Radar and the Air War – Part 10 – by John Heys G3BDQ

A Personal View, by John Heys G3BDQ.

Part 10  Counter Measures

Whenever a new weapon is invented and used, a way to nullify its efficiency soon appears. The Stone Age Spear with its flint tip resulted in some high IQ’d genius inventing the shield. So it has been through history, and certainly both sides in WW2 engaged hundreds of scientists and technicians to invent increasingly sophisticated radars and equally sophisticated countermeasures. These countermeasures were not always electronic and could be physical attacks upon equipments and systems. The CH station at Ventnor (Isle of Wight) was heavily bombed and put out of action for a few days but despite such an attack the antenna towers mostly survived.  Self-supporting towers only succumb to bombing when they suffer a direct hit, and even then often just one support leg close to the ground will be destroyed leaving the other three to continue supporting the structure. Bomb blasts close to a structure like a lattice tower do little damage because of the open structure of the tower

The Jamming of CH Radars

The CH superhet receivers suffered jamming from CW, Pulse and Frequency Modulated interference. CW was combated by two narrow band filters in the receiver passband – (Intermediate Frequency Rejection Units or ‘IFRU’). Pulsed jamming was countered with Intentional Jitter Anti-jamming units – known as ‘IJAJ’. FM problems used an Anti-Jamming Blackout Unit -‘AJBO’.

Transparent colour filters over the CRT displays were used to minimise or eliminate unlocked transitory interference. There was also the option of Frequency change and loudspeakers were connected to the CH receiver outputs, which helped to identify the type of interference being received.

The RAF set up a special electronic countermeasure organisation known as J-Watch to monitor all jamming transmissions. There was initially a J-Watch station established at CH Station Ringstead (Hampshire) set up in a hut close to one of the wooden receiver towers with its own range of antennas positioned partway up the tower.  Similar J-Watch stations were located at Stoke Holy Cross (Norfolk), and Dunkirk in Kent. These units employed a variety of receivers including R1155, Hallicrafter S20 and SX27 or 28 and ‘specials’ made at TRE from modified ‘Gee’ equipments etc. The operators at a J-Watch station could monitor frequencies between 20 and 3000mHz. After a time the Dunkirk station moved to Dover Hill (behind Folkestone), Ringstead moved to Durlston Head near Swanage and the Stoke Holy Cross unit went to Bawdsey. The J-Watch stations employed movie cameras to capture the interference on film to help in analysing its nature.  Three German aircraft equipped with jammers often flew along the South Coast, one of which was shot down on the night of 5th/6th June 1944 (D-Day).


This was a very cheap but effective jamming system. It was nothing more than using a microphone located in an engine nacelle to broadcast the wideband noise on the frequencies of the German ground control stations, which having received the information from the ground radars, directed nightfighters to the British bomber stream. The broadcasts were for a time on HF, and the 1154 bomber radio transmitter could be tuned to the German control frequency and so radiate a powerful noise signal on top of that frequency. To overcome Tinsel the Luftwaffe began to use VHF and eventually Medium Wave broadcast frequencies for their fighter control.


The Dictionary states that a mandrel is the shaft of a lathe or a miner’s pick. I cannot see a connection, but perhaps there never was one.
My first introduction to his jamming equipment was in early 1943 when I was on the Australian no. 466 Squadron based then at Leconfield near Beverley in East Yorkshire.  At that time the Squadron flew Wellington bombers, one of which was fitted with a Mandrel jammer. I was intrigued and had a good look at the jammer’s innards and a study of its handbook.  The equipment was quite bulky and measured about 3 X 2 X 2 feet. It had a sweep oscillator and wideband valved amplifier that could put out a strong signal over the 30 to 215 mHz range. I think its output power was in the region of 500 watts CW.  The key to its success was its modulation which was derived from a small over-run noise diode valve. White noise has an enormous bandwidth so Mandrel proved effective against the German Freya ground radars. Mandrel became operational during December 1942 and at first was deployed in Defiant aircraft and used as a screen ahead of our bomber stream. The Defiants from no. 515 Squadron were eventually replaced by the much more effective Mosquito, and towards the end of the War as many as seventy Mosquitos from RAF 100 Group (The electronic countermeasure Group) were used for the Mandrel screen. As many as eight Mandrel sets were installed in each Mosquito and in addition there were powerful ground based Mandrel stations along our coasts. Sadly the Germans changed the frequencies of the Freya radars to ones that were very difficult to jam at first, but these radars working in the 148 to 196 mHz band were successfully countered by new Mandrels which transmitted a very stable 500 Hz or 1000 Hz pulse repetition frequency. These PRFs of the German equipments were maintained to either of these frequencies to within a few Hertz.


