German FuG 202 /FuG 220 Lichtenstein airborne radars
by Hans H. Jucker, Schwerzenbach, Switzerland

Nachtjäger Messerschmitt Bf 110, 2Z + OP 6
Interception Radar: Lichtenstein FuG 202
Weapon: 4 x MG Cal. 7.8mm, 2 x MK151, Cal. 20 mm

Landing on 15th March 1944 at Dubendorf Airbase (Switzerland)
Crew: Pilot Oberfeldwebel Helmut Treynogga
Radiooperator Unteroffizier Heinz Schwarz
On the night of 15th March 1944 a German Night Fighter crew defected to
Switzerland with an Messerschmitt 110G. They approached intact with anoperating Lichtenstein FuG202 interception radar at Dubendorf airbase.
(see photography of the airplane after the landing). According to the authentic landing report, the ME 110 had taken off from Echterdingen in order to attack a British bomber formation approaching Munich. The crew
told they have lost its bearing due to radio failure.

1

The purpose of this paper is to show the result of investigations
and conclusions on the German Lichtenstein FuG202 and FuG220
night fighter radars.
The Lichtenstein FuG202 radar equipment combines a relatively narrowcone of search, around ±20°, with high sensitivity to change in directionof the target (e.g. 1.4:1 in pip size at 5° off axis), so it is well suited toassist a fighter to keep on the tail of a bomber taking evasive action, once
the fighter has get approximately on an overtaking course. The greatest
practical range of the set is approximately 2.5 km; the minimum range isvery short around 150 meters. The range is limited by the height only up
to 1000 m. Above this height targets are visible beyond the ground clutter.
The Lichtenstein FuG202 equipment consist of transmitter S, receiver E, T/R unit
W, multi-element direction finding antenna
array, motor driven antenna switch andindicator unit SG. The pulse modulated
radar operates on 50cm wavelength, theinstrumented range is 8 km but the maximum range on a large bomber is approx.
4 km. The antenna array consists of sixteen end-fed half-wave dipoles with sixteen reflectors connected in four sets eachof four dipoles with reflectors. A switching
device feeds the antenna sections in varying phase to swing the antenna polar diagram left, right up and down. The indicator has three CRT’s, one has a circulartime base and the other two have normal
straight traces, horizontal and verticalrespectively

In the switching unit the relative phases inwhich the four antenna sections are fed
are varied. In this way, the beam is rotated in a split angle of ± 7° about theaxis of the airplane. The split rate is 25
Hz. The cross - over point is only 1.1 dB(77% down from the maximum. At thesame time, the angular sensitivity is very
good out to 25° from the axis of the airplane. The antenna gain is 14 dB.

All four sections are used at once, giving abeam 25° wide (one half power) with thedirection of maximum gain 7° off the axisof the airplane. There is one appreciableside lobe, pointing 35° to the other side of
the axis. Its maximum is 8-10 dB down
relative to the maximum of the main lobe.

2

The transmitter operates on a frequencyof 490 MHz and has a peak output powerof 450 watts, approximately at a pulse
repetition rate of 2700 Hz. Two RS394
triodes are used in an anode modulated
push-pull oscillator. Tuned anode and gridlines (see photo with the original RF circuitry) are used and cathode tuning is also
provided. The antenna coupling loop feeds
the RF power over the T/R unit into theantenna. The high voltage supply consistsof 1 µs pulses from the separate modulator unit. The peak modulator voltage isapproximately 1500 volts and the modulator power is 2 kW. The repetition frequency of 2700 Hz is obtained by dividing
down from a 19 kHz master timing oscillator which drives all the timing circuits in
the FuG202 equipment.
The Lichtenstein FuG202 is unique among German radars in its use of a superregenerative receiver. The receiver consists of three units, an R.F. unit, a quench generator and a wide band amplifier, the latter including an automatic gain stabilisation (a.g.s.) circuit. The sensitivity of the receiver is 30µV at 70 ohm’s for signal plus noise equal to twice noise and the bandwidth is 3.5 MHz
at 6 dB down. The receiver is tuneable from 479 to 499 MHz.
The R.F. unit consists of a triode oscillator and a double diode detector. The plate supply to thetriode consists of cycles of the quench voltage at a frequency of 454 KHz, having no steady D.C.
component. The magnitude of the applied quench voltage is varied by the setting of the gain control and by the automatic gain control system. Full wave detection is used, the detector output
being fed to the broad band amplifier.
The voltage gain from the antenna input to the detector output is approximately 160 under average operating conditions, but depends on the setting of the gain control. The receiver can be tuned
in flight by the operator, with variable capacitor 5.

