Altimeter transmitter/receiver RT7-APN1.



 This altimeter is designed for measuring constantly the height of the aircraft above ground, terrain during flight. It is also suitable in conjunction with an Automatic Pilot System.

There are more principles of altitude measurements, but the APN-1

a  FM CW radar altimeter.

It is supplied with a double Range Indicator, type AYD 3. One for  till 400 ft and one for 4000 ft.  One ft is equal to 0,3048  meter.

So 400 ft is 122 m, 4000 ft is 1220 m.


Also a Limit Range switch is provided,

type SA 1, to switch for the desired altitude of the aircraft.

When no Automatic Pilot is used, the  Limit Range switch can be used in conjunction with a Limit Indicator, consisting of 3 lamps, red, white and green. All these items are at the dasboard. When not using the Automatic Pilot, a dummy connector is put in the receptable connector at the front of the transmitter/receiver J-106. When not placed and still Automatic Pilot is connected to this receptable by a cable, the change over switch on the Automatic Pilot box must be set on manual. When switched to automatic, these lamps have different functions.

In my installation that dummy is placed and the functions of the lamps are discribed just below.

At the Limit Range switch , the height of the aircraft , which is chosen to fly, or providing a flight with save height, can be chosen with the knob on it. When now the red lamp in the Limit indicator, is lightened, the aircraft is flying beneath the value on the Limit Range switch. When white, the height is about the desired value. When green, the height is above the value of the Limit range switch, so save height. See also the first picture below.

See below the block diagram of the APN-1 Altimeter.




Inter connection diagram picture.


The APN- 1 is a frequency modulated  CW Altimeter. It , just like another altimeter,  SCR 718, determines the time required for a radio wave to travel from aircraft to earth and return. A different method of time measurement is used , which depends on the observed difference in frequency between transmitted and received signal.


If frequency of a radio transmitter is changed rapidly at a constant rate (frequency modulation), the transmitter will change frequency in the time required for a radio wave emitted by it to travel to earth and return. The higher the aircraft, the longer the time required for a round trip and the greater the difference between the transmitter frequency and that of the reflected wave when it arrives at the aircraft. This frequency change is the proportional to the altitude of the aircraft.

If the rate at which the transmitter frequency carrier varies,  is known , e.g. FM modulation, and this  signal is also modulated at a rate of 120 cycles,  is CW modulation, his frequency is varied from his center frequency of 400 Mhz , between 420 and 460 Mhz at lower altitudes of a maximum range of 4000 ft,  the elapsed time corresponding to any observed frequency difference is then established. This frequency difference (elapsed time)  is converted, in an electronic  circuit, which delivers a proportional current to drive  a meter instrument, the Altitude Indicator.

The output of the transmitter is a FM modulated constant carrier of 0,1 watts, which is radiated from a small half wave dipole, located, often,  just under the wing of the aircraft. Each wing carries one dipole antenna, one for transmitting, one for receiving.


The principle of the altitude measurement in the APN-1 will be discribed in the following presentation  below.


Above the C is constant all the time, so  when T in increasing, H is increasing too.






Above: when T is increasing, rate of change carrier frequency  is stable,  the beat frequency is increasing.




Above means: FM modulation of 40 Mc, CW frequency , and  and velocity factor (300.000 km/sec) is stable, then the altitude is proportional to the beat frequency.



Above the principle block diagram of the APN-1.

Below some pictures of my APN-1 unit. Just click on the picture to get a larger one.



My  Altimeter unit RT7 APN-1.

Below the transmitter/receiver. Above left the Altitude Indicator and Range Switch unit. In the mid the 3 lamps  of the Limit Indicator. Just below at that, the 2 dipole antennas.


Transmitter/receiver box. Just watch the inserted dummy connector at the left, when the Automatic Pilot is not in use.


Left the height indicator with 2 scales on it. With the Range switch, switching between 40 ft and 400 ft. Below the 3 lamps of the Limit Indicator.


One of the small halve wave dipole antenna.

Comparison of FM and Pulsed Altimeters.

The APN 1 is a FM altimeter, a SCR 718 is a pulsed altimeter.

They are designed for different purposes. For instance for bomber aircraft, flying at great altitude and for fighter planes flying often at lower altitude. Fighter planes use often the FM units, while bomber planes use the pulsed altimeters.

The FM unit has a very small fixed error while the pulsed unit has negligible  percentage error. The FM unit is intended for better measurement  at low altitude, while the pulse unit is better at very high altitudes. Often you can see at pictures, taken in bombers, like B17 or B 29, the indicator unit of the SCR 718 , type I-152, is to be seen. See picture below.