A British jammer developed to be used against the German SN2 AI (air interception) equipment was codenamed Piperack.  It was carried by some of the aircraft of 100 Group to give some protection from nightfighters and so limit
Luftwaffe successes against our bombers. Piperack was installed in various aircraft including the American Fortress and Liberators and our Halifax planes.  Each of these types when adapted by 100 Group could carry six Piperacks operating on a range of frequencies covering all those known to be used with SN2. The smaller Mosquito aircraft were fitted with just two of the jammers.

A receiver given the name Serrate was developed by TRE at Malvern which allowed an aircraft to home in on the radar pulses emitted by the German Lichtenstein (Emil-Emil) nightfighters when their SN2 AI sets were working. The code name Serrate was derived from the display on its CRT display. It had two screens one giving a vertical and the other the horizontal direction of the enemy, and its traces had serrated edges like the blade of a saw.


Window, also called ‘Chaff’’ by the Americans and Düppel by the Germans, was the codename of a relatively simple way to confuse or ‘blind’ enemy ground radars by using thousands of strips of metalised paper strips that could be released from an aircraft. Each strip was made from a very thin metal foil coated on one side of black paper strips, each strip being half a wavelength long on the operating frequency of a German ground radar system. The paper backing minimised twisting and a possible short circuiting of the foil conductor which would then be de-tuned. The paper backing additionally allowed the use of a much thinner foil layer and helped preserve the then precious aluminium. The Germans knew the theory of this technique but had not used it in case we retaliated in kind. We too held this point of view for a considerable length of time. Each combatant was afraid that its defensive radar would be put out of action.  The RAF held back the use of Window until the night of July 24th 1943 when it was released during a highly successful raid on Hamburg.  I clearly remember that warm and sunny afternoon when out on the airfield  at Elvington seeing the ‘bombing-up’ by the armourers and loosely tied up bundles of metalized paper strips being loaded into each Halifax bomber close to the chutes that each aircraft carried for the dispersal of leaflets (which were all codenamed Nickel ). We realised at once what this new ‘weapon’ was all about and, noticed that some bundles held strips of a different length, the shorter ones being designed to blind shorter waved radars.  The bundles of Window were dropped down the chutes by either the Wireless Operator or the Flight Engineer at intervals during the run over Germany. This first raid using Window was a great success and out of a force of 728 aircraft only 12 were lost.

Other British Electronic Countermeasures

Cigar was a jammer used against German VHF R/T Nightfighter Control. There was an airborne version of Cigar.

Airborne Grocer was used to combat the German Lichtenstein AI. It had an automatic scanning and re-transmission arrangement and was carried by Fortresses and other Mandrel aircraft. It was modified to jam the Wurtburg radars and had been developed from Ground Grocer.

Boozer was an oddly named RAF tail-warning receiver to counter AI attacks from behind.
Carpet  was a jammer used against enemy ground radars and also had both ground and airborne versions.

Corona was effective in jamming the German R/T Nightfighter Ground Control and allowed false instructions (in German) to be given to Luftwaffe crews.

Dartboard was originally called ‘Lightup’ and was a very high powered jammer used against German Medium Frequency Nightfighter control transmissions from Stuttgart.  It was actually the 800 Megawatt transmitter (Aspidistra) located near Crowborough.

Drumstick was yet another jammer used to over-ride German Nightfighter control.

Jostle IV was an airborne device to combat HF Luftwaffe Aircraft Control.

Moonshine was an airborne responder which when used by just one aircraft could trigger the German early warning radars suggesting that a large bomber force was approaching.

Perfectos allowed airborne homing on to German IFF transmissions by our nightfighters.

Ping-Pong was ground controlled DF equipment used to locate enemy radar stations.

Rayon was an RAF jammer used against the German  Ottokar  nightfighter control.

Shiver was a modified British Transmitter at Droitwhich and later at Start Point in Devon, and it was used at certain times to counter the German ‘meaconing’ of the MF Beacons used by the RAF. ‘Meaconing’ is the masking of a radio beacon or beam by automatic re-radiation on the same frequency.

German Jammers and countermeasures included Flensburg an airborne bomber used against the Monica radars used for a time in our bombers.

Naxos was an airborne receiver that let German nightfighters home on to our H2S transmissions.

There were no doubt other German electronic countermeasures but I do not know of them.

This is the end of my long series relating to the Radar Airwar. It has involved me in considerable research but I found the ‘hunt’ for information fascinating. No doubt my pieces will contain some mistakes so let me now apologise should you discover them.

John Heys (G3BDQ) – Vital Spark April 2008.

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