Licht/FuG 223.






  Right from it's introduction the pick-up range of the FuG 202 Lichtenstein (AI-radar) of 4 km was deemed insufficient.  A successful interception continued to be dependent on the accurate Himmelbett method of control.  With the introduction of Window, and the subsequent shift in German CONOPS to Wilde and Zahme Sau, the requirement for an AI-radar with a longer pick-up became even more apparent.  In the spring of 1944 German industry was working to perfect the FuG 218 and 220, which would have a pick-up range of about 8 km.  
  At the same time the company RPF by Vienna was attempting an altogether different approach.  The short pick-up range of the Lichtenstein was mainly the result of the low power output (1.5 kW), due to the lack of the cavity magnetron.  If more power could be directed at the target and hence reflected, the pick-up range would increase.  RPF accordingly devised a concept (Licht), where a ground based radar would illuminate the target and the reflected energy would be picked up by the Lichtenstein receiver.  Proof-of-concept trials were conducted using a modified Würzburg Riese (8 kW) and a FuG212, with a light, single engine (low RCS) aircraft (Fw 58) as target.  The trials were most successful.  At a distance of 40 km between the target and ground radar, a pick-up range of 3.5 km was realized with the FuG 212.  At a distance of 15 km between target and ground radar, the FuG 212 picked up the target at 8 km.  Since both the target and the fighter were hit by the same pulse, synchronization and hence range measurement was also possible.  


The principle in "Licht Verfahren".

  During Zahme Sau the Bomber Stream would be tracked by multiple search radars, but seldom, if ever, by W-R type radars.  The search radars worked on a much lower frequency than the W-R, so RPF designed the system FuG 223, which it intended to produce in two versions.  X-Halbe lang working in the frequency band of Freya (10 - 36 kW), Freiburg (15 kW), Mammut (200 kW), Wassermann (100 kW) and Jagdschloss (150 kW) and X-Halbe kurz, working in the frequency band of W-R (the figures in parenthesis indicates the power out-put) .  The antenna for the system were mounted at the wing-tips like those of the FuG 227 Flensburg.  If I read the German description correctly, no synchronization was attempted with the X-Halbe - for good reasons.  It would have been extremely difficult to coordinate the action of one fighter on a Zahme Sau mission with one specific ground radar.  Please see map below.  The concept of the X-Halbe seems to have been, that the Bomber Steam, floodlighted as it was with radar energy, would reflect sufficient radar energy for a usable signal to be picked up.  Synchronization with a Jagdschloss type radars, and hence range measurement, would have been quite possible just by tuning the X-Halbe frequency to that of the Jagdschloss.  
  The X-Halbe lang covered the frequency bands shown below with the corresponding ground radar.  
Frequency (MHz). Ground radar.
60 - 95 Freya LZ and Köthen.
75 - 120 Freya LZ, Köthen and Freya.
110 - 175 Freya, Freiburg, Mammut, Wassermann and Jagdschloss I.
160 - 250 Freiburg, Jagdschloss II
  The X-Halbe Kurz covered the frequency bands 250 - 450 MHz and 400 - 600MHz, the last of course being the band of the W-R.  The first band was only used by the Kriegsmarine.  
  The pick-up range of a X-Halbe of a bomber being illuminated by a Wassermann M can be illustrated as follows.  Assuming that the RCS of a bomber was 10 times that of the Fw 58, this would result in an increase in pick-up range of 1.8.  Since the power output of the Wassermann is 12 times that of W-R this would increase the pick-up range with 1.85.  With a distance of 40 km between the target and the Wassermann, this would result in a pick-up range of 11.5 km - if my math is right.  This would be as good as or better than the FuG 220.  And since the aircraft in the stream were illuminated by multiple radars this is a conservative estimate.  

The map above depicts a fictitious situation in late 1944.  A bomber stream of about 600 aircraft is penetrating German airspace.  Depicted on the map are the location of Stellungen within 100 km.  If all radars in that area are tracking the stream - and especially the lead portion of the Stream as was the German procedure - the leading aircraft could theoretically be illuminated by about 50 Freyas, five Jagdschloss and one Wassermann.  But please bear in mind that only a portion of the energy is reflected back at the transmitter, or towards a night fighter equipped with X-Halbe, and that not all ground radars would single out the same aircraft for tracking.
When the war ended one pre-production model of the X-Halbe had been completed.