Poor Man’s MWLW Wideband Noise Reduction


Many radio signals in
the medium and long wave bands (MW/LW) but also shortwave (SW) are
infested by noise of wide variety and of such levels that weak stations
are virtually obliterated. The worst noise is the wideband variety which
stretches across several hundred kilohertz across the band. However the
very fact that the noise is wideband in nature allows it to be
suppressed using a little known and inexpensive method of using a second
receiver tuned just beside the frequency you want to listen to. The
principle is illustrated in the drawing. The second receiver effectively
isolates the noise by adding it in anti-phase to the wanted signal.

As shown the loudspeaker outputs of the two radios are connected in
phase while the loudspeaker in the second radio is disconnected or
removed. With the second receiver tuned a little higher or lower than
the main one, the loudspeaker in the main receiver is fed with the
difference between the two signals. Using the volume adjustment on the
second radio, a setting can be found that should result in considerable
reduction of wideband noise. If an external antenna is used, it may be
connected in parallel to the receiver inputs. Ferrite rod antennas need
to turned in the same direction. Best results are obtained when two
identical radios are used.


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