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Radio Broadcast
FIG. I
A highly developed shielded receiver
It has recently been brought to the writer's attention that there is now available for amateur use a novel coupler employing a separate secondary load coil, which may be applied to this circuit. If the circuit is employed and the apparatus shielded, an extremely sensitive and selective receiver is available for long distance Radiophone reception, which should enable us to enjoy broadcasting without undue interference from nearby radio telegraph stations.
With the apparatus generally in use, a great many of us are bothered by interference from nearby commercial and amateur stations while receiving Radiophone broadcasts. In cases where the interfering stations are located close to the receiving station the antenna effect of
the receiver wiring is very marked. Upon first thought one would think this phenomenon helpful, whereas in truth it is really detrimental.
Let us consider the case of the receiving set acting as an antenna. Unless the receiver, especially one having a large wavelength range, is equipped with "dead-ending" devices on the windings, the adjustment of the inductances, more particularly the primary, will have very little effect upon the intensity of the incoming signals.
Assuming the normal intensity of a near-by spark station when tuned in to have a value of loo, if we disconnect our antenna, it will be found that we can receive the station over a comparatively wide range of adjustments with
THE "ULTIMATE" COUPLER
THE SEPARATE SECONDARY LOAD COIL
an intensity of perhaps 10. This would be sufficient to interfere seriously with broadcast reception. In the case of the usual commercial spark station at a distance of 5 miles, and on a wavelength of 600 meters, and a broadcasting station at a distance of 10 or 15 miles on 360 meters, the above example is a fair estimate of the relative intensities.
Thus we see that the least intensity with which we can normally receive the commercial station even without the antenna, is 10 when we are tuned to the broadcasting station on 360 meters. It follows that we shall be unable to tune the spark station out entirely no matter how selective the receiver.
Now, if we can prevent the receiver itself from "picking up" any signals, we shall at once greatly reduce the intensity of the interference and the selectivity will depend entirely upon the inherent characteristics of the antenna