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In tune

Question: While listening to the radio (on a sound system) tuned to 98.6 FM,
I realised that the radio signal could be switched on and off as I moved to
different parts of the room.

It was so sensitive to my position that by sitting on a chair about 6 metres
from the radio it was possible to switch the sound on and off simply by leaning
forwards and backwards. What caused this phenomenon?

Answer: It sounds as if you are using an aerial which is in the same room as
your sound system. In addition to receiving the signal direct from the
transmitter, such an aerial will also pick up signals reflected from the walls,
ceiling and floor of the room, and from objects in the room, including a human
body.

These various signals will interact to form an interference pattern, with the
result that the signal strength will vary with the position of any people and
the aerial in the room. The wavelength of a signal at 98.6 megahertz is about 3
metres, so a significant phase change can be produced by a person in a room
moving less than a metre.

The signal is reaching the aerial directly from the transmitter and also from
your body. When these two signals reach the aerial they add together, and the
signal the aerial picks up is a result of this addition.

Because the signals are sine waves of the same frequency, the strength of the
signal produced by adding them depends on the relative phases of the two signals,
the phase difference being
caused by the fact that the two signals have travelled different distances from
transmitter to aerial because they took two different paths.

If the path lengths differ by an odd multiple of half a wavelength, then the
signals arriving at the aerial will be exactly 180 degrees out of phase and the
resultant signal, if the two original signals are of exactly equal strength,
will be zero.

This shows how you are able to vary the strength of the signal, but how is it
that you can switch the sound on and off? Well, most FM receivers include a
circuit which mutes the sound output when the signal falls below a certain
strength—this is intended to eliminate background hiss when tuning between
stations.

What is happening is that, as you move about the room, you are varying the
signal above and below the strength at which the muting circuit operates, and
hence turning the sound on and off!

Of course, all this illustrates the importance of using a good outside aerial
for good-quality FM radio reception. Unfortunately, satisfactory results will
rarely be obtained with an aerial in the room.

Richard Davis

Merlin Communications International

London

Answer: The reason for your problem, in which you are not unique, is that FM
radio signals, of wavelengths of about 3 metres, are being reflected off the
walls, floor and ceiling of the listening room and, of course, off you.

By trial and error, you can place your body in a position to cause this
cancellation effect or in a place where you can hear the signal clearly.

However, this phenomenon is not to be confused with the complaint, often
heard from radio listeners, that their signal goes off-tune when they move away
from the radio. That problem, quite widespread at one time, was caused by a
well-known make of radio suffering from cross talk between its AGC (automatic
gain control) circuitry and its AFC (automatic frequency control) circuitry.

In this case, moving the body changed the signal level reaching the radio by
the means described above, but the radio then altered the tuning position by
itself in an attempt to compensate for the problem. Needless to say, this was
considered by the owners of the radios concerned to be a fault of FM reception,
and thus the responsibility of the broadcaster—in my case the BBC.

Charles Hope

formerly of BBC Engineering Information

Woodside, Surrey

This week’s questions

Double trouble: I recently purchased a box of eggs, each of which was
guaranteed to have two yolks. And the claim was proved correct. How does the
supplier ensure that each egg has two yolks?

John Crocker

Solihull, West Midlands

Golden age: What factors influence the intensity of autumn colours. I’m
thinking particularly of areas of Canada, Maine and Japan that are renowned for
the wonderful colours of their leaves in autumn.

These colours are considered to be far more intense than in other areas: is
this anything to do with the species of trees in these areas or the soil in
which they grow?

JoAnn Marks

Halifax, Nova Scotia, Canada

Golden gripe: Autumnal red and gold leaf colours are reputedly caused by
cold-induced changes in the leaves’ sugars.

In Jamaica we see similar colour changes in almond leaves in the intense heat
of July. It’s not the cold, so what is the reason for this?

Edwin Besterman

Kingston, Jamaica

Metal detector: A member of my family is allergic to aluminium, and we have
been advised by his doctor to remove aluminium cooking utensils from our
kitchen.

We have tried using a magnet to help us to determine the difference between
stainless steel and aluminium, but it wouldn’t even stick to some items that we
knew to be steel. Is there another way of finding out which items are aluminium
and which are stainless steel?

Andrew Easter

Wakefield, West Yorkshire

Mercury rising: On a recent flight, I was studying a card listing items that
were prohibited by airlines. I was amazed to see that I couldn’t take a mercury
thermometer on a flight. Why on earth not?

Rick Eraho

Cleckheaton, West Yorkshire

Blue blooded: I have heard that some sea creatures such as horseshoe crabs
have blue, copper-based blood. Why is this, and what advantage does this kind of
blood give them over creatures with the more common red, iron-based blood, like
ourselves?

Do any creatures have blood that is based on metals other than iron or
copper?

Kiyotaka Tanaka

London