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Thread: IM Plinths

  1. #11
    Senior Member Denis's Avatar
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    Default Re: IM Plinths

    Quote Originally Posted by Carnaby View Post
    (Note fighter stations using VHF or Tuneable Beam Approach would have the beacon set well to one side, but Hunsdon wasn't one of those.
    Lots of things dont add up at Hunsdon it would appear Graham Remember that concrete rendered brick cable pit that is 800 yards from the northern end, and off to one side, of the 210/030 secondary runway? That by rights should not have been there either!
    There was a Beam Approach Flight with Airspeed Oxfords stationed at Hunsdon for a while if that ties in with anything. Thanks for your help so far chaps!

  2. #12
    SuperMod Carnaby's Avatar
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    Default Re: IM Plinths

    Quote Originally Posted by Denis View Post
    There was a Beam Approach Flight with Airspeed Oxfords stationed at Hunsdon for a while...
    Back to the drawing board, Denis, and I find from my master list AIR 2/5509 (Status of Beam Approach, December 1944):
    Hunsdon had a Category 2 BABS Mk.1F (aka Airborne Interception BA), Cat 2 meant it was being installed. Marker beacons were not needed with BABS as the radar operator could read off the distance-to-go.

    BUT,
    It didn't have Standard Beam Approach - it had VHF BA, Cat 1A (Installed and operational), and for what it's worth the crystal frequency was 6450.5860.
    This means that it almost certainly would have offset markers. All SBA and VHFBA used vertical aerials for the Main Beacon - which gets you onto the runway centreline. All SBA aircraft had a corresponding vertical aerial for the R1124 Main Beam receiver.

    SBA marker beacons had a horizontal aerial, which meant that all aircraft had to have a horizontal aerial for the Marker Receiver, R1125.

    Fighter aircraft (limited space) used their R/T set with its standard vertical aerial as the BA receiver. They had no need (or room!) for a separate marker receiver / aerial, and consequently had to rely on 'stray' pick-up from this vertical aerial. If the marker (horizontal aerial) was in its usual place - on the centre-line - there would be no pick-up as the aircraft flew over. Hence the marker transmitter was positioned at the side of the approach and squirted its signal across the path so that the vertical aerial would detect something.

    Think we may have it. (The BATF was No.1532, which then moved to Wittering - hence it must have been a Fighter Command VHFBA unit rather than the overwhelmingly common SBA training flights.

    Finally I've found my SBA training manual which states the Inner Marker should be 150 yards from the threshold. Hence the 500 ft estimated looks about right. I'll go and have a beer!
    Last edited by Carnaby; 19-08-2011 at 19:06. Reason: more info

  3. #13
    Senior Member Denis's Avatar
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    Default Re: IM Plinths

    Whoopee doo!
    Graham, you are a star have a beer on me!

  4. #14
    Senior Member Denis's Avatar
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    Default Re: IM Plinths

    As an aside, is there an image available from anyone of the type of installation we are talking about?.. especially something out of copyright?

  5. #15
    SuperMod Carnaby's Avatar
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    Default Re: IM Plinths

    4
    Quote Originally Posted by Denis View Post
    is there an image available ... especially something out of copyright?
    Yes and No!
    A marker beacon for SBA looked like this. The 'kennel' contained a low power transmitter. The overall length of the horizontal aerial was about 4 metres.


    However I suspect the marker for the VHF BA was different and possibly looked like a much smaller version of the system here in photo 1

    The Main Beam or Localiser for VHF BA was very different from the SBA device. About 150 yards off the 'far' end of the runway were two wooden towers about 6 m high and 2m apart facing the runway. The towers were three, rather than four sided. A wooden bar was fitted between the towers a couple of feet from the top and this supported the aerial and two reflectors for the beam system.

    The aerial length for all VHF BA units was about 1.2m.

    There was a later version of VHF BA which was quite different, known as the Type 2, or Hecht, after the designer. I haven't got to grips with this one yet.

  6. #16
    Senior Member Denis's Avatar
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    Default Re: IM Plinths

    Graham, I am in debt to you Thanks again.
    I wonder if Paul B or Buccaneer could knock me up a rough drawing from the description in their sketching programme..hint hint I would like something for the website....

  7. #17
    SuperMod Carnaby's Avatar
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    Default Re: IM Plinths

    Quote Originally Posted by Denis View Post
    I wonder if Paul B ....
    That has been going through my mind for a while regarding these copyright problem photos I have!

  8. #18
    SuperMod Peter Kirk's Avatar
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    Default Re: IM Plinths

    You could trace them as a type of isometric drawing?

  9. #19
    84OTU Chris Lowe's Avatar
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    Default Re: IM Plinths

    Can give it a go Denis but Paul is best at it.

  10. #20
    Senior Member Denis's Avatar
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    Default Re: IM Plinths

    We are not 100% sure what it looks like yet Chris, according to Grahams description it is a bit of a mix of the two known types. Three sided towers, the aerial, the two reflectors would need to be as spot on as possible, but then I have the possible remains of a four footed mast, if the four angle iron stubs in the ground are indeed the tower supports!
    All a tad cloudy at the moment

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