AM image at 100 khz?

[Tom Wells]

This morning on the way to work (north side of Chicago) I decided to check on WOKY AM 920 in Milwaukee.
It was critcal hours, and in the mud, equal to WOKY, was WGN AM 720 in Chicago.
This radio has an actual tuned RF secton on the AM, so overload/selectivity is not in question, and I was 10 miles west of WGN's site.
The radio has an IF of 262.5, so I can't think of any way to come up with this internal to the radio.

We have WAIT Willow Springs AM 820 as a pretty strong signal here, so I must have heard a 100 khz image intermod of 820.

But isn't that a bit far removed for an "outside the radio" image?

 

[amfmsw]

It sounds like harmonics. They can do some pretty neat, and damaging things.

In my area, there are stations at 1340Kc and 1450 across the street from each other in city A, and another in city B 20 miles away at 1230. Outside of city A, 1230 is very respectable and clean. As you get near city A, 1230 begins to "beat", until it's not listenable and replaced with a combo of the 1340 and 1450. I figured that one out to be the first harmonic of 1340, 2680. 2680 - 1450 = 1230, and interference.

Another is interference to nearby 98.3 in city B. In the same city A as above, 98.3 is at the very end of it's city grade signal. 96.9 and 1450 originate from the same building and stick, creating a spur. 96.9Mc + 1.45Mc = 98.35Mc...close enough. In fact, when 98.3 is off for maintenance, 96.9 is CLEARLY heard on 98.3, and when 1450 is strongly modulated, BOTH can be heard on 98.3 in City A.

It's possible that a nearby FM is reacting with 720Kc to create a spur at 100Kc. Grab a calculator and have fun.

 

[Grrrradio]

I'm not an engineer, just a nerd with too much time on my hands who's curious... is it possible that your 1230 situation was a chain of two difference products? (1450 - 1340 = 110; 1340 - 110 = 1230?) Or is this "too many steps" from a physics point of view? Since I'm not an engineer, it's FAR more likely that you're right with your 2680 theory, I'm just curious how you went about excluding the other possibilities.

Also, a weird FM/AM story of my own... This may just be an issue with the radio, but if it's not, I think I may have received an honest-to-God SEVENTY-EIGHTH harmonic off an AM station some years ago. I used to live right in the main lobe of a nine-tower directional AM on 1130. This station was 50 KW daytime (might have only been *pattern* directional on the 25 KW nighttime power, but at the very least the conductivity was better in my direction on the daytime signal.) And I lived all of three miles away, with the towers just barely at eye level on the horizon due to slight terrain differences. Even before it went HD, this station nuked everything from 1060 to 1200 on a Walkman if I stood out on the driveway, with nothing between me and the towers... and sometimes bled into our phone lines.

One early afternoon, I was trying to tune in a college station 80 miles away on 88.1 (theoretically 31 dBu at my location), on a radio which would ordinarily get a weak but listenable signal from it but had recently suffered a broken whip antenna. Instead of Modest Mouse and Guided by Voices, I heard sports talk... To confirm my suspicions, I alternated between 88.1 and 1130 for some time, and sure enough, it was an exact match. There was nothing else on 88.1 in the local area that would have carried the sports format heard on 1130, and a quick check of the car receiver confirmed that there was no tropo or skip in at the time. Like a moron, I didn't listen to the 88.1 interference long enough to get 1130's ID... I figured that the only possibility was their 78th harmonic on 88.14!

In retrospect, I have some uncertainties -- first off, the sound was pretty clear (not strangely garbled, as it intuitively seems would happen if you tried to demodulate something with a different scheme than it had been modulated in), and it even seemed to have some treble well above 5 KHz. Secondly, I did own a rather drifty Ramsey FM-4 kit at the time, but I can't remember if it was on, and it would have been about 100 feet away inside the house (I was outdoors.) So my questions are... is there something about an FM detector that allows it to render "clean" audio from an AM signal it's somehow picking up? Could the events described have possibly originated from a mixing product with the Ramsey kit, if it were on and had somehow drifted to 87.0 or 89.2... even though the thing was only 250 mw TPO and hooked to a (probably poorly resonant) random wire?

 

[amfmsw]

The harmonics at that multiple stage would be microscopic. No, it simply sounds like a front-end overload of RF that the Walkman couldn't reject.

At then WPGR in Philly, the station had a 50kw three tower array in a dog-leg. About 1kw went out the NE, and a theoretical 150kw out the SW. There were homes built about 150 yards from tower 3, right in the main lobe. In one house, you could hear the station in the toaster, wall outlets and switches, oven, and of course the phones. But the owner complained to the FCC when a plumber, working in the basement, cut a copper pipe in half, only to have sparks jump from end to end of the now two piece pipe. It was pure RF.

At then WIBG in Philly, the chