Q John, N7GXD, asks, “What causes the subharmonic or image signals that I hear when I am tuning below 400 kHz with my Kenwood TS-140S? What can I do to clean it up?”
A If it is any consolation, the problem is not confined to the TS-140S. At very low frequencies, most modern digital receivers don’t receive very well or at all. This is due to a combination of factors such as filter roll-offs, PLL lock range, spurious mixing of oscillators and mixer responses. If you were to go through the LO and first mixer stages and carefully shield (keeping appropriate impedances in mind) and reroute connections (wires and cables only), you might be able to get rid of some of the extraneous “noises,” but the rig will never hear as well there as it does above 1 MHz. This is one of the reasons we pay special attention to low frequency sensitivity figures in all the radios we currently evaluate for QST “Product Reviews.” Only a few do well below 1 MHz; most are mediocre and some are downright pathetic (although those usually have AM broadcast rejection influencing the design).
If you are interested in low frequency reception, your best bet is to get a radio specifically designed for that purpose rather than one that just happens to sort of work there.
From QST August 1999
