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Old 2nd Jul 2020, 1:19 am   #1
AdrianH
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Default Early Radar question? Long question.

I have been watching Youtube videos on the start of radar and I am afraid that the video and Wiki have left me asking questions.

In one video https://www.youtube.com/watch?v=iNOK64T2KOA the initial test starts at 04:30, in the video it starts talking of fading initially and in the back of the van shows a vertical blip on the screen and one sees it disappear off to the right as the range gets to 8 miles.

https://en.wikipedia.org/wiki/History_of_radar describes the experiment as "A Handley Page Heyford bomber flew along a path between the receiving station and the transmitting towers of a BBC shortwave station in nearby Daventry. The aircraft reflected the 6 MHz (49 m) BBC signal,and this was readily detected by Arnold "Skip" Wilkins using Doppler-beat interference at ranges up to 8 mi (13 km). This convincing test, known as the Daventry Experiment, was witnessed by a representative from the Air Ministry."

OK now for my confusion, I would associate a moving blip on a straight oscilloscope line moving from left to right as being a pulsed form of signal measuring the delay from the echo, but I could not imagine the BBC pulsing their transmission for the experiment so can only assume it was a AM broadcast transmission.

If Doppler, how would the receiving gear not get swamped by the local transmitter,?

The only thing that comes to my mind is that an aeroplane travelling directly towards or away from the receiver and in line with the transmitter, at say 120mph (160Kph) could produce a change in received frequency of approx 54Hz, but that would be hard to differentiate between the transmitted short wave signal.

So how was the aeroplane detected and could such an experiment be done today?

Adrian.

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Old 2nd Jul 2020, 7:55 am   #2
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Default Re: Early Radar question? Long question.

I'm not au fait with the technicalities, but to answer your last question, I seem to remember that a modern day repeat of the experiment was done on the TV Coast program when they visited (I think) Orfordness.
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Old 2nd Jul 2020, 8:00 am   #3
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Default Re: Early Radar question? Long question.

In "The Secret War" "To see for a hundred miles" there is a reconstruction of that experiment. Arnold Wilkins is played by himself, so it seems to be accurate. The dot just went up and down.
 
Old 2nd Jul 2020, 8:32 am   #4
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Default Re: Early Radar question? Long question.

I believe the purpose of the experiment was to prove that it was possible to detect a reflected signal from an aircraft. The receiver received a signal direct from the transmitter and a signal reflected from the aircraft. Because the path the reflected signal takes is different from the direct signal, and is constantly changing, the phase difference between the two signals causes the signal at the receiver to vary in amplitude as shown by the dot on the scope going up and down.

I think the aircraft was carefully chosen to have a wingspan that matched the wavelength of the transmitter.

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Old 2nd Jul 2020, 8:43 am   #5
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Default Re: Early Radar question? Long question.

"If Doppler, how would the receiving gear not get swamped by the local transmitter,?"

Two aerials and signal paths were used. These were combined in anti-phase to minimise the standing carrier. The resultant signal was then due to the difference between the ground wave and the reflected signal.
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Old 2nd Jul 2020, 10:38 am   #6
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Default Re: Early Radar question? Long question.

Doppler gives you a beat between the reflected signal and the transmitted signal. It was also common to use separately sited transmitters and receivers.

The effect is quite common on communications links, look up 'Aircraft flutter' for separate TX/RX sites

It's also used in radar speed guns/cameras with TX and RX in one unit

David
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Old 2nd Jul 2020, 10:58 am   #7
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Default Re: Early Radar question? Long question.

Quote:
Originally Posted by lesmw0sec View Post
"If Doppler, how would the receiving gear not get swamped by the local transmitter,?"

Two aerials and signal paths were used. These were combined in anti-phase to minimise the standing carrier. The resultant signal was then due to the difference between the ground wave and the reflected signal.
In the videos and yes I appreciate it was made as an information video but no connections to any outside antennas seen, and I would have expected anti-phase antennas to also reduce the wanted signal as well. This is what I get with watching videos in the early hours of the morning, I start to wonder how things are done, it keeps the grey matter working at some level.

David, I am assuming you mean an audio beat note that would be down in the low audio frequencies, the sort of thing used at temp traffic lights by the old 10Ghz Doppler units, I must get my old PW Exe out of moth balls.

The thing that has me stumped from watching the video, if using any of these methods was how were they driving the oscilloscope trace at the time, what was driving the X and Y axis.

Oh warning to all, when I get these inane things going around in my head I have to try and understand in practical terms how things were/are done.

