Audioclinic (Mar. 1980)

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Edge-Wound Voice-Coils

Q. Several loudspeaker manufacturers make a point of stating that their speakers employ edge-wound ribbon voice-coils. What is the advantage of ribbon voice-coils over the ordinary round-wire voice-coil?

--Name withheld.

A. One advantage of the edge wound voice-coil is that it is light and thus does not add mass to the speaker system. Also, such a coil can radiate heat more efficiently.

Receiving Distant FM Signals

Q. Is there a device for FM/FM stereo which is analogous to devices for AM that make it possible for AM radios to receive signals up to 1,000 miles away?

-Merritt E. Tilley, III, Syracuse, N.Y.

A. There is no such device for FM, and the reason is found in the differences between FM and AM propagation. AM signals, because of their relatively low transmitting frequencies (not because they are AM), can reflect back to earth and be heard at great distances. The ionosphere acts almost like a mirror. Note, however, that this action does not occur in daylight hours when the signals are absorbed rather than reflected back to earth.

Such signals can be reflected to a given location at surprising strengths.

This makes it possible for even relatively poorly designed AM radio sets to "hear" these signals, provided they are not located on channels adjacent to or shared by strong local stations.

The mechanism by which FM signals can at times be transmitted over greater than normal distances is different.

Signals can be reflected from meteor showers, for instance, or from atmospheric dust. The latter is known as "tropospheric scatter" or "forward scatter." Meteor showers are not reliable; signals tend to fade in and out so rapidly that the program is really not usable. Tropospheric scatter can be quite effective, but to take full advantage of it calls for a really directional antenna, and a means of getting the exact antenna elevation as well as azimuth.

The most common reflections are those associated with the approach of a weather front. It often happens that warm air is trapped below a layer of colder air, making it impossible for the warm air to rise as it normally would.

This creates a region in which there is a change of "optical density." Under these conditions, reflections must occur to the degree that this change of optical density occurs. Because Syracuse, where you live, is located on relatively flat terrain, you should, during spring and fall, sometimes hear signals from as far away as Michigan, Canada, etc. These signals will in some cases be so strong that you will wonder why you do not hear them all the time.

In general, FM signals are not reflected back to earth and if the receiving antenna is not in the line of sight of the transmitter, the signal will not be heard. These signals do not tend to follow the curvature of the earth; they travel out into space.

The most reliable method to use if you wish to hear a somewhat distant FM station is to raise your antenna.

This means that your line of sight is now greater than it was with the antenna in its original position. Hence, you are now capable of receiving more distant signals.

It is also a good idea to have an antenna with directivity. One of the properties of such antennas is that they have gain (like an amplifier) over the conventional dipole antenna. Because the antenna favors one direction rather than "hearing" signals in a full circle, it tends to exclude signals which arrive at the antenna from other directions than that of the desired signals.

Given a good outdoor antenna which has good directivity, you should hear signals from as far as 150 miles away at any time, provided there are no major obstructions between you and the desired stations. You need to use a relatively "selective" tuner or receiver so that local signals will not wipe out "alternate channel," weaker signals.

Of course, any good antenna will strengthen local signals as well as the weaker, desired ones. Making the antenna directional, however, will tend to attenuate local signals unless their transmitters are located in the same direction as that of the desired, distant signal.

(Source: Audio magazine, Mar. 1980; Joseph Giovanelli )

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