RADAR: RDF 289 – Performance, accuracy and operational trials
R.D.F. INSTALLATION TYPE 289
Performance, accuracy and operational trials
SECRET 1940-1945 (expired classification)
A report in our archives, dated 17 July 1941 and signed by Max Staal, describes a required redesign of the time-based unit of the “Dutch” RDF set (type 289) and discusses the RDF 289 operational performance:
Method I – Circular Time Base
A. Time Base
A sinusoidal alternating voltage is produced by means of a circuit of conventional design. The frequency is 15 kc/s, corresponding to a measuring range of 10,000 metres (10,940 yards). This alternating voltage is directly applied between the horizontal deflector plates of the Cathode Ray Tube whereas it is applied between the vertical deflector plates with the phase displaced 90 degrees but with the same amplitude. The result is an illuminated circle on the Cathode Ray Tube.
Again, the angular velocity of the line connecting the luminous spot with the centre point of the circle corresponds ONLY to the oscillator frequency (15.000 kc/s) and is independent of the radius of the circle and consequently of the voltage applied to the Cathode Ray Tube. From the 15,000 c/s oscillator (afterwards referred to as the “Mother Oscillator”) is also derived the modulating pulse for the transmitter.
[Refer to the first RDF 289 manual for its design]
B. Calibration
The incoming signals are applied across the gun voltage of the Cathode Ray Tube, thus modulation this voltage and causing an instantaneous shortening and enlarging of the radius of the scan. Consequently, the angular difference between the beginning of the groundwave and the beginning of the reflected one is a determination of the range of the object which has caused this reflection.
C. Accuracy
The accuracy depends on:
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- The frequency stability of the mother oscillator.
- The accuracy with which the time base is a pure circle.
- The degree of eccentricity of the time base circles with respect to the measuring scale.
The estimated range accuracy under service conditions taking into the above sources of error is +/- 40 yards in 10,000.
Method II – Linear Time Base
A new system has become necessary since it is difficult to obtain a satisfactory deep modulation of the scan with the Cathode Ray Tubes now available.
A mother oscillator has been constructed as described above, also producing the modulating pulse of the transmitter. The frequency is again 15,000 c/s.
A. Time Base
The 15,000 c/s oscillation is applied to the horizontal plates of the Cathode Ray Tube only, after it has been delayed in phase by means of a radio goniometer. Now a straight line appears on the Cathode Ray Tube, in this case, however, the luminous point is not moving with a linear velocity.
B. Calibration
By means of the radio goniometer, one changes the phase of the alternating voltage across the horizontal deflectors with respect to the original oscillation to such an amount that the beginning of the reflection is exactly in the middle of the luminous line (the centre point on the Cathode Ray Tube). Thus the range is proportional to the applied phase difference.
C. Accuracy
This depends on:
-
- The stability of the mother oscillator.
- The accuracy of the dividing scale on the radiogoniometer with respect to the actual angle introduced.
This accuracy, however, in this case, is absolutely INDEPENDENT of what type of Cathode Ray Tube is used. This is an improvement on Method I, where it sometimes occurs that no pure circular scan can be obtained due to faults inside the Cathode Ray Tube. In [this] case II, a comparatively small Cathode Ray Tube may be used as only the middle of the screen is used.
Also coupling between the radio-goniometer (which alters the phase linearly with angular position) and the fire control system is simpler and moreover absolutely independent of the construction and possible faults of the Cathode Ray Tube.
Note: with this system also the accuracy in the measured range is +/- 40 yards up to ranges of 10,000 yards.
Sea-Trials on Type 289
General Performance.
No serious difficulties occurred; it became clear, however, that the transmitter valves 4316A (Standard Telephones & Cables Ltd) show a tendency to become soft rather quickly. For this reason, the trials were continued with 4 Philips transmitter valves.
The degaussing of the ship showed to have some influence on the focussing of the C.R. Tube [Cathode Ray Tube]. This C.R. Tube, however, was not magnetically screened, so that is very likely that a screen of mu-metal will cure this fault.
Changing the valves of the first detector involved only small corrections in the tuning (in practice, no correction is necessary).
When changing the transmitter valves, a slight variation in wavelength might occur. The transmitter can easily be re-tuned to its original value, but if the variation is only slight, the first detector can be simply tuned to the new wavelength.
This apparatus proved to be stable; when it was left unused for some days and switched on again, practically no retuning was necessary.
It takes about twenty seconds to put the set in working order.
Measurements.
The set installed in the [HNLMS] “ISAAC SWEERS” has a circular time base. The transmitter pulse has a length of about 3 microseconds, corresponding to 450 m on the circular time base. The circle is modulated by adding the signal voltage to the anode voltage of the C.R. Tube. In front of the C.R. Tube is a circular range scale. The distance to a certain object is given by the angle between the beginning of the transmitted pulse and the beginning of the received reflected pulse.
Measurements made with a Swordfish class aeroplane showed that from 4,000 m (4,400 yards) an echo that can be followed continuously may be expected. For bigger distances, the echo sometimes disappears. Up to 6,000m and more, however, weak reflections may be expected.
The aeroplane mentioned above was flying at a height of about 4,000 feet. A convoy with a balloon barrage was followed up to 6,000 m. A destroyer gave a good reflection at 3,000m, whilst a submarine gave a very weak echo at 2,600m.