You never really know what you’ll hear on the ‘graveyard’ frequencies, do you? While 1430 is usually KASI around here (I’m about 15 miles from their studios in Ames, Iowa), that’s no guarantee I’ll actually hear it at night when their power drops. Tonight’s demo starts off with an unknown AM station to the South and East of me, possibly KZQZ out of St. Louis.
A note about the Pro-1B. As you may remember from my earlier demo, the magnetic loop is bi-directional, so there’s no forward gain. It is, however, very capable of some pretty deep nulls of stations that are off to the sides. By rotating the loop to the Southeast/Northwest, I was able to effectively null out KASI in favor of our mystery oldies station. You will notice a ‘chug’ in the signal on the magnetic loop that isn’t there on the SAL. I think it has something to do with the smaller antenna being more susceptible to phasing differences between the two sidebands. The radio probably could’ve corrected this if I had turned on the synch detector, but its an interesting observation nonetheless.
UPDATE: Tim Tromp, a DXer in Michigan who has some of the most amazing DX catches you’ll ever hear, has a much better explanation for the ‘chugging’ sound heard on the magnetic loop:
The chugging is an interesting radio phenomena and can be heard throughout the AM dial. The chugging (or “whoosh whoosh”) that you hear is caused by two (or more) stations who’s AM carriers are very close to the exact same frequency, but are slightly off from one another. The slightly offset heterodynes beat against one another causing a “sub audible het”. The resultant effect is this chugging sound which can be avoided by listening in LSB or USB. The slower the chugging, the closer the two heterodynes are to one another. The effect is most evident on the graveyard channels which makes them very difficult to listen to at night and the cause of the “roaring” sound you hear on those channels at night. Of course when the two co-channel hets are more than a couple hundred hertz apart, the chugging turns into an audible tone when listening in AM mode.
Be sure to check out Tim’s blog at http://midx.wordpress.com/, and his YouTube channel at https://www.youtube.com/channel/UC5h8mmMqUgjOobqoxOd3wmA.