Circuit of the FuG 202 superregenerative receiver unit

The output of the detector is fed to the broad band amplifier having 5 stages. Its bandwidth is 3.5
MHz at 6 MHz down, the overall voltage gain 4000 times at 100 kHz falling to 2000 at 500 kHz. The
broad band amplifier also contains the automatic gain stabilisation circuit. This circuit provides anegative voltage which is applied to the quench generator to control the quench amplitude applied
to the superregenerative detector and thus control the gain of the detector.

3

The quench oscillator consists of an oscillator operating at 454 kHz, whose output is fed to the a.g.
s. control stage. The broad band amplifier output is fed to the other input of the a.g.s. control stage. The output of the a.g.s. control stage supplies a variable quench voltage of 1 - 40 volts as plate
voltage to the triode oscillator of the R.F. unit. The time constant of the whole a.g.s. system isabout 120 ms, the range of the gain control 60 dB. The a.g.s. system greatly reduces the vulnerability of the receiver to jamming.
The video output of the broad band amplifier for any kind of input signals consists of 0.2 µs pulses
at the quench rate of 454 kHz. Thus the pip on the screen appears filling in, and there is no breakin the baseline.
The indicator consists three scopes (about 2½” diameter) alongside each other, a circular range
scope reading up to 8 km, an azimuth scope with the pips on opposite sides of the vertical baseline, and an elevation scope with the pips on opposite sides of the horizontal baseline.

4Magnetic Deflection System for the Circular Range Scope

The presentation in range tube is more than simple deflection type. Thereis also intensity modulation the brightness of the trace being an increasing
function of the output of the video amplifier.
5

Lichtenstein FuG 202 airborne radar, schematic diagram
Tastenstufe
4 x LD2 parallel

6

Sender
2 x RS394

The sweep frequency is that of the 19 kHz master oscillator frequency, theelectron beam in the circular range tube makes 7 revolutions per transmitted pulse. In addition to the circuit for intensity modulation, there is abrightness circuit which applies a square brightness pulse for the durationof the first revolution, making it alone visible. Thus targets more tan 8 km
away cannot ever be seen. In the range tube, an 8 km scale, with 100 mdivisions, is engraved on the face. The deflections are outwards.
The D/F scopes display a sweep cutof 2 km only. The sweep delay contains for this purpose an adjustablephase shifter for the 2700 Hz sine
wave. Its can be adjusted in flightby the operator within the 8 km of
the range scope.

Lichtenstein FuG 202 Airborne Intercept Radar
Developed 1941 at the Telefunken-Laboratory Zehlendorf-Berlin
Hypothetical calculation of the effective detection range against
flying targets:
System Parameters:

4

R =

R

Maximum Detection Range in meters

Pt

Transmit Peak Power (750 w = 58.5 dBm)

G

Antenna-Gain (G2 = 28 dB)

λWavelength (0.612 = -4.3 dB)

σtRadar Target Cross Section in Square Meters(4π )3Sphere Surface (twice) (= 33 dB)

MDS

Signal plus noise equal to twice noise (-80 dB)

58.5 dBm 28 db (-4.3dB) 0or10or16 dB
Pt G2 λ2 σt---------------- = ----------------------------------------------(4π)3
MDS
33 dB
(-80 dBm)
1 m2 (ME109 small fighter)=< 1’700 meters=< 3’000 metersfor σt 40 m2 (B24 or Lancaster bombers) =< 4’300 metersRfor σtRfor σt 10 m2 (HE111/HE177)

R

7

Nachtjäger Messerschmitt Bf 110 G-4, C9 + EN
Interception radars: Lichtenstein FuG 202 & FuG 220
Weapons: 2 MG 151 Canons Cal 20 mm in the nose
2 MG FF Canons, Cal 20mm firing upward

