Here's one that has me scratching my head...
Let's see if anyone has been here and done this before:
I have a Harris RF-550 communications receiver (rack-mount commercial jobbie, early 80's vintage, digital tune by flip-switches .1 to 29.999 MHz AM/LSB/USB/ISB/CW, definitely not a cheapie) that I use for general RX and SWL work. Antenna is a random wire in the 60' range fed through a dual-core Guanella 4:1 balun against a ground rod at the feed point.
The antenna works -REAL- good above 800 kHz or so and the S-Meter in the Harris is calibrated in dB from -20 to +120 relative to 1uV. I have two local AM stations that come in at 100 dB over 1uV, and that's a very manly signal, probably better than 50 over 9 on a conventional S-Meter. A 'normal' signal in the AM band is between 50 and 70 dB over 1uV on this setup.
Here's the question:
I have a spur on 1040 kHz from the -two- strongest AM stations that happen to be on 1170 and 1300 kHz. A little mental math will tell you there's 130 kHz difference between 1170 and 1300, and if you subtract this from 1170 you get 1040, which is exactly where I hear the spur. Simple enough? I can't tell if there's another spur at 1300 plus 130, as there's a fairly strong local station on 1430.
If I understand my superheterodyne theory, when you mix two signals together you should hear their product at the "difference" between the two frequencies. Sure enough, if I tune 130 kHz there's a clean mix of the two stations. Tune the first or second harmonic of either, and they sound pretty good. Which tells me that if the RX is overloaded, it's not by much. Right? Tune the fundamental of either and they sound good, flip the filter switch to 'wide' (20 kHz) and they're (by AM standards) extremely Hi-Fi.
If I swap the balun for a 9:1 single core jobbie, it knocks 5 dB off everything and the spur at 1040 kHz goes away. But, the difference signal at 130 kHz remains. The spur on 1040 kHz doesn't bother me, (and my apologies to anyone at WHO in Des Moines for not hearing you at night) it just puzzles me as to why it's there.
On my main rig (an Icom IC-756 on a 136' Coaxial Windom) the "Spectrum Scope" shows a peak in the 'grass' on the bottom of the display with a ragged spike at 1040 kHz, but no audio is heard other than the 'scratch' from another local station on 1050 kHz. In other words, the 'spur' really is there on more than one receiver. (and the IC-756 will play the two-station mix on 130 kHz, same as the Harris RF-550)
Can anyone tell me why?
Note: I'm familiar with high side / low side injection in a mixer stage, but I still can't figure how mixing 1170 with 1300 kHz comes out at 1040 kHz with a 130 kHz differential. (?)