Adrian
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Old 2nd Jul 2020, 2:09 pm   #8
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Default Re: Early Radar question? Long question.

Memory stretching time...
Davantry at 49m was used with two aerials spaced 1/2 a wavelength apart and the output nulled to give, as near as possible, zero output. Aeroplane flies over and disrupts the balance, result a wiggling spot. To quote "Secret War", "And you can see this on TV whenever a low flying aircraft goes over", said by the wonderful William Woollard.
 
Old 2nd Jul 2020, 2:46 pm   #9
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Default Re: Early Radar question? Long question.

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Originally Posted by merlinmaxwell View Post
Memory stretching time...
Davantry at 49m was used with two aerials spaced 1/2 a wavelength apart and the output nulled to give, as near as possible, zero output.
Only in two directions, of course. In directions orthogonal to these, they add.

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Old 2nd Jul 2020, 3:03 pm   #10
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Default Re: Early Radar question? Long question.

Quote:
Only in two directions, of course. In directions orthogonal to these, they add.
Oh yes, enter the aircraft!
 
Old 2nd Jul 2020, 3:15 pm   #11
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Default Re: Early Radar question? Long question.

OK I have just spent over an hour watching 'The Secret War' episodes on Youtube starting from the Lorenz beam and then going onto Radar. I have just watched the part where Arnold Wilkins re-enacts the original demonstration with a scope and Racal RA-17 (I think).

I notice how there is no X-Y plot in this demonstration but a single, central blip oscillating up and down going slower as the aircraft comes over head etc.

So this is a lot easier to understand, I could see the oscilloscope Y input being on either the AGC line of the receiver or possibly on the demodulated output line from the detector. So It shows how some producers embellish demonstrations in video's.

So thank you all for your responses/answers.

Must now read up on the Lorenz beam!

Adrian
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Old 2nd Jul 2020, 3:31 pm   #12
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Default Re: Early Radar question? Long question.

It makes it very difficult for anyone technically competent to watch anything intended for the general public... where artistic people try to cover scientific things.

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Old 2nd Jul 2020, 4:18 pm   #13
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Default Re: Early Radar question? Long question.

I would treat the reconstructions with a large pinch of salt.
Page 30 of the excellent but hard to find Book, RDF1 by Michael Bragg explains that a twin channel receiver was used that had been developed for long distance short wave propagation investigation. Two sets of horizontal dipoles 12’ above the ground were spaced 100 feet apart. Each receiver was connected to a dipole but one had a phase shifting device inserted between it and the aerial. The theory was that both aerials would pick up the ground wave but due to the distance apart would not be identical. By a modification to the receiver and adjusting the phase shifter the deflection of the ground wave on the crt display could be reduced to a small dot.
It goes on to say that many accounts of the Daventree experiment contain errors from the minor to ludicrous. An authoritative description is in Watson Watts 1958 book, three steps to victory.
If you want to read these few pages from Bragg’s book, too long to relate here I will scan them and send them via email if you pm me.
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Old 2nd Jul 2020, 4:35 pm   #14
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Default Re: Early Radar question? Long question.

A while ago I succumbed to an RSGB advert in RADCOM and bought a book from them. It was supposed to be about someone and their involvement in early radar development. He certainly was and they said he was, but that was it. Nothing technical at all. It was simply a book about domestic arrangements and being kept moving around the country. I gave it away.

It did not live up to what was suggested.

I've never bought another book from them.

David
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Old 2nd Jul 2020, 4:36 pm   #15
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Default Re: Early Radar question? Long question.

Quote:
I could see the oscilloscope Y input being on either the AGC line of the receiver or possibly on the demodulated output line from the detector.
They would amount to the same thing in the end.

Quote:
It makes it very difficult for anyone technically competent to watch anything intended for the general public.
I do find that the "Secret War" series tried quite hard not to do that (perhaps they didn't and just made a good show without pandering to the un-brained), mind you it is over 40 years old now, the general populous must be getting more un-understanding now (or should that be derstanding?).
 
Old 2nd Jul 2020, 4:45 pm   #16
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Default Re: Early Radar question? Long question.

Terry I will pm you shortly, I find this sort of information very interesting. I wonder how things were done try to think of a simple way to do it and then see if there is technical information to check if I am on the correct track or not.

RSGB unfortunately after trying again for 12 months have stopped subscription, I spend very little time on air and find no interesting articles in the monthly magazines, I am finding more things of interest going through the old magazines on worldradiohistory.com.

Again thanks all for the help.

Adrian
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