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Thread: Effect of Window on H2S

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    Default Effect of Window on H2S

    I have examined numerous wartime records on the use of H2S and I realize I have not found any comment or discussion on the effect of Window on the performance of H2S. H2S was the primary tool for the navigation team for determining “fixes” of aircraft on deep penetration raids when they were outside of Gee range. However it would seem to me that Window would have a profound effect on the signal in the vicinity of the aircraft. The only comment that I have seen on this was in the ORB of an Intruder Mosquito crew who commented they encountered a Ju.88 over the target (Dessau) but lost him “in the tube” in the mass of bombers below. No comment about Window.

    Chaffe is Chaffe. From my experience with marine radar systems, window must have impacted all radar systems.

    Has anyone encountered any comments in the records on this possibility?

    Jim
    Last edited by JDCAVE; 15th September 2021 at 20:15.

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    Default Re: Effect of Window on H2S

    Window was cut to specific lengths to target the wavelengths of German radars. There were all earlier-generation technology not the centimetric radar used by H2S and RAF night fighters in this period. I'm no expert on the subject, but chaff is not just chaff but was and is sized to be used in a specific manner. It is possible that the centimetric radars could see through Window.

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    Default Re: Effect of Window on H2S

    Yes. Window was sized for specific radars, but that does not mean it won’t affect other radars. H2S was a first generation airborne radar and was not a high quality technology. There were considerable issues with it.

    My question remains: has anyone found commentary in the wartime documents about the influence of window on H2S, and also AI radar on Mosquito a/c?

    I have a 48” open array radar on my boat and the “anti clutter” filtering for rain and sea state are not perfect. I’m pretty certain my radar would be completely snowed up if I encountered window at any size. FWIW, overhead aircraft are strong targets on marine radar.

    Jim

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    Default Re: Effect of Window on H2S

    Jim,
    Ian The Radar is the bloke that will probably be able to answer your question.
    We tried (in the 1970's, under field (SPPTA) conditions) to 'invent' a mobile met radiosonde kit in 38 Grp Tac Comms Wing. It worked technically, but we had no means of transferring the response frequencies to 'Met' numbers. It also 'played hell' with the TacATC a/f radar (our 50x50cm sheet of bacofoil - as a reflector - looked very large, and upset a lot of the parameters!!).
    Why don't you get one of these kids balloons gas canisters and release a number of balloons (off your boat) - each with a different length of 'chaff/duppel' hung underneath, and see what results you get?
    Could be good fun?
    HTH - but I refuse to admit to responsibility!!
    Peter Davies
    Last edited by Resmoroh; 17th September 2021 at 17:20.
    Meteorology is a science; good meteorology is an art!
    We might not know - but we might know who does!

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    Default Re: Effect of Window on H2S

    Thanks Peter. Sorry for the belated reply. I have reached out to this fellow for his opinion:

    https://www.radartutorial.eu/html/author.en.html

    Incidentally, it turns out the radar on my boat is operating on the 3 Cm band, so a narrower band radar than H2S. I’m pretty sure, it would be “jammed” by clouds of window.

    Further your suggestion about balloons with window…I wonder if a test could be done with a plastic kite with, and without a “window” tail? That might be doable near my moorage.

    Jim
    Last edited by JDCAVE; 19th September 2021 at 19:39.

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    Default Re: Effect of Window on H2S

    Received from “The Radar Tutorial”:

    “Dear Mr. Cave,

    Of course, Window strips tuned to the wavelength have the greatest effect due to resonance. But they can also have an effect on other wavelengths. But in the order of magnitude of (estimated) only 10%. Just as an example: with weather radar, raindrops are not in resonance with the radar's wavelength. Nevertheless, the cloud will be seen by radar.
    However, at that time the receivers were not sensitive enough. 10% of the magnitude can often be too weak. Therefore, I cannot estimate whether the H2S could see Window clouds outside the resonance wavelengths or not”

    Jim

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    Default Re: Effect of Window on H2S

    Jim,

    Interesting question, although I should start by mentioning that it is not that relevant to compare your modern marine radar with wartime radars. Not only did most radars operate on longer wavelengths, but the technology was less refined with fewer filters etc to avoid clutter and so on. The comparison between modern equipment and wartime ones is not that relevant even if it might seem so.

    In answer to your question, most German WW2 radars operated on a wavelength of around 50cm. so that is what most Window was cut to affect, ie around 25cm long as a half wavelength. Trials were carried out to establish how Window would affect a range of British radars and there was no effect at all on 10cm, so British Window would not affect H2S at all. It would probably be seen by British radars operating on 1.5 metres, but would not have a significant effect on their performance.

    So in answer to your question, Graham is correct and it is not true that "chaff is chaff" and it doesn't affect all radars equally. British Window had no effect on 10cm radars including H2S which is why you've not found any reports on how it was disrupting this equipment, because it didn't.

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    Default Re: Effect of Window on H2S

    Thanks Ian. My request in my original post: “Has anyone encountered any comments in the records on this possibility.”

    I’m not saying you are incorrect. But I want to see the wartime documents. I haven’t seen them. That was my original request.

    WRT modern radars, the clutter filters are not particularly relevant to the question, as I don’t think the anti-rain or anti-sea filters would filter out interference from window on my radar. My radar operates at 9400 MHz on the “X” band (approximately 3 Cm wavelength). That said, I think Wolff's comment "However, at that time the receivers were not sensitive enough. 10% of the magnitude can often be too weak. Therefore, I cannot estimate whether the H2S could see Window clouds outside the resonance wavelengths or not.” is the best I have to go on right now.

    Jim
    Last edited by JDCAVE; 21st September 2021 at 06:34.

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    Default Re: Effect of Window on H2S

    I don't understand why you prefer uncertain answers to Ian's clear and definitive one. Could it be that the reason you haven't seen any comment about such interference because such an effect doesn't exist? Not just H2S worked on centimetric frequencies, but the AI radar in later Mosquitos did too, and these operated alongside and amid the bomber stream perfectly successfully. This would not have been possible if Window jammed the AI.

    Had there been any interference, H2S would not have been successful, and hence either it would not have been adopted or the use of Windows would have been abandoned. Both continued in use quite happily. QED.

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    Default Re: Effect of Window on H2S

    Quote Originally Posted by JDCAVE View Post
    Thanks Ian. My request in my original post: “Has anyone encountered any comments in the records on this possibility.”

    I’m not saying you are incorrect. But I want to see the wartime documents. I haven’t seen them. That was my original request.
    Jim,

    Sorry to be pedantic, but you say above that your original request was "I want to see the wartime documents" but with respect that was not your original request. To quote you, you asked "Has anyone encountered any comments in the records." I have seen such documents and that is why I provided the information you asked for, You said nothing in your original post about wanting to see the documents. So please be clearer in what you ask for, rather than coming across as rather ungrateful for people trying to help you.

    The information I provided came from records at The National Archives at Kew. I can provide the file references for you to examine yourself, but it will take a little time for me to check the exact file numbers and provide this information. The files in question are not digitised, so not remotely available so you will only be able to see the documents if you are able to visit Kew in person. If you can confirm you are able to do this, I'll try and get the file numbers for you as soon as possible.

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