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

 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 above the ground, can be read on the scale of a circulair trace  of a CRT screen. Indicator 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 ft.

It is a pulsed altimeter type. The transmitter has a pulsed output signal.

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.

On the low range altitude , 5000 ft range,  98,356 pulses of transmitter power are sent out each second. (Reference stage),

The transmitter signal has a speed, equal to the light speed.  The electron beam must travel in (1 divided by 98,356)  10.167 uSec around it circular trace on the CRT screen. Direction clockwise. This is precisely the time required for the transmitter pulse to travel to earth and back to the plane at an altitude of 5000 ft. The scale at the CRT screen is calbrated in 0 – 5000.

So if the reflected puls is at 2000, the altitude  is 2000 ft then. When the trace select is at “one time”

This system is a kind of 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. Two pulses are shown on a circular trace  on the CRT.  The transmitter puls or reference puls, zeroed at ground level,  is at the zero point of the scale of 0 – 5000. The reflected signal is shown then somewhere furtheron the trace  the circular trace of the indicator.

Actually there are 2 scales .  To be chosen bij a switch, “one time”and “10 time”.  Eg. 0- 5000 ft and 0 – 50.000 ft. This switch is located at the above-left of the frontpnel.

In both scales, the zero point must be calbrated with the airplane at ground level bij de the knobs “zero one time”and zero “10 times”, located at the right side on the frontpanel.

The accurancy is 50 feet + 0,25 % in all ranges.

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 the transmitter stage, only an oscillator 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 or the transmitter puls. It can also be zeroid at the beginning of the scale. 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.

In the picture below, the transmitter puls  shown at the zero point on the 5000 ft scale. Switch on indicator front at “one  times”.

Note that the puls is somewhat wide.

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In the picture below, the transmitter pulse is shown  in the ‘” times ten” position as a small pulse..


Here the original antenna, type AT-4. One used for the transmitter and one for the receiver. They were  mounted just under each wing of the airplane. Sometimes under the fuselage.

The dipoles antennas for transmitter and receiver, are mostly places each under the wings of the plane. The type number is AT-4. See picture below.


Some pictures of the altimeter indicator, at the lower part of the picture.

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Here above a picture of the I-152 indicator at the Navigator position of a B 17 bomber. Mostly the indicator was placed at the Bombardier position.



Picture left, the  indicator I-152 at the pilot position of a fighter airplane. Normally it was used on bomberplanes, but now a special situation, The fighterplane is also used to protect the bomber squadrons, so flying at large altitude.