Landed on 28th April 1944 at Dubendorf airbase (Switzerland)
Crew:
Pilot:

Oberleutnant Wilhelm Johnen

Radiooperator: Leutnant Joachim Kamprath
Gunner:Oberfeldwebel Paul Mahle

The German ME110 C9EN airplane had taken of from Hagenau to attack
night bombers in Southern Germany. Having established contact, the experienced crew managed to destroy an Avro Lancaster. After some problems with the left DB605 engine, the pilot made an emergency landing at
the Swiss Dubendorf airbase. The top secret night-fighter was equippedwith the brand new Lichtenstein FuG220 interception radar as well as its
predecessor the Lichtenstein FuG202. The aircraft also carried the secret“Schräge-Musik” - two upward firing 20mm Oerlikon canons.
8

Attack procedure of the ME-110 Nachtjäger
with the „Schräge Musik“ - weapon

Installment of the „Schräge Musik“ twin canon M.G.
FF Oerlikon in the already cramped cockpit of theME110G-4 night fighter.

9

The German Lichtenstein FuG220 intercept radar was installed in twin engine
night fighters. In addition to the numerical coding, it had the code LichtensteinSN2. The Lichtenstein FuG220 intercept radar has a relatively wide cone ofsearch, around ±45°, direction finding for flying targets was good, being 1.65:1
for 10° off centre at an operational frequency of 90 MHz. The forward coveragainst flying targets was excellent and the ambiguities not yet serious. The
Lichtenstein FuG220 had an instrumented range of 8 km, however the greatest
practical range of the set was limited to the flight altitude, beyond ground clutter
appeared on the scopes. The minimum detectable range of the set was limited by
the T/R switch around 900 meters. For to improve the limited near range resolution of the Lichtenstein FuG220 an additional Lichtenstein FuG202 was installedin the Messerschmitt Bf 110 G-4, C9 + EN night fighter.
The Lichtenstein FuG220 equipment
consist of transmitter S, receiver E,
T/R unit W, multi-element direction
finding antenna array, motor driven
antenna switch and indicator unit SG.
The pulse modulated radar operates on
90 MHz, the instrumented range is 8
km. The antenna array consists of four
half-wave dipoles, each with a parasitic
reflector, arranged in the form of asquare. In order to obtain direction
finding a split system is used, the main
lobe of the polar diagram being
switched either up and down for elevation or left and right for azimuth
beams. The indicator has two CRT’s,
both with normal straight traces, one
for azimuth, the other for elevation.

Lichtenstein FuG220 Antenna
Antenna Array of the German ME 110 C9EN airplane landed on 28th April
1944 at Dübendorf airbase (The drawing was taken occasional the investigation at Dubendorf airbase).

10

The antenna array consists of four half-wave dipoles, each with a parasitic
reflector, arranged in the form of a square. In order to obtain Direction/Finding a split system is used, the main lobe of the polar diagram being switched either up and down for elevation or left and right for azimuth
beams.
To accomplish the beam switching, the antenna array is fed with one pairof antenna array lagging in phase by 90° with respect to the other pair sothat the beam is deflected in that direction. The delay is then switched into the circuit of the other pair so that deflection in the opposite direction
occurs. This switching is carried out by a motor driven capacity switch at a
rate of approximately 25 Hz. An additional manual operated switch (seethe schematic below) is used to accomplish direction finding either in azimuth or elevation.

