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.