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THE BERLIN GERÄT
"SECRET A. D. I. (K) Report No. 188/1945
THE FOLLOWING INFORMATION HAS BEEN OBTAINED FROM P/W
AS THE STATEMENTS HAVE NOT AS YET BEEN VERIFIED, NO
MENTION OF THEM SHOULD BE MADE IN INTELLIGENCE
SUMMARIES OF COMMANDS OR LOWER FORMATIONS, NOR SHOULD
THEY BE ACCEPTED UNTIL COMMENTED ON AIR MINISTRY
INTELLIGENCE SUMMARIES OR SPECIAL COMMUNICATIONS.
THE BERLIN GERÄT.
1. A recent German Army prisoner, a doctor of Physics of
Vienna University, had worked for fifteen months up to 23rd
October 1944 in the High frequency research laboratory of the
G.E.M.A. G.m.b.H. at Berlin and latterly at Wahlstatt, near
Liegnitz. During that period one of his tasks had been to
determine the characteristics of the transmission lobe from
the aerials of the Berlin Gerät.
2. The Berlin Gerät is a German development of the British
H2S, on a wavelength of 9.1 cm., but employs a totally
different aerial system developed by Siemens. Whilst P/W's
description of the apparatus given in the following
paragraphs, is considered to be reliable, he had no knowledge
of its eventual operational use.
3. Acknowledgements are due to A.D.I.(Science) for their
collaboration in the interrogation.
AERIAL SYSTEM
4. The sketch in the Appendix to this report gives an
impression of the Berlin aerial unit.
5. The four rods forming the aerial array are composed of a
plastic called Trollitul and are of circular cross-section
about 4 to 5 cm. in diameter at their base, tapering somewhat
to a rounded end. The rods lie parallel to the plane of a
circular metal plate of about 1 metre diameter, and about 15
cm, clear of it.
6. The energy to be radiated is led to the aerial rods by a
concentric feeder which forks into too branches where it
enters the metal plate and again fork; making 4 branches to
feed the four aerials. At each of the forks the concentric
feeder widens into a funnel shape called a transformer piece,
the sloping side of which is a half wavelength long, that is,
4,5 cm.
7. A Trollitul dome some 40 cm in depth covers the aerial
array and fits, flush to the edge of the circular metal plate.
8. The whole unit including the aerials can be made to
rotate. This prisoner had never seen the apparatus fitted
either to an aircraft or a ship and he did not know the speed
of rotation; he has an idea however, that the axis of rotation
was at an angle to the geometrical axis of the cylindrical
disc, so that in an aircraft the transmission beam would be
thrown slightly downwards or in a ship, upwards.
TRANSMISSION LOBE
9. The Berlin Gerät has a half-value lobe 10° in width in the
plane of the four aerial rods and 35° to 37° in width at
right-angles to the plane. The lobe was measured by the normal
method; the aerial unit, however, was not resting on a metal
surface as it would have done if built into an aircraft. Under
these conditions the lobe was symmetrical about the axis of
the Aerial array. P/W presumed that if the aerial system were
built under the fuselage of an aircraft shadow effects would
cause the lobe to be asymmetrical or to be deflected.
10. The experiments which P/W had conducted were in the open
air; he had found that when rain covered the Trollitul dome
with a layer of moisture, no transmissions could be detected,
even at a range of 20 metres, along the line of the axis of
the aerial rods. He thought that in these circumstances the
whole lobe was strongly deflected.
PRESENTATION.
11. This P/W had read the regular reports of the "Rotterdam
Sitzungen" - a special committee on centimetre radar - and one
of these reports contained a description of the Berlin
presentation, including photographs taken in an aircraft
flying over Kiel Bay.
12. From this report he could remember that the presentation
was on a circular screen; coast-lines of the mainland and of
islands showed as white ribbon-like stripes and towns appeared
as white areas, whilst individual ships in Kiel bay could be
seen as small elongated white blobs.
13. He had the impression that distant towns, although
slightly distorted in the picture, still retained their
approximate shape. He thought that in the photographs of the
presentation an area of about 60 km. in diameter was
represented; he did not know, however, at what height the
aircraft had flown.
RECEIVER MAGNETRON.
14. It was stated by P/W that a weak point of the Berlin
apparatus was the receiver valve, which frequently broke down.
This valve was a magnetron contained in a glass envelope, with
a solid metal anode in which four or six holes had been
drilled. An impression of the valve, which P/W believed was
called the MD2, appears in the sketch in Appendix I.
USES OF BERLIN GERÄT.
15. Apart from its use in giving a panorama of the ground
over which an aircraft was flying, P/W knew of no other air
uses of the apparatus. He understood, however, that the device
was to be installed in U-boats as an aircraft warning device.
A.D.I.(K) and
U.S. Air Interrogation. S.D. FELKIN
24 February 1945. Wing Commander"
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