rule out a whole bunch of other possibilities, we don't have an independent noise source. If it proves very difficult to locate the grounding issue for the prop, I wonder if just relocating the antenna to a position that has an aharmonic distance relationship to the prop will work around it? I probably shouldn't be suggesting such kludges...I'm just curious to see if the modulation effect is working the way I imagine.

If I park 200M away from the Tower the problem is unchanged.

This is interesting because the VHF antenna has a clear line of sight to the VHF antenna on top of the tower.

At this range I can taxi in circles with the engine RPMs high enough for the problem to be there and the noise starts at +-20 degrees of the nose pointing towards the tower. The rest of the 320 degree rotation the noise is not there at all. When the noise starts coms go from 5s (everywhere else) to 2s for those 20 degrees.
I initially felt the at the signal was going "getting chopped" as it went through the prop, however I can put the aircraft in positions where there is an absolutely clear LOS between the transmitting and receiving antennae and it still happens if the station is aligned with the nose of the aircraft.

One of the major problems I have is that I am not a LAME, as this is a Charter category aircraft I am not free to take things apart myself, I need to bring ideas to the engineers or avionics guys who then charge me a fortune to see if they can fix the problem.

At the risk of sounding unkind, it does seem to me that in most cases today the standard procedure is to replace boxes until the problem goes away, often the problem is determined to be "the box" because a new one fixed it (sometimes because they broke the old one removing it). I am at the point now where they have replaced pretty much all the replaceable components in the system and I still have the issue and seem to be running out of ideas.

There also seems to be a lot of contrary theory amongst the techs.

There are 2 competing companies that have both had a go.

One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.

The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.

Does anyone actually know which one of them is correct?

Also we have 2 different brands of radio on 2 different length coax cables on 2 different types of VHF antenna located on 2 different parts of the fuselage that both have the identical problem! Harmonics?

Both radios have been swapped with other units. Not specific to just one unit.

hangflyer

Could you tell us a little more about the aerial change, please? For example why did you decide to fit a new one and when was it fitted? Thanks.

Thanks HF for the additional information. Some other contributions have been very useful, one or two not quite so IMHO (that's all I want to say on that :suspect: )

There is a small amount of truth in each of them but neither would get a satisfactory pass in the real world with my old radio theory tutor.

The first statement might be a consideration if we were dealing in micro-watts over a considerable distance. Unless we have a seriously desensitised reciever (two on them in fact) I suggest that the shielding effect of the fuselage would be minimal given the transmit power of the radio equipment used in the tower ground installations which I would expect to be 'significant'. This theory is further eroded by the fact (if I understand correctly) that even with the aircraft on the ground in close proximity to the tower such that line of sight conditions between the transmitting and receiving antennas, the problem is still evident.

The second statement has some credibility in that if the shield of coax cable to the antenna is bonded to the skin of the aircraft (metal is assumed here) and thus to the airframe (grounded to use the more common terminology) at the antenna feedpoint the entire aircraft becomes the ground (some would say counterpoise) against which the whip of the antenna either radiates or receives a signal. This is the sort of system I would expect in this aircraft (the turtle deck is metal (aluminium) isn't it ?????) .

There is usually some directivity in that the radiation patten of the antenna would normally be the strongest in the direction of the largest area of ground plane. In this instance I think the directivity effect would probably favour signals in front of the aircraft if at all. Others might argue for signals from the rear of the aircraft being stronger in both transmit and receive. A beer or several are needed here to arrive at the correct answer by concensus. But I would be astonished beyond measure if either possibility were serious enough to produce the effect we are seeing here.

The prop modulation possibility as presented and argued by others earlier today is very interesting and seemingly credible. However I am troubled by the intermittent nature of the problem, and that the onset of the problem has been lifted by quite a few hundred RPM as changes were made to engine equipment which improved the performance. Prop modulation is possible perhaps, but surely there would have to be other contributing factors.

Finally, one especially puzzling aspect is that, as I understand it, the noise is stronger on the radio which has the newer, higher gain antenna. That again really suggests some sort of RF interference which is being received by that antenna at a higher received sig strength than the other one. (No it doesn't help a lot - at least I don't think it does - but it's just another question without logical answer)

What is the answer then? I don't know. I'm baffled like almost everyone else but somehow, someway, the cause will be identified and the solution found.

FOR
(all written in hast - with apologies to grammatical and/or technical purists - it's been a long hard day, this might be better attempted after a good sleep) :zzz:

Does anyone here remember Fred the Wheel Tapper from an RAF poster of the ‘60s. Fred’s job was to find cracks in the wheels of railway carriages. He’d whack them with his hammer and if they didn’t sound right he’d have them changed. This cost the railway company a fortune until someone discovered that Fred’s hammer was cracked.

So, of all the radios you’ve tried, have they been through the same shop with the same test equipment and same tech? Just curious. :hmm:

full opp rudder
 

Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.

Lots of clues there.

Need to work with what we know, use good radio theory and experience.
When I have had weird radio problems more often than not it is something earthed that should not be, or something that is poorly grounded / bonded/ shielded that should be.

I'm still suspicious of the ignition lead shielding (which prevents wave energy emissions). It's a simple test procedure for your AMO and an inexpensive fix.

ampclamp

Also this:

More clues here, which have perhaps been glossed over...

The noise only occurs on an active frequency. Note the effect of squelch off on an unused frequency...

Brain working overtime but not coming up with much :ugh:

- GY

Ampcamp said One othe big clue is that it happens at -every- distance to base station so almost has to be some modulation effect as opposed to an independant (fixed level) rfi source.

Both the intemitant and RPM sensitivity can be explained by prop vibration, which if I understood a previouse post correctly, is basically a "in the direction of spin" vibration (torque load/unload) that occurs above a certain speed. This onset speed was raised slightly when the ignition system was improved.

My theory is that this vibration is intermitantly grounding/ungrounding one (or both) of the blades, or less likely some mechanically related structure.

Only when ungrounded does the prop blade create a reflection/multipath/interfernce situation.

The signal does not have to go directly "through" the prop for this to happen, think tall buildings and ghosty (analog) TV reception. It does have to be mostly in line, hence the 20 degree range of the effect.

The handheld not seeing the noise can be explained by the effect being larger at "magic" ( ~ 1/4 wavelength multiples) distances from the prop.

Worse with new antenna could be that a slight difference in (electrical/effective) distance to prop.

One interesting test would be to take the handeld outside and slowly move toward the plane when noise is happening.
Not sure one could (safely!!!) get close enough to get the effect or not though. Results of handheld connected to antenna would also be usefull.

multi-path
 

Not an a/c man (did you guess ?) but is the prop metal ? Reason I ask is that if the spacing between the blades is right then it might be giving you multipath which would modulate the signal. Far enough from the tower and the fraction of the received signal intercepted by the blades would decrease so the effect would vanish. The rpm dependency would have to be explained by agc timeconstant or something. The directionality of a hand-held (unless you want to risk life and limb would prevent that happening to it.

Key clues to my mind:

- Non-airframe grounded receiver does NOT 'hear' noise => Noise is not an RF transmission
- Opening squelch on the a/c radios on an open frequency does not 'hear' noise => Infers something is happening to the transmitted signal BEFORE reception by the a/c radios (and reinforces this is not an additional RF signal)
- Only occurs to stations when within +/-20 degrees of being on-the-nose => Something in the front of the a/c is responsible! (Thinks big whirling thing out front...) What angle does the prop diameter subtend to the tower station at 200m. Does the angle of interference appear to change?
- Noise is not always present => See post above by MurphyWasRight! I'm with you on that idea.

Back to thinking some more :8

- GY