RADAR: Manual of the RDF 289 SET
R.D.F. INSTALLATION TYPE 289 (manual)
TOP SECRET 1940-1945 (classification withdrawn)
[A translation of the Dutch original document dated 1941]
N.B.
- This manual is intended as a guide for commanders and those officers who must take note of it by virtue of their service.
- It must always be kept locked away when not in use.
- It may not be taken from the ship or the establishment to which it was issued without the permission of the C.O.
- It was compiled by the Dutch R.A.P. service and is the property of the [Netherlands] Ministry of the Navy.
The [RDF 289] installation comprises a transmitter, receiver and antenna.
The antenna is connected to the transmitter and receiver by a high-frequency cable. The antenna consists of 2 Fishbone aerials, which are connected in parallel. The transmitting antenna is also used as a receiving antenna.
The device itself consists of 3 panels, which are screwed into an iron frame.
The first panel (leftmost) contains the transmitter and the first stage of the receiver.
The second panel (middle) amplifies the received signal. This panel also contains an oscillator for 15625 c/s, an oscillator for 1000 c/s, a pulse modulator, and a valve to block the receiver.
The third panel contains the power amplifier and the cathode ray tube.
A power supply unit supplies the required DC voltage. This is 1,400 Volts for the transmitter and 400 Volts for the rest of the device. The whole is powered from the alternating current network 220 Volts, 50 per[iods]/sec.
OPERATION.
The transmitter transmits for a very short time, namely a few microseconds. This transmission pulse is emitted by the antenna. An object located in the path of electromagnetic radiation will reflect part of the captured energy. The reflected beam is received by the receiving antenna. The time that has elapsed between the transmission of the pulse and the reception of the reflected pulse is directly proportional to the distance of the object from the antenna. Electromagnetic waves (e.g. light waves) propagate at a speed of 3.108 m/sec. Suppose an object is located at 10,000 metres. The time that the transmitted pulse needs to travel the distance is: 10,000/3.108 = 33.33 micro sec.
The total time elapsed between transmitting and receiving the pulse is 66.67 microsec. For an object at a distance of 5,000 metres, this time is 33.33 microsec. Measuring distances is thus reduced to measuring times.
This time is measured using a cathode ray tube. On the screen of the tube, we see a circle. This is created by a luminous point that moves around at a constant speed.
Point A marks the point where the transmit pulse begins. Point B is where the pulse ends. The transmitter operates during the time that the luminous point needs to move from A to B. The reflected pulse is marked by points A’ and B’.
The time is now determined by the arc AA’ – arc BB’ ([example] 90 degrees).
The circle frequency is 15,000 per/sec. The time that elapses between the transmission and reception of the pulse is 90/360 x 1/15,000 sec = 16.67 microsec. In this time the electromagnetic wave (in air) covers a distance of: 16.67 x 3.108 x 10-6 = 5,000 m. The measured object is therefore at a distance of 2,500 m. (The pulse must namely cover this distance there and back).
DESCRIPTION.
TRANSMITTER
The transmitter contains 4 so-called apple valves. These are connected in parallel. Between the plate and the grids of these valves is a so-called “Kolster circuit”. [Editor: A Kolster circuit is an LC circuit with a very high Q (quality factor)]
The transmitter valves can be changed by unscrewing the cover of the transmitter and pulling the valves out of the valve bases.
When changing the valves, it is possible that the wavelength changes. This must then be corrected. The wavelength in the rear R.D.F. cabin is approximately 70.5 cm [~426 MHz]. That of the device in the front R.D.F. cabin is approximately 69.5 cm [~432 MHz].
A wave meter is present to measure these wavelengths. This device consists of two parallel brass rods soldered together at the ends. A brass short-circuit block can be moved along these rods.
If we hold this device near the transmitting loop and move the short-circuit block, the grid current will decrease at a certain position of this block. We now read the wavelength at the position of the short-circuit block.
Another method is to weakly couple the wavemeter to the transmitting loop and hold it close to the neon stabilizer valve. When the tuning is correct, the stabiliser valve will light up red. The grid current meter is the second meter from the top.
If the wavelength is not the right value, we remove the 4 transmitting valves from the device and unscrew the end plate from the Kolster circuit. Below are some mica plates. If the wavelength is too long, we remove a mica plate. If the wavelength is too short, we add a mica plate. Then we screw the device back together and measure again. Of course, instead of a full disk, we can also insert a smaller disk of mica to accurately adjust the wavelength.
When screwing on the lid of the Kolster circuit, we must ensure that no mica remains between the middle leg and the lid, which would prevent good electrical contact.
The transmitter valves are the weakest part of the device. It happens that the filament of one of the valves breaks. The transmitter can operate with 3 burning valves. Usually, the wavelength changes slightly. It is best to replace the broken valve with a new one.
A second error is that a transmitter valve becomes “soft”. This means that air has entered the balloon. In this case, the valve gives a purple light. The transmitter is now not working and this valve must be replaced with a new one. If this happens during operation, the high voltage must be switched off immediately. The legs must be removed from the old valve and screwed onto the pins of a new valve, to have as many spare valves ready as possible.
The transmitting loop and antenna loop (see Fig. 3) must be set at a certain distance from each other to get maximum energy into the antenna.
The distance from the antenna loop to the receiving loop must also be set as favourably as possible. The tuning of the antenna (see Fig. 3) can be done by moving the short-circuit block.
The anode current of the transmitter is read on the top of the 4 meters on the left of the transmitter panel. The grid current is read on the 2nd meter. This is roughly set by setting the transmitter loop and antenna loop at a certain distance and subsequently tuning the antenna frequency setting.
Note that the pulse length, visible on the cathode ray tube, is approximately 400 meters, so approximately 3 microsec. long. This length depends partly on the coupling between the antenna loop and the transmitting loop.
