The German proximity fuse.

The development of the US proximity fuse by the US in WW Two is
regarded as a unique allied triumph.

Little known however is that the Germans independently developed and
successfully test fired almost 1000 rounds of a similar proximity fuse
near the wars end that if introduced into service would have had a
dramatic effect. The allies estimated that the availability of the
proximity fuse would force them to abandon use of the B-24 Liberator
due to its lower flying altitude compared to the B-17.

The Allied Proximity fuse was used on both Anti-Aircraft Artillery and
anti-personnel howitzers where they were set to explode approximately
50 feet above the ground. At that height they would produce a lethal
zone over a terrifyingly wide area. When used against aircraft it
seemed to increase effectiveness of a round by 3-7 or more.

The proximity fuse was fielded as an AA weapon from ships in the
Pacific from June 1943 where it was reasoned that secrecy could not be
compromised as dud rounds would fall into the sea. (I have no
knowledge of its use against the Japanese Troops on islands). It on
one occasion apparently helped shoot down 90 of 120 attacking Japanese
planes.

It was first supplied to Britain to help overcome the V1 cruise missile
fired at London where it in combination with radar and computer
directed guns reduced the mean number of rounds expended to destroy a
V1 from 4000 to 180.

Finally there are records of it used against German troops during the
Ardennes Offensive (Battle of Bulge). It was reasoned that the
Germans would not be able to reconstruct the fuses in time to make use
of them. In fact the 'folklore' on the Internet is that they
captured some 20,000 but did not recognize them and also that they
recovered duds and reasoned that they were triggered by the Earth's
magnetic Field. (Note the magnet field theory probably came from the
troops themselves before being analysed by more technical branches of
the German forces)

The allied fuse workings.

Technically the Allied fuse was not radar: it did not send out a pulse
and listen for an echo. It had 4 tubes. One tube was part of the
oscillator. When a 'target' that was about a =BD wavelength in size
came within a few wavelengths it would load the amplifier and the anode
current would increase. Two additional amplifiers would detect this
change and then triggered the 4th valve (a gas filled thyraton) to set
of the detonator. Contrary to other reports it apparently did not
trigger on Doppler shift either or on frequency change. There were
many shock hardening techniques including planar electrodes and packing
the components in wax and oil to equalize the stresses.

The German fuse workings.

The fuse was based on electrostatic principles.

The circuitry of the German fuse is not precisely known to me as I do
not have the schematics however the details are in allied files refred
to I do not have a circuit layout drawing. It is known that the nose
of the shell was electrically insulated and isolated from the rest of
the shell. It was built by the company Rheinmetall. The program was
halted in 1940 then restarted in early 1944 and then terminated again
due to being over run by the allies at the point that it was ready for
production.

Initial fuse testing demonstrated a sensitivity of 1-2 meters and a
reliability of 80% when fired against a metal cable target. A circuit
adjustment yielded an increase to 3-4 meters and a reliability of close
to 95%.

Further work showed a 10-15 meter sensitivity. This was with 88mm
canon shells. The shell to all intents and purposes ready for
production.

References are "Truth About the Wunderwaffen" by Igor Witowski who
cites "Proximity Fuse Development - Rheinmettal Borsig A.G.
Mullhausen. CIOS report ITEM nos 3 file nos XXVI -1 (1945)

Capacitance based fuses became highly developed after the second world
war due to their high resistance against jamming techniques.

It is unlikely that the shell could have been easily degraded by
jamming or chaff. (unlike the Allied shell).

I can speculate as to several ways that this might work. It is
referred to as a "influenz zunder" based in electrostatic
principles.

Method 1: Bridge Cicuit. The shells external capacitance is made
part of a bridge circuite with an internal reference capacitor in the
other arm. Any disturbance caused by an large object such as an
aircraft would cause a current to flow across the bridge that would be
amplified.

Method 2: QT or charge transfer methods. A high speed vibrating
contact charges the shell and then discharges it into a known
capacitance which is then measured.

Method 3: making the shell body part of a resonant circuit and
detecting frequency changes.

I can not find the precise reason for the abandonment of the work in
1940 however it probably relates to the 'fuhrer befehle' or fuhrer
directive that with few exceptions all work that could not be put into
production within 6 months were to be terminated to increase resources
for those that could (in order to support operation Barborosa). It
was at this time that the Germans also abandoned their magnetron and
microwave development teams and programs. Many programs suffered
severely due to this; something that was to have far reaching
consequences for the German war effort.

What would have happened if the proximity fuse was not abandoned in
1940 but development continued such that it entered service in 1943?