I’ve been following the observations Chris Smolinski has been making with his netSDR radio on Medium Wave (AM) broadcast frequencies. Quoting Chris,

There are six graveyard channels, 1230, 1240, 1340, 1400, 1450, and 1490 kHz. These channels were set aside as local channels by the North American Radio Broadcasting Agreement, which went into effect in 1941. The term graveyard comes from the weird mix of sounds often heard at night, as dozens of stations mix together. Graveyard stations are restricted to 1000 watts maximum, and some use well under that at night, sometimes under 100 watts.

Indeed, when monitoring these freuqencies for several hours on a number of these frequencies, several carriers can be seen +/- 50 Hz from the expected center. For examples see here, here and lastly here. The first two were recorded without the use of a high stability local oscillator, so some drift occurs over the length of the traces. The last one however was performed with the netSDR radio locked down with a highly stable 10 MHz frequency source. You can instantly see how much more stable this trace is.

The last trace also has an interesting phenomenon, four of the carriers (between 1.229970 MHz and 1.229980 MHz) appear to be echoed on the other side of the dead center frequency (between 1.230020 and 1.230030 MHz). See this image below:

Reflective echo patterns, where signals can appear as mirror images around a center frequency, is a common problem with Quadrature Sampling Detector (QSD) radios. One  such radio is the “SoftRock” SDR (software defined radio) kits -I have one and it will feature here again soon with an explanation of why the reflective echo effect can happen. However, the netSDR radio is a Direct Digital Sampling (DDS) style SDR, which, as far as I know, should not have this problem.

Closer observation shows that these 4 carriers are not actually mirror images around the center frequency. Look at the figure below:

Here I have copied the waterfall plot boxed in white from the top right  and shifted it down underneath the “echoes” on the left. You can see that these carriers align perfectly. But I have not flipped the section and made it a mirror image. A mirror image does not align well at all, so this is clearly not carriers being reflective across the center frequency. These carriers just simply appear to repeat with identical frequency shifts relative to each other, on the other side of 1230 kHz.

What could account for this?