RECEIVER (First detector)
The first stage of the receiver (also called the first detector) is located on the transmitting panel.
The received signal has a frequency of approximately 426 Mc (432 Mc). The auxiliary oscillator is tuned to approximately 413 Mc (419 Mc). This gives us a difference frequency of 13 Mc.
After checking the tuning of the transmitter, the auxiliary oscillator must first be tuned. This must be set to approximately a 2.7 cm longer wavelength than the transmitter wavelength. This is done by opening the door of the first detector and holding the wave meter at the anode loop of the auxiliary oscillator. We move the short-circuit block until the anode current of the auxiliary oscillator increases. This is the bottom meter of the 4 meters on the transmitter panel. In this way, we find the wavelength of the auxiliary oscillator. Then we turn the tuning of the first detector until the noise comes on the receiving half of the circle on the cathode ray tube. The current of this valve can be read on the third meter from the top.
The sensitivity can be adjusted with the feedback knob.
CENTRE PANEL
The centre panel contains the medium-frequency amplifier. The bandwidth is approximately 3 Mc [error in the manual – should be 13 Mc]. The gain can be adjusted with the knob marked “gain”.
The centre panel also contains the oscillator for the circle frequency, 15,000 c/s. This can be switched on or off by the button marked “circle freq.”
The size of the circle can be adjusted by the corresponding button. The block valve serves to block the receiver. This means that the receiver only amplifies for half the time. This time corresponds to the sensitive half of the circle. The noise in this area can be adjusted with the “gain” button of the intermediate frequency amplifier. With the “Phase shifter” button this range can be shifted.
This panel also contains the pulse modulator. This gives a positive voltage peak, which is placed on the grids of the transmitting valves. During this peak, the transmitter is active.
Finally, this panel contains an oscillator for 1000 c/s. With this tone, the transmitter can be modulated. The occurrence of reflections can not only be seen on the cathode ray tube but can also be heard in headphones. This tone can be switched on or off by a switch. A rectifier is mounted on the centre panel, which supplies the DC voltage for the cathode ray tube (approximately 800 Volts).
Finally, the centre panel contains a m.A. meter with a switch, which can be set to 8 positions:
| Position | Part | Meter indication |
|---|---|---|
| 1 | The first valve of the mid-freq. ampl. EE50 | 1,0 |
| 2 | The second valve of the mid-freq. ampl. EE50 | 1,1 |
| 3 | The third valve of the mid-freq. ampl. EE50 | 1,2 |
| 4 | Pulse modulator | 1,65 |
| 5 | Block valve | 1,1 |
| 6 | Oscillator 1000 c/s | 0,35 |
| 7 | Amplifier 15,000 c/s | 1,25 |
| 8 | Oscillator 15,000 c/s | 0,3 |
THIRD PANEL
This panel contains an amplifier that further amplifies the signal coming from the intermediate frequency amplifier, after which it arrives at the cathode ray tube. The DC voltage of the tube is modulated with the signal. This amplifier contains: a mixing valve amplifier terminal and a power amplifier (2 EL35 in push-pull). The currents are read on the m.A. meter. This meter has a switch that can be set to three positions:
| Position | Part | Meter indication |
|---|---|---|
| 1 | Mixing valve | 12 m.A. |
| 2 | Amplifier valve EE50 | 10 m.A. |
| 3 | Power amplifier valves | 55 m.A. |
A so-called phase shifter is connected to the cathode ray tube and creates the circular time base on the tube. The voltage on the horizontal and vertical plates is 90° phase shifted. This can be adjusted by the button marked: “setting circle”.
The filament current of the cathode ray tube is adjusted with a variable resistor. If a new tube is placed in the device, one must check and adjust the value of the filament current of the new tube.
Under the cathode ray tube, there are 4 marked buttons: vertical shift, horizontal shift, focus and brightness.
A round distance scale is fitted to the front of the cathode ray tube, running from 0 to 10,000 metres (some 10,940 yards). The centre of the luminous circle must coincide with the centre of the scale division. The zero point of the scale must coincide with the start of the pulse.
The distance is now given by the arc running from the beginning of the transmit pulse to the beginning of the received pulse. This distance is found at the scale division. This distance can be turned into a range transmitter that transmits the distance to the [Hazemeijer fire control of the twin mount 40 mm] Bofors [L60] machine guns.
POWER SUPPLY UNIT
The power supply unit supplies 400 Volt DC at approximately 500 m.A. and 1,400 Volt DC at approximately 60 m.A. The voltages are adjustable using 2 control knobs. The voltages can be switched off using switches B and C.
Behind the door on the left side of the unit are 4 rectifier valves. The fuses are also located here.
Replacing valves.
- Changing of the transmitting valves, see manual, page 4. et seq. [above]
- First detector.
The valves can be changed when we open the door on the left side of the device. First, the plug of the connection cable with the medium frequency amplifier must be pulled out. Then the door can be opened. The valve of the auxiliary oscillator is very easy to exchange. However, the valve of the first detector is much more difficult to change. The best approach is to unscrew the entire first detector from the transmitter panel. For this, 3 connection terminals on the back of the transmitter panel must be unscrewed. Then the 8 screws that fix the device to the transmitter panel must be unscrewed. The lamp of the first detector is now easy to exchange. - Mid-frequency amplifier. This amplifier contains 3 valves (EE50). These can be easily exchanged on the front of the centre panel.
The other valves that are located in the centre panel can be exchanged if we open the door in the centre panel.
Sources
- Handbook R.A.P. Apparatus Type 289 (the Dutch apparatus), Very Secret (expired).
- Drawings: Museum Waalsdorp Van Soest archive; folder Von Weiler