Two antennas are used, one under each wing.
One is connected to the transmitter, the other to the receiver.
The APN-1 had sinusoidal FM, using a voice coil driven membrane
as variable capacitor in the transmitter. The AM210, introduced in 1952, had the same frequency range, but triangular FM, made by a dc motor-driven butterfly capacitor. The advantage is a constant beat frequency, proportional with altitude, and less sensitivity for Doppler shift.
In this graph, the solid line is the transmitted frequency versus time, whereas the dotted line is the frequency of the received echo.
BW = peak-to-peak frequency deviation
T = time to deviate from min to max,
dF = frequency difference between received and actual transmitted signal
c = speed of light ( 3.10 ^ 8 m/s = 983 feet / us)
The altitude H is then found from : 2 x H = c x dF x T / BW
A dc motor produces the FM with two, centrifugal contact-stabilised speeds:
4000 rpm (T/BW = 3.75ms/40MHz) 0 ft— 2000 ft
800 rpm (T/BW = 18.75ms/40MHz) 100ft—10000 ft
The combined length of the coax cables to the antennas, and the height of the antennas above the tarmac are over 20 feet, so -even with an ac-coupled amplifier- the altimeter measures straight from 0 altitude. In fact, the dF amplifier in the AM210 has a pass band from 500Hz to 50kHz, the gain rises with the square of frequency to compensate for the weaker echo at higher altitude.
The circuit diagram of the AM210 with some description is here
Much more background ( in Dutch) is here
The wiring (under) side is shown with the transmitter in the gray box top left, and the power supply on the right.
The unit was known in Sweden as PH-11/A
*) The 420-460MHz band was abandoned in 1963, when radio altimeters went to the 4200-4600 MHz band
Instrument landing systems
Radio Altimeters based on CW FM were developed during WWII, like the AN/APN-1 in 1943 . A small trans- mitter inside was frequency modulated with approx.100Hz, shifting the carrier frequency continuously between 420 and 460MHz. *)
The receiver measured the frequency difference between the transmitted signal and the signal returned from the ground, having the transmit frequency from a few us earlier.
AM210 radio altimeter