It will be noted that depending on position of switch U1 the beam will beswitched either up and down (elevation) or left and right (azimuth) thatspeed. It will be seen that the cable F297 from the transmit/receive unit,
enters through condenser C1 on the first stator of C2 of the motor driven
capacity switch. The rotor here has 360° vanes so that a constant coupling is maintained whilst revolving. The second part of the rotor has verysmall vanes which couple with alternate sides of the second stator as it
resolves.
11

As there is a length of phase shift cable F285 between the two halves ofthe second stator, output from these halves are alternately delayed by anamount dependent on the length of the cable.
This delay is used to obtain switching of the polar diagram main lobe. The
two output cables F279 and F280 are connected to switch U1. Connections
to the actual switch mechanism are made through low pass filters for harmonic suppression of the transmitter signal.
Calculation of lobe deflection by the antenna switch
∆φ =
90°

2π d sine θ-------------- =2π d 1.415 sine θ 0.4416------------------------------λ∆φ
λsineθ = ------------- =26.20°
2π
d
∆φ

=

= rad

1.5704= ∆φ

λ 2.5
rad 90° = 1.570 λ 2.5----------------------------2π= sine θ 0.4416=θ

d 1.415

Phase Angle to steer the Beam to ∆φ(90° lagging)sineθ =

Deviation of Beam from Broadsided=Dipol Radiator Spacing 1.415 meters (measured on the ME110 C9EN)λ=Wavelength 2.5 meters

FuG220 Antenna Pattern
Diagram for the operationalfrequency of 90 MHz.
The Lichtenstein FuG220
radar was originally designed for operation in the
87 to 92 MHz frequency
band. Therefore the D/F ratio is optimal between 90
and 91 MHz only. At lowerand higher operational frequencies the D/F ratio does
deteriorate increasingly.

12Lichtenstein FuG220 Transmitter
The pulse transmitter found in the Messerschmitt Bf 110 G-4, C9 + EN
night fighter operates in the frequency band between 87 - 92 MHz with apulse repetition frequency (prf) of 292 Hz. The prf is derived from an audio frequency generator. To avoid trouble on the receive picture throughsynchronism with other FuG220 transmissions, a control in the indicator
unit provides two other alternative prf’s of 295 and 298 Hz. A square wave
is derived from the 292 Hz audio frequency and fed into the hard tube
modulator driver and final stage.
The figure on the left shows the final
stage of the pulse modulator. The driverpulse of Rö 8 is supplied over pulse
transformer U4 to the grid of Rö9 and
Rö10 (LV13 all-glass triodes). The 15
amps anode pulse current of Rö9 and
Rö10 flows through the primary windingof the pulse transformer U5 and induces
positive-going 1µs output pulses of 2000
volts in the secondary winding of thepulse transformer U5. Toroid type magnetic cores are used for the pulse transformers. The 2000 volts pulses are applied as high voltage to Rö1 and Rö2 ofthe plate modulated oscillator stage. The
load impedance of the transmitter is approximately 1000 ohms. The peak plate
current is typically 2 amps.

Rö1 and Rö2 are connected in a push-pull
RF oscillator circuit, feed back taking
place from anodes to opposite grids
through condensers C62 and C69. The
oscillator frequency is determined by inter-electrode capacities in conjunction
with the anode circuit inductance. Thisconsists of a single turn of heavy gaugewire, part of which is bent to form atransmission line. This transmission line
has an adjustable shorting bar. The line
provides variation of operating frequency
by moving the bar. The RF peak power istypically 2 kW. If the “NahauflösungsRelais” R2 is closed (activated by aswitch on the indicator) the transmitter
pulse will be reduced from 1 µs to 0.6 µs.

The operational frequency of the pulse transmitter found in the Messerschmitt Bf 110 G-4, C9 + EN night fighter has to be pre-adjusted in the
depot, it could not be changed in flight by the operator.

13

Lichtenstein FuG220 Receiver
The schematic (freehand-drawing) was made occasionally the investigation after
the landing in Switzerland, it shows the circuits of the Lichtenstein FuG220 interception radar.

It is a classical 12-tube super-heterodyne receiver with two RF-preamplifiers, a
local oscillator with mixer, four IF amplifier stages, a detector and a video amplifier. The IF-frequency is 4 MHz (-3 dB down) at a bandwidth of 2.2 MHz. The receiver sensitivity is better than 5µV (- 90 dBm) for signal plus noise equal totwice noise.
The frequency of the local oscillator may be changed by a variable capacitor. The
capacitor is controlled by a selsyn motor linked to the remote tuning unit, andprovides a small variation (± 2MHz) of local oscillator frequency whilst in flight.
The receiver gain can be controlled also in flight by a potentiometer on the indicator.

