Metox R203 and R600 VHF Radar Detectors

by Erik Ludwig

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Part 2.2
Intro

Following its invention in the 1930s, WWII was the first war in which radar was deployed by Navies, Armies and Air Forces. The Metox receivers played their modest role in this 'war within the War' Let us briefly examine what radar is, before we deal with radar receivers.

Radar
A radar transmitter emits short pulses of radio energy in the direction of an object, for example a ship, which reflects part of the energy back to the sending ship where a receiver (radar receiver, not the Metox receiver) detects this 'echo', then delivers it to the radar indicator unit where a CRT display shows the echo as a vertical deviation of the timebase. The distance between the origin of the timebase and the echo (the time difference) gives the distance between the radar and the ship which reflected the echo. (Note: this description relates to the original 'A Display' format. Various other display formats were developed later, including the 'Plan Position Indicator' or PPI, in which received echoes are displayed in the form of a map centred on the radar station.)

In one second the pulse travels 300,000km, which corresponds to a two-way path of 150,000km. Most German radars used 500 pulses per second (PRF - pulse repetition frequency 500Hz) which gives a maximum range scale of 300km (150,000 divided by 500). The German Navy also used 1250Hz (120km) and 3750Hz (40km).
A distant ship will reflect the incoming radar pulse in all directions, with only a very small part of the energy being reflected towards the sending station. To combat this the designer can increase the power of the emitted pulses and concentrate these pulses in a beam as narrow as possible. This entails the use of very high frequencies (low wavelengths), allowing the use of small narrow‑beam, high-gain antennas. The German Navy opted for the use of dipole arrays and vertical polarisation. The British Navy probably opted for Yagi beam antennas and horizontal polarisation. Vertical polarisation, when used against sea targets, has the disadvantage of increased interference from the surface of the sea (sea clutter) as compared to horizontal polarisation.
In the 1930s final amplifier valves for very high frequencies did not exist and had to be developed. The German Navy had opted for a rather high carrier frequency for its radars, 168MHz, and had to use the only available valves, two TS 1 triodes in push‑pull to obtain a maximum output of 1kW pulses. New transmitters running 8kW output became available in October 1942. During the later war years radars could be fitted with 125 and 400kW transmitters. Many German surface ships used the 168MHz frequency till the war end, but around August 1944 a considerable number of ships, ranging from MTBs to the heavy cruiser Prinz Eugen, were fitted with 9cm radars, panoramic indicators and small antennas in radomes. radomes. 
The British Navy started at the very low frequency of 43MHz with a power of 20kW, soon increased to 70kW. These first radars were followed in 1941 by radars on 90MHz having outputs of 350kW during long-pulse operation and 1000kW during shortpulse. Another new radar, type 284, became available in 1941, on an even higher frequency (600MHz) and 25kW.
The German Navy radars were practically all 'revolving', that is to say their antennas could be manually rotated to find azimuth by monitoring maximum echo strength. Only the 9cm radars had continuously rotating scanners.
The British Navy started at the very low frequency of 43MHz with a power of 20kW, soon increased to 70kW. These first radars were followed in 1941 by radars on 90MHz having outputs of 350kW during long-pulse operation and 1000kW during shortpulse. Another new radar, type 284, became available in 1941, on an even higher frequency (600MHz) and 25kW.

The German Navy radars were practically all 'revolving', that is to say their antennas could be manually rotated to find azimuth by monitoring maximum echo strength. Only the 9cm radars had continuously rotating scanners
After the Battle of the Atlantic the Captain of the Prinz Eugen stated his belief that British radars had a greater range than those of the German Navy. He also recommended that radar should be used sparingly, since a radar transmission discloses your position, and that it should be switched off after a few minutes.
At the beginning of the war the German radar range against sea targets was only 8km. Improvements increased this to 25km, just below the horizon distance, and the use of bigger antennas allowed the maximum to go up to 40km at the end of the war.
The radar waming receiver is not connected to the radar installation. It is a separate receiver which is just one of the many communications receivers on board a warship. Its particular feature is its frequency coverage: VHF and UHF The purpose of the radar receiver is to find enemy signals on the very high frequencies, especially radar pulses, but also radar jammers or enemy ship-to-ship communication. It is a passive device. The enemy cannot hear it because it does not enit a signal of any kind.
WWII Radar Warning - On Submarines After the war numerous authors dealt with the so called 'Metox affair', where the receiver was used on board Gerrnan submarines for passive warning against approaching British aircraft. In this application the Metox acts as an efficient passive defensive device. It hears the the ASV2 pulses before the aircraft gets an echo of the submarine

WWII Radar Warning - On Surface Ships  

The Metox receiver was considered to be capable of a receiving range of at least twice the geometric (horizon) distance of about 25 to 30km. This specified capability was verified at the acceptance test of the receiver. This means that if the radar of ship A gets an echo from a ship B at 20km distance (before the horizon), the Metox at B tuned to the radar pulses of A must still hear these pulses from ship A when B has moved to a new position 60km from ship A (beyond the horizon), where the radar of ship A has already lost the echo of ship B. The above mentioned factor of two is conservative since the strong kilowatt pulses under normal tropospheric propagation can be easily received at ship B at more than 60 km. The superior performance of the Metox receiver compared to the receiver of the radar equipment is explained by the difference between detecting a two-way, greatly weakened, reflected signal and detecting a direct non­reflected signal.

  The Metox receiver is therefore a passive indicator of enemy ships at certain bearings (when using appropriate antennas) but is not, in principle, capable of giving precise distance information. Approximate indication of distance can be obtained by monitoring the received signal level, for example by using the magic eye fitted in the last R600 production series. Radios also play a part in electronic warfare. Let us examine the radio equipment available, for example, on board the heavy cruiser Prinz Eugen which escorted the battleship Bismarck during the Battle of the Atlantic. Prinz Eugen (call sign "PG") carried several radio systems, each service having its own radio station called a radio centre (Funkzentrale). The B-Dienst service comprised five receivers to monitor enemy signals between 75kHz and 25MHz and two 200W transmitters, the latter possibly for fake transmissions and jamming between 100kHz and 75MHz. The R203 and R600 were installed in the observers' hut on the foretop. Two radio centres, each equipped with two 800W transmitters and three receivers, took care of the telegraphic traffic of coded messages on long and short waves. The weather service used three receivers to take the WX bulletin.
Two special services took care of traffic with other German Navy ships and with the embarked aircraft during reconnaissance flights on long and short waves. Intercom VHF AM mobile transmitter/receivers were used to communicate between the various work stations on the ship and for shorthaul communication with Bismarck. directional hydrophone arrays should also be mentioned. The Prinz Eugen had ten wire antennas switchable between the radio centres. Various HF and VHF radiators were available. The wire antennas could be rapidly fastened to the ship's superstructure, between the central tower mast (28m) and the after mast (50m), the distance between the two masts being about 35m. A 12m high vertical fan shaped antenna was used for the long waves. Before entering port all antennas were taken down.
Finally, a few words about the radio traffic on both sides. Since each had broken their opponent's coding sys­tems, they were obliged to monitor day and night, the frequencies of the German (Paris) and British Naval Commands (London) and the frequencies of the ships participating in battle actions. In order to preserve some secrecy of radio traffic, the radio operators in both shore and ship stations could change frequency several times during the transmission of telegrams. In 1941, with near minimum solar activity, propagation over about 2500km was poor, with dead bands and deep fading. It seems the Germans sed also a relay station (code name: ‘Anton') when needed.

 

by Erik Ludwig 
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Updated: 03/16/07 © hgs 03/02
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