Classically, the height of an aircraft is measured with a barometric altimeter. Drawbacks are with this type of altimeter are that the indicated height may vary with the barometric air pressure and that it indicates height over sea level, not taking the elevation of the underlying land into consideration. This makes flying blind in a mountainous area particularly hazardous.
In the 1920/30's it was discovered that radio waves could be bounced back from the earth's surface. Since the speed of radio waves is known (= speed of light), if the time lapse between transmitting the wave and receiving the bounced back wave could be measured the height over the earth can be calculated.
A number of different schemes were tried and it was discovered that a frequency modulated system worked quite well. In an FM system the transmitter frequency is continuously wobbled up and down. The receiver will first receive a direct signal from the transmitter, and a reflected signal from the earth below:
The reflected signal has taken slightly longer to reach the receiver than the direct signal. Since the reflected signal was transmitted, the frequency of the transmitter has changed, so the direct signal will have a slightly different frequency. By comparing the two frequencies, a measure of elapsed time, and therefor height over ground, can be obtained:
Because the difference in the direct and indirect frequency is relatively small, a low frequency "tone" is generated in the receiver that can easily be converted into a DC signal for an indicator instrument.
Based on this principle, the Luftwaffe adopted the Siemens FuG 101 system early in the war. The FuG 101 operates from 337 - 400 Mhz (75 - 89 cm wavelength). The system consists of a transmitter unit S101, receiver unit E101, power supply U101 and indicator instrument AFN/101:
The picture shows the S101 top left, the E101 top right, the U101 bottom left and the AFN/101 bottom right. Both the transmitter and receiver are combined with their dipole antenna into stand alone units:
These units could easily be placed inside the wing of an aircraft, with just the antenna protruding below the wing. Here is the Me110 of the Battle of Britain museum in Hendon:
Here the transmitter is placed on the left, the receiver on the right. The distance between the transmitter and receiver had to be kept between 1,4 and 2 meters. The transmitter and receivers were placed in special wing boxes that could be swung open for easy access:
The transmitter has a small motor running at 4000 rpm which wobbles the frequency of the transmitter. The motor configuration is such, that when running in one direction, there is a five times greater frequency variation than when it is running in the other direction. This allows the system to have two height measurement ranges; one for 0 - 150 meters, the other from 0-750 meters. The motor can clearly be seen on the next picture with the transmitter on the top left:
The AFN/101 instrument displays the height over ground directly:
The instrument has a switch to switch between the 150 and 750 meter ranges and a small tuning knob. When pulling and turning this knob, the speed of the wobble motor can be adjusted (as this may vary slightly with the power supply voltage in the aircraft). A red calibration mark at the 60 meter point allows for the motor speed to be set at the required 4000 rpm.
As discussed earlier, the transmitter and receiver units were placed in the wing of the aircraft. The power supply was mounted on a typical mounting frame and could be placed elsewhere in the aircraft:
My FuG 101 is a slightly later and improved FuG 101a version. I have a "Prueftisch" testing rig which holds all the units.
The transmitter and receiver slot into the top of the testing rig with the antennas pointing backwards. The U 101 fits on the frame left bottom and the AFN/101 instrument slots into a special bracket on the bottom right. A number of switches and meters allows the performance of the system to be checked.
Unlike in the aircraft, the rig allows the transmitter to run without radiation RF energy. In this case the speed control of the motor can be tested and also the sensitivity to supply voltage changes. When switching the installation on, you hear the motor in the transmitter above all. I have the antennas pointing out of a window towards a thicket of wood about 50 meters away. Under good conditions (when the trees are in full leave and nice and wet), I can actually just about get an echo and a reading on the intrument. Most of the times however the instrument varies wildly because it will not pick up a reliable echo.
In practice the system worked pretty well over solid ground and water. When flying over woods, the signal would show variations (just like in my current test setup), so care should be taken. The great advantage of a radio altimeter is that it shows true height over ground. If you fly over hilly terrain, the instrument will show you so you can pull up to a safer height. The manual states an accuracy of +/- 10% and states that 0 and 3 meter height are clearly distinguishable. The FuG 101 could therefor be a valuable tool for blind landing and low level flying.
The FuG101 was a successful device, so much so that it was used for a number of years after the war by the French air force. (In fact my testing rig has french text plates on it from this period). French versions of the FuG 101 units can still sometimes be found in the surplus trade.