The Common T/R Unit
This unit is mounted at one end of the receiver. The essentials of the circuit are aquarter-wave line using lumped elements, terminated by a spark gap at the receiver input.
This spark gap a soft tube (Telefunken
LG 75 Nullode) passes current duringtransmitter pulses and consequently
the feed to the receiver offers a highimpedance during the pulse. Subsequent receiver paralysis is thus much
reduced.

14

Near range resolution of the FuG 220 receiver

The front end of the FuG 220 receiver isprotected by an LG75 gas-discharge
tube. The gas filling in the tube shallquickly ionize on the leading edge ofthe transmit pulse and bypass the
RV12P2000 receiver preamplifier tube
by a low impedance. In the ideal casethe gas filling shall quickly de-ionize
after the transmit pulse is gone. Unfortunately the gas filled TR tubes are notideal switches, they have some delay
time to ionize and a much longer delay
time to de-ionize. This time delay limits
the near range resolution of the radarsystem.

Remark
A later modified version of the Lichtenstein FuG 220 interception radar incorporates the so-called “Nahauflösungs”- fix. The meaning of “Nahauflösungs” isnear-resolution, and its function is to improve the minimum range by suppressing the receiver while the transmitter is operating.
For better overall resolution the transmitter pulse-length can be reduced and it
was possible also to change receiver bandwidth by bringing in damping resistors
across the IF tuned circuits. Also the gain controlling line of the receiver was
modified in that the time constants are considerably reduced to allow the suppressor grids to follow the suppressing waveform. Allegedly an improvement inminimum range to 200 m was claimed for the modification.
Lichtenstein FuG220 Indicator
The indicator unit contains an Azimut CRT-Display as well as an Elevation CRTDisplay tube, both displays provide simultaneously range indication by a marker
generator (see figure below).

15

Original Lichtenstein FuG220 Indicator

Schematic of the Lichtenstein FuG220 Indicator16Lichtenstein FuG220, Indicator Function
Positive triggering pulses from the modulator are applied to the grid of Rö 1 andthe resulting heavy anode current, rapidly discharge C1-2. After each pulse, thecapacitor recharges slowly producing the required time-base voltage.
Tube Rö 2 is used to provide a brightening waveform. Tube Rö 3 together withthe tuned circuit in its cathode forms the start the calibrator circuit. The cathoderings each time the modulator output trigger is received, the output is fed to thegrid of tube Rö 4 and Rö 5. When calibration is required those suppressor grids
are brought to ground by the antenna switch motor. The waveform is fed on theX-plates of the CRT’s to provide pips of varying brilliance as the time base writing
speed changes.
The video output of the receiver is connected to the appropriate Y-plates of thedisplay unit CRT’s by means of the antenna switch unit.
The standing anode voltage of the receiver output tube is sufficient to deflect thetime base trace in a direction perpendicular to its length so that the resulting
screen picture consists of three traces the centre one having calibration dots, theouter ones having signals shown as deflection from the centre trace.

Indicator Waveforms

17

Lichtenstein FuG 220 Airborne Intercept Radar
Developed 1943 at the Telefunken-Labaratory Zehlendorf-Berlin

System Parameters
Frequency:
91 MHzTransmit peak power:
2 kW
Receiver Sensitivity:- 90 dBm
Antenna Gain:
5 dB
Antenna split frequency:
25 Hz
Pulse Length
1 µs
Near Resolution
1000 Meter
PRF:
292, 295, 298 Hz
Instrumented Range:
8 km

Hypothetical calculation of the effective detection range against
flying targets:
Parameters:
R
Maximum Detection Range in meters
Pt
Transmit Peak Power (2000 Watts = 63 dBm)
G
Antenna-Gain (G2 = 10 dB)
λWavelength (3.192 = 10.35 dB)
σtRadar Target Cross Section in Square Meters
3(4π)
Sphere Surface (twice) (= 33 dB)
MDS Signal plus noise equal to twice noise (- 90 dB)
Pt G2 λ2 σt63 dBm 10 db 10.35 dB 0 or10 or16 dB
R = ---------------- = ----------------------------------------------MDS
33 dB(-90 dBm)
(4π)3
4

R

for σtR
R

1 m2 (small fighter)=<

3’200 metersfor σt 10 m2 (HE111/HE177)=<

5’800 metersfor σt 40 m2 (B24 or Lancaster bombers)=<

8’000 meters

18

The figure on the left shows the original data
sheet of the Telefunken LG 75 gas filled TRtube installed at the receiver front-end of
the Lichtenstein FuG 220 airborne radar - asfound in the emerged ME110 C9EN aircraft.
According to the data sheet the recovery
time of the LG 75 is specified with 1km(equal to 6.6 µs).

The figures on the left and below show theTelefunken LG 75 TR-tube (in Germany was
the tube designated as a so called Nullode)

LG 75 Nullode with removed sprayshieldThe transmitting triode LD15 used in thetransmitter of the Lichtenstein SN2 radar(left on figure) was derived from the Telefunken LD5 triode.
The plate dissipation of the LD15 was 25
watts and the plate peak current 1.5 amps
for pulses ≤ 1µs during keying.

19

To avoid arcing on higher flight altitudes the triode LD15 (left on figure)has a larger glass-body than the LD5(right on figure) this allowed a wider
spacing of the double ended anodeterminals (the two top pins) from theother tube terminals.

The power triode LV13 (left on figure) was
designed during WWII by Telefunken as
high–vacuum switch tube for pulse modulator
application in the Lichtenstein SN2 airborne
radar system. During the pulse interval theswitch must conduct a high current. Therefore for maximum efficiency, the effective
resistance of the switch must be as small as
possible, so that the potential difference
across the switch tube, called the “tube drop”
is small during the conduction period.
For pulse operation ≤ 3µs the LV13 triode
was able to switch an anode current of 15
amps at 2000 volts by an effective switch
power resistance of ≤ 20 ohms. Telefunken
could reach this design goal with a large areabarium oxide cathode. Two LV13 triodes inparallel were used in the modulator of the
Lichtenstein SN2.

The triode RS394, left on the figure, wasonly used for the first series of the Lichtenstein FuG202 transmitters, in early 1941.
The RS394 looks rather like a laboratory
design model than a typical rugged German
Airforce tube. The anode dissipation was 30
watts, the tube could be pulse operated up
to approx. 500 MHz.
However, the manufacturing process of theRS394 was to complicated for mass production, so Telefunken re-designed later theRS394 into the LS30 tube.

20

The Telefunken P07S1 CRT, left on
the figure, was used as range tube
on the indicator unit of the Lichtenstein FuG202 intercept radar.

References:
Kdo. Armeeflugpark, Oberst Carl Högger, Bericht über den am 15.3.44
notgelandeten deutschen Nachtjäger ME110 2 Z
Kdo. Armeeflugpark, Oberst Carl Högger, Bericht über das am 28.4.44 in
Dübendorf notgelandete deutsche Nachtjagdflugzeug ME110 C9ENETH Zürich, Institut für Hochfrequenztechnik, Prof. F. Tank, Ing. H.K. Jenny, Untersuchungsberichte über Navigations- und Radarausrüstungen aus
fremden Kriegsflugzeugen, August 1945
Ernst Wetter, Geheimer Nachtjäger in der Schweiz, ASZM Verlag, 1989
ISNN 3-274-00016-7
Telecommunication Research Establishment, Malvern, Report No. T. 1538,
Memorandum on Lichtenstein Apparatus, 2nd September 1943
Royal Aircraft Establishment, Farnborough, Report on FuG 202 Lichtenstein Equipment, December 1943
Telecommunication Research Establishment, Malvern, Report No. T. 1897,
FuG 220 Lichtenstein SN2 Equipment, July 1944
Telecommunication Research Establishment, Malvern, Memorandum No.
5/M.104, Airborne Jamming of the extended SN2 Frequency Band, 14th
February 1945
Ries Karl, Deutsche Luftwaffe über der Schweiz 1939 – 1945
ISBN 3-87341-022-2
U.S. Air Material Command Dayton OH, Summary Report No. F-SU-1109ND, The High Frequency War - A survey of German Electronic War, 10th
May 194621