VHF conditions

In reply to G3RMD:

Forgive me for possibly being a “party-pooper” but isn’t long distance VHF propagation primarily the result of tropospheric scatter at lowish altitude? i.e. from a common scattering volum at 2-5km AGL. The dust cloud’s reported altitude at the time of 1st reports of poor propagation was 55000ft or about 16.5km which seems too high to affect troposcatter. I’ll accept there would have been possibly some precipitation into lower levels from the high altitude cloud but the changes in propagation being described seem to be happening in advance on the dust getting down into the lower atmosphere.

I’ve certainly noticed the lack of aircraft. The flight path in/out of Edinburgh Airport lie about 3km to the North of here and the aircraft are low enough to be lowering their undercarriage when I can see the never ending procession of 737/757/A319 short haul jets flying past the shack window. The TV in my bedroom uses the Craigkelly transmitter North of the Forth overlooking Edinburgh. As the planes fly between me and the mast we used to get visual patterning from reflections on analogue TV. On digital TV there’s a click and the audio disappears for a second. Since the grounding of flights we’ve had no clicks.

I’m just off out to check the Mrs. FMF’s new car (it’s white) for signs of dust fallout. Especially as it’s raining hard which is why I’m sat inside and not on my way up a hill! :frowning:

Andy
MM0FMF

In reply to G8ADD:
http://www.flightglobal.com/articles/2010/04/16/340727/pictures-finnish-f-18-engine-check-reveals-effects-of-volcanic.html

This may interest you Brian.
73,
Frank

In reply to MM0FMF:

In reply to G3RMD:

Forgive me for possibly being a “party-pooper” but isn’t
long distance VHF propagation primarily the result of tropospheric
scatter at lowish altitude? i.e. from a common scattering volum at
2-5km AGL. The dust cloud’s reported altitude at the time of 1st
reports of poor propagation was 55000ft or about 16.5km which seems
too high to affect troposcatter. I’ll accept there would have been
possibly some precipitation into lower levels from the high altitude
cloud but the changes in propagation being described seem to be
happening in advance on the dust getting down into the lower
atmosphere.

Andy
MM0FMF

That is just the question, Andy. Troposcatter seems abnormally poor and some of us are theorising that it is normally augmented by aircraft scatter; in normal circumstances there would be a considerable number of aircraft between me and the Angus beacon, I have made a number of VHF contacts by undoubted aircraft scatter using the flight paths to Birmingham International, the flutter pattern is unmistakable, and I have sometimes used several aircraft to complete. At a greater distance I would expect weaker effects but from a number of aircraft merging together.

73

Brian G8ADD

In reply to G8ADD:
Hi Brian,
Angus beacon up and down in deep qsb at present. At about ‘normal’ strength when up in qsb, but reverts to ‘very weak’ when down. Some flying resumed? Sorry about rather vague signal strength descriptions. I am describing a signal that is, at best, about 6db above noise floor from this qth.
73,
Frank

In reply to G3RMD:

According to News sites, some Scottish airports and English airspace to as far south as Newcastle was open this morning but was expected to close at 13:00 as another ash cloud approaches. Traffic was apparently quite light. This seems consistant with your report, Frank, assuming that there is an aircraft scatter component to normal propagation. The forecasts suggest that there will be a depression over Iceland by the end of the week that will possibly move the plume to the Northwest, so a resumption of normal traffic may happen, and possibly normal propagation with it - keep watching, folks!

73

Brian G8ADD

In reply to G8ADD:

It’s a shame I wasn’t able to listen at home today. I have always suspected that my signal from GB3ANG was entirely by aircraft scatter however difficult it was to believe given the consistency of that signal. So far this episode seems to back up the theory. Not a peep since the aircraft were grounded. The QSB Frank mentions should have been even more pronounced for me if that’s so.

73, Richard

In reply to G4ERP:

I’m desperately trying to figure the geometry of some of these paths. I feel somewhat embarrassed at not being able to take something as gospel from you Richard when I consider how long you’ve been active on VHF!

My first question is wouldn’t there be quite observable amounts of Doppler on the signals if aircraft were involved. The second is the geometry problem. It feels that for you in Cheltenham to be hearing GB3ANG from Dundee way the aircraft should be midway between you and the beacon. That would put them somewhere north of the M65 up as far as Newcastle and I don’t believe there were any aircraft flying there today.

However, I’m fascinated by the possibility of proving such a reliable propagation method so any good websites/articles you can suggest will be devoured with great interest up here. And, with the availability of “RADAR” receivers such as the SBS1 or websites showing real time tracking info then the possibility of using such scatter for scheduled microwave contacts seems incredibly tasty :slight_smile:

Andy
MM0FMF

In reply to MM0FMF:

No doubt there will be Doppler shift if the aircraft track is not transverse, and if only one aircraft is involved the Doppler shift will be measurable. However, if the scatter is the resultant from numbers of aircraft then there will be a spread of frequencies involved, some aircraft will be moving more or less directly towards or away from the receiving station, others will cross at an angle. If the scatter is from a particular path then all the aircraft contributing will be travelling at much the same relative speed and Doppler is no longer a problem. It is complex.

This brings me to the geometry problem. I really don’t see that the aircraft need to be halfway between the transmitter and receiver, they can surely be anywhere where both transmitter and receiver are visible to them (figuratively speaking!) This suggests that aircraft in a significant volume of space are involved. On a day that produces good contrails you can look up from my garden at any time of day and see several active contrails at the same time, even though buildings and trees block out the view of the horizon. Then there are the reflections that might pass from aircraft to aircraft before arriving.