Just right down to be seen the I-152 indicator at the Bombardier position in a B -29 aircraft.


Here to be seen in the background, the APN-1 transmitter/receiver, in a fighter plane.

In the week of the 28-th of march, 2018, I changed the instrument panel of the altimeter. I putted the antennas on a seperate base. With 2 m of coax cable to the receiver/transmitter. It all worked better, so the demo.

Below the pictures.


The new instrument panel.


The transmitter antenna at 400 Mc.


The receiver antenna.


The red lamp is lighten, that is correct, because the needle of the altimeter indicator is below the installed value of 50 ft at the limit switch.


 The back side of the panel. Note the rare connectors at the instruments.

The demo is going very well, if you move the receiving antenna. The indicator meter and the lamps are responding very quickly. This is correct, because at the functioning of the altimeter  at ground level  is always very unstable.  At bigger altitudes it is more stable.


BC 788 C

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  1.  BC 788 C altimeter transmitter/receiver.

 BC 788 C altimeter transmitter/receiver.

 SCR 718-C.

This is an altitude measuring system, consisting of a transmitter receiver type BC 788 C, SCR 718 C and an indicator box with CRT. The height in feet of the aircraft obove the ground, can be read on the indicator screen (CRT), type I-152-C.

The SCR 718A is an American radar equipment designed to give a true measurement of height above terrain at altitudes from zero to 40.000 feet. 40.000  ft is about 12 km.

It is a pulsed altimeter type.

The principle is obtained by measurement of the time of travel of a radio pulse from the airtcraft to the ground beneath it, and back to the aircraft.

This system is a pulsed radar system. The transmitter sends a pulsed signal from the antenna dipole down to earth. The receiver is picking up the reflected pulsed signal. The reflected signal is shown then on the circular trace of the indicator. When the aircraft is at sufficient height, there is to be seen 2 lobs on the circular trace on the indicators screen. On reference lob at the zero point and one lob somewhere at the trace, corresponding with the height, the aircraft is flying. At the  take off of the aircraft, when getting a height of about just below 50 feet, the reference lob has to be chosen by a knob at the front of the indicator, just at the zero point of the trace. The altimeter is now calibrated.

So there could be chosen 2 ranges on the screen, one till 500 ft and one for 5000 ft height.

The accurancy is 50 feet in de low range and 150 feet in the extended range.

The power supply is 115 volt by 1500 – 2400 Hz. For this supply I use a home build power supply of 115 volt 1500 Hz. This power supply can be used for all kind aircraft equipment which desire a power frequency between 1500 and 2400 Hz.

The transmitter frequency can be tuned from 420 – 440 Mhz. The pulses are transmitted with a P.R.R. of 98,356 per second for each puls transmitted one complete timebase circleat the indicators screen  is effected.

For the transmitter stage, a radio tube type 6J6 is used. A pulsed power of about 8 – 10 watts is delivered to the dipole.


I managed to let the whole installation working a bit. I get a reflection on the screen, the reference lob. But, because the installation is always at ground level in my case, no heights can be measured. But no trouble, it is nice to show this in working order. So it is not fully calibrated at all.



 The front of the altimeter installation, which is in working order.

Left the transmitter/receiver BC 788-c and right the indicator unit I-152 c.

Note, that the  height scales on the screen are circular for an accurate reading.

front BC 788

Here the front of the transmitter receiver. To let it work and having a indication on the screen of the indicator, 2 small whip antennas are used, one for the transmitter and one for the receiver. The connectors in the mid are left for the connection with the indicator and the right one for the power supply of 115 volt – 1500/2400 Hz.

indicator 1

This is the indicator I-152 c. Note the circular trace on the screen. There 2 ranges for reading the altitude. From 0- 500 ft and one for 0 – 4000 ft. ( about 1,2 km and one for 12 km). In this picture, the reflection point near zero is shown on the 500 ft scale.


In this picture, the reflection near zero is shown on the 5000 ft scale. Switch on indicator front at “10 times”.


Here the original antenna, type AT-4, used in my other altimeter equipment, the RT7-APN 1. One used for the transmitter and one used for the receiver. The were mostly mounted just under each wing of the airplane. Sometimes under the fuselage.

Some pictures of the altimeter indicator.

3957-2cmyk (1)

Here a picture of the I-152 indicator at the Navigator position. Mostly the indicator was placed at the Bombardier position.


Picture if indicator I-152 at the pilot position of a fighter airplane. Normally used on bomberplanes, but now a special situation, the fighterplane is flying for large distances, so flying a large altitude.