Chuff, chuff, chuff, chuff chuff, chuff, squelch! (1)

Christopher Bartram <cbartram@...>

The 'steam train effect' on VHF signals is caused by the signal scattered by a
moving reflector - typically an aircraft - interfering with a signal propagated
over a static path. The 'chuff, chuff, sound is usually caused in an AM receiver
(which includes SSB in this context) by the receiver AGC trying to follow the
relatively slow (~1Hz-ish) changes in signal strength caused by the the two
signals adding and subtracting as their relative amplitude and phase changes.
A similar thing happens in FM receivers below limiting but the mechanism is

Given the usual velocities of aircraft, the doppler shift on the signal
scattered by one is likely to be in the region of 1kHz at 10GHz. That's too
fast for most AGC systems to follow, and the chuff-chuffing becomes a tone. In
practice, because we tend to hear only the signal scattered from the aircraft,
the norm on the microwave bands is to hear a frequency shifted signal, but not
to notice the interference effects. I have heard chuffing on the microwave bands
from (relatively slow moving) cars, and even pedestrians, but they tend not to
high enough to provide useful extended propagation :-)

I'm pretty sure from his description that Tony noticed simple scattering from

Vy 73


(1) FWIW the subject line is taken from the chorus of a 1970's song - the
title of which I've long forgotten - by a very funny man called Mike Absolom.
The song, which had the heroine tied to a railway track, contained one of my
all- time favourite lyric lines: 'She said to me you virile brute, don't be so
eclectic, British Rail have changed their chuff-chuff trains to electric ...'

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