However, if we just take the example of today, aircraft flying perhaps to Scottish airports, certainly to Newcastle, should be visible in Cheltenham, after all, the tops of thunder clouds can be seen a couple of hundred miles away…and of course there is some refraction to take into account!

As I have used aircraft reflections to complete contacts on two meters I would imagine that the method will work on microwaves, otherwise radar is a myth!

73

Brian G8ADD

In reply to G8ADD:

Hi, both.

Well, I did say I was finding it difficult to believe so when I got home I switched on the RX. Not a peep out of GB3ANG for about 20 minutes then it popped up out of the noise and rose to it’s usual strength before disappearing again. I’ve just been working here in the shack again for the last 20 minutes and once more absolutely nothing.

73, Richard (going back to building racing cars…)

In reply to G4ERP:

http://www.flightradar24.com shows all the aircraft currently using Mode S transponders within range of the website connected Mode S receivers. I’m not up on aircraft spotting but I think most commercial craft now carry Mode S transponders.

By adjusting the map and clicking on the aircraft you can get the position and height. From that it should be possible to see which aircraft are likely to aid with aircraft scatter propagation. ISTR that an aircraft 200 miles distant needs to be at 30000ft AGL for it to be above your horizon i.e LOS. I need to check those figures but I’m too tired after a hard day at the programming coal face and further time at home with a bit of CPD (Python and SQL should you care) to check them now! A bit of SOHCAHTOA and Pythagoras should do the trick so that given your own location plust the height and location of a plane you can get a scatter footprint. Turn the beam the right way and Bob’s your uncle!

Andy
MM0FMF

I can just hear GB3ANG this morning but it is definetly not as strong as it used to be.

73 Graham G4JZF

In reply to MM0FMF:

Hi Andy
I must have ago at aircraft location for contacts. Its used by a number on the microwave bands.

If its any help I listened to the Angus beacon and it seems roughly normal strength to me here so either you are all imagining things “down south”, conditions are just poor or the effect is real. Aircraft reflections are quite distinctive when you hear enough of them, all very strange. I was out /P on 23cm last night and conditions were very poor on there, everyone was complaining.

One thought, there was no attenuation through the atmosphere off the moon, the Arecibo signals were great onto a single yagi in the garden on 432.
Robert
GM4GUF

In reply to GM4GUF:

http://www.ringbell.co.uk/info/hdist.htm

…confirms my memory that planes 200miles distant need to be 30000 ft (actually 213 miles) to be above your horizon.

Planes don’t need to be midway between the observer and the beacon, they just need to be visible at both locations at the same time. Given that few planes fly above 41000ft in the UK that gives a maximum range of 248miles from either site to the plane.

With that calculator and some schoolboy maths should now be possible to calculate a range of plane positions that are visible from both the observer and GB3ANG. Fire up Spectran and your radio, beam at GB3ANG and watch the trace. You should be able to see when ANG is “heard” and whether there is any Doppler on the signal. A quick butchers at the real time radar site will show if there are any planes in the right place. If there’s a signal and a plane in the right place then it’s QED and time for a pint! :slight_smile:

Andy
MM0FMF

In reply to MM0FMF:

… and that’s exactly what Graham G3VKV has done for me (except that he used Argo). He lives a couple of miles south of me and has a similar path to GB3ANG over G/CE-003. For both of us no planes coincided with no GB3ANG.

He sent me two plots this morning. One is from Feb this year showing a multitude of traces but all have one of two distinct Doppler shift characteristics indicating maybe that there are two routes from which he and I benefit in Cheltenham. The second plot is from yesteday and it has but a few traces. Interestingly, all have the same Doppler characteristic indicating possibly that only one route was open.

As far as I’m concerned, it’s QED - although I suppose I ought to wait until I get home this evening and hear the return of a more consistent GB3ANG.

73, Richard

In reply to MM0FMF:
Hi Andy,
I have a very good write up on the topic of aircraft reflections at vhf, which I will send to you as an email attachment. It is on a different pc, so will send it this evening when I fire it up.
Been golfing this morning, instead of observing, but since switching on about an hour ago the Angus beacon has been pretty solid copy, with usual qsb. I have been looking at doppler using Spectran, and the carrier is wandering up and down a few HZ, almost constantly. Not been monitoring very long so no sensible measurements taken, and as I have never looked at the beacon using a FFT waterfall display I have no history to compare with.
What started me off on this wild goose chase, was the fact that I was very surprised not to hear the beacon at all on May hill, when it has been a solid S5 signal in the past.
73,
Frank

For a real world illustration of radio LoS to en-route aircraft I’ve linked to a screenshot taken a while ago from my radarbox display. The maple leaf shaped affair illustrates the max range of aircraft received via RF. Dotted lines mark the 200KM and 250KM radii. The antenna is a collinear with masthead pre-amp just above the gutter. As you can see there’s not much traffic kicking about after 10PM.

It’s also a good indication of how useless my VHF coverage is to the south from my home qth due to the local terrain :wink: (the spiky bit at the bottom is an anomalous return btw)

73 Chris
M1DTJ

In reply to G8ADD:

Just an update at this location, Have been monitoring now regularly since my previous posting of lowest 2 m conditions and it’s effects on all beacons…

last night GB3ANG rose out of the noise and made a s3 at times then fell again
that was the first time since last weekend I had seen . Tonight there is a dramatic change all is ‘near’ normal 'ang peaking s5 at times and back at a constant pattern and on0vhf has appeared again. The only missing signal is EI2WRB to the west…

2 x 17 ele Tonnas / masthead amp.

Ian

In reply to G4ERP:

"As far as I’m concerned, it’s QED - although I suppose I ought to wait until I get home this evening and hear the return of a more consistent GB3ANG. "

QED.

Richard