The 6m Aircraft Reflection that breaks the rule

We know for certain. The reflected signal from an Aircraft will always fall in frequency due to the fact that the reflected signal path decreases in Length as it approaches the TX RX path and increases as it goes away.

The previous Captures have all proved this to be true.

This Capture of G4FCC signal at my QTH on 6m is the exception. What is the Aircraft doing in this instance at the start of the trace?
The clue could lie in the fact that Sywell Airport lies to the north of our signal path and within a few Kilometres of my QTH. The signal is short lived so could well be reflected from an Light Aircraft manoeuvring to land.

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Modern Ham Radio – Funny

WSPR is a mode which was purposely designed for QRP Propagation Study. The whole point is to radiate a known level constantly repeated signal and analyse the resultant Spots of that signal by disparate stations around the globe.

“But hang on there I see that T1WIT is getting Spots from America I had better turn up the Power until I do” is the logic often used. No propagation to W1LOUD using 100mW means that conditions are not favourable at that time, a valid piece of Data for Propagation Study. Turning up the Power until you get results invalidates all Data, as does not knowing what your Output Power really is.

Personally I can set a stable measured 10mW Output from my 100W Commercial rigs if I need to. I would much rather build a QRP Radio that consumes a few Milliamps for a few Milliwatts Out.
Ignoring the Computer consumption, on WSPR we have many stations with an HF rig consuming 60 to 100 Watts to produce 100mW of RF. What makes me smile is the fact that a Commercial Step Attenuator is then added to the set up to dissipate the power just generated.

The Elecraft AT1 or some other commercial Attenuator is listed in the equipment used as a modern ‘badge of honour’.elecraftat1

 

Well at least the AT1 is available as a kit so you can have had a hand in this Modern Farce.

More Scatter on 6m

G4FFC lies due East of me. The main UK Air Corridor cuts our path approximately midway. He is there on 6m so an ideal chance to experiment.

In this Capture a single aircraft crosses our path, on frequency WSPR decoded without interference from the crossing plane. Poular oppinion has it that Aircraft Scatter will kill WSPR.

6mrefl-g4ffc

Early in this transmission a plane crossed the path followed by another which tracked our path closely for a long period before finally crossing and leaving. WSPR again managed to decode without problems.

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By beaming along the Corridor I reduced the direct signal from FFC to zero. I found an aircraft that produced a 60Hz straight shift. WSPR decoded at the shifted frequency. The aircraft must be at an angle to both stations that causes a shift that is related only to the speed of the plane, or on the East West route to Ireland which would place the aircraft perfectly aligned on our path and West of me. Some aircraft reflection from a plane crossing the path can be seen crossing the transmit frequency

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Tests will continue as time permits.

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Radar Aircraft Scatter and all that

With the help of Colin G6VAK’s Grabber I was able to see my WSPR signal on 6m arriving at his QTH, strong enough, but not decoding. I could see my signal was accompanied by multiple lines descending in frequency and modulated by WSPR. I consider the WSPR decoder is confused by these.

I assumed them to be Aircraft reflections and set out today to duplicate the effect over a shorter range on 70cm with the help of G8EUX.
At my QTH I am well provided with high altitude Aircraft on the North South UK Air Corridor and also the Daventry DTY Navigation Beacon Chain.

G8EUX QTH is 14 Km to the South and Transmitted a slightly drifting WSPR signal which barely showed on the S meter. Under these conditions any reflections were rare but soon we were treated to a steep strong trace, modulated by WSPR which apparently crossed our signal path and went through flutter and zero beat. The steepness meant less time within the Bandwidth of the WSPR signal and it decoded OK.

At first the straight lines always descending in frequency and crossing the transmission frequency may cause confusion. The explanation lies in the fact that we have two stations and one moving Aircraft. As the Aircraft approaches the signal path the distance decreases. After crossing the path the distance increases. The same holds true from which ever way the the Aircraft approaches. The offset depends on the rate of increase or decrease.

The capture shows the 70cm signal with multiple reflections and one long strong one probably flying on the DTY beam.

dtypath70cm

The map shows the Track of the DTY Navigation Beacon chain to/from the Luton Control Area, the North South Air Corridor and the 70cm path between G8EUX and G3ZJO.dty-NSpath

G6AVK QTH lies 127 Km distant to my South East and the path is largely parallel to the North South Air Corridor. Consequently the trace of many Aircraft Reflections is less steep and spends time almost parallel to the wanted WSPR signal. Unlike the 70cm trace which carries only a few phase changes from the modulation, the 6m reflections were modulated with an obvious image of the Direct Path signal.

G6AVK had a sample of this stored.6mrefl-g6avk

The capture shows 2 transmission periods from me with a blank 2 minutes when G6AVK transmits. The second period is the one that has an Aircraft which is on an almost parallel track to the signal path. The reflection from it quickly arrives at the transmission frequency. At this time we can assume that the Aircraft is at the midway point between G3ZJo – G6AVK, flying either North – South or South – North. At this point Doppler Shift is Zero. The plane moves toward one station at the same speed as it moves away from the other, result Zero.

G4FFC at my QTH, some multi reflections here.

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The Radio Path crosses the UK Jet Motorway.

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In this one, a parallel tacking aircraft produces a parallel reflection. The decoder in WSPR is confused by this and produces no decode.

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Interestingly this is Early Radar at work. The first Radar tests used a Fixed Radio Transmitter, [Daventry](G8EUX) and Aircraft [Heyford Bomber] (Thanks RAF or who ever) and a Receiver Station [Watson Watt/Wilkins] (G3ZJO).

Where did this first historic test by Watson Watt take place? Why right below our 70cm Test Aircraft track and only a short distance West of G8EUX.

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