Neptun (radar)
(Neptune) was the code name of a series of low-to-mid-VHF band airborne intercept radar devices developed by Germany in World War II and used as active targeting devices in several types of aircraft. They were usually combined with a "backwards warning device", indicated by the addition of the letters "V/R" , meaning Forward/Backward). Working in the metre range, Neptun was meant as a stop-gap until scheduled SHF-band devices became available (for instance the FuG 240/E cavity magnetron-based FuG 240 Berlin AI radar).

Transceiving antennas used for  on twin-engined night fighters usually used a  (stag's antlers) eight-dipole array with shorter elements than the previous 90 MHz SN-2 radar had used or as an experimental fitment, the 90°-crossed twin-element set Yagi based  single-mast-mounted array.

Variants 

FuG 216: Experimental series to plan the further development. Installed in Fw 190 A-6/R11 and Bf 109 G-6 The aircraft were used by NJGr 10 until March 1944, after which some machines of 6./JG 300 () were equipped.
 Manufacturer:  (FFO, German for airborne radio research institut in Bavaria)
 R1 version (backwards warning device)
Frequency: 182MHz
Power: 1.0kW
Transmitting and receiving antennas each consist of twin dipoles, mounted under and above the wings, respectively
 Single display device with distance readout
 V version (for single engined night fighters)
Frequency: 125MHz
Power: 1.2kW
Range: 500 to 3,500m
Antennas in the form of spikes or (Fw 190) as "antlers" on right and left wings

FuG 217: Installed mainly in Ju 88 G-6, only a few Bf 110 G-4, He 219 or Me 262 received the . It could be combined with the additional  device to automatically measure the target distance and fire the guns at a set range.
 Manufacturer: FFO
 R2 version (backward warning device)
 J2 version (for single-engined night fighters)
  V/R (combined night fighter and backward warning device for two-engined fighters)
Two switchable frequencies: 158 and 187MHz
Search angle: 120°
Range: 
Spike or "antler" antennas

FuG 218: mass-produced
 Manufacturer: Siemens / FFO
 R3 version (backward warning device)
 J3 version (for single-engined night-fighters)
 V/R version (combined night fighter and backward warning device for two-engined fighters)

Six switchable frequencies: 158 to 187MHz
Search angle: 120°
Range: 
Weight: 
R3 and J3 with spike antennas and V/R with "antler" antennas.
 G/R version (combined night fighter and backward warning device for two-engined fighters)
Only one single device built, replacing the 2kW transmitter with a 30kW transmitter. Range increased to up to . This device was intended for the Dornier Do 335. "Antler" antennas.

See also 
 List of WW II Japanese airborne radar systems

References 

 (1) TME 11-219 Directory of German Radar Equipment
 Gebhard Aders: Geschichte der Deutschen Nachtjagd, Motorbuch publishing corporation, 1977, 
 :File:Neptun FuG 217.pdf

Aircraft radars
World War II German radars
Military equipment introduced from 1940 to 1944