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 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.

Posted in AAF Altitude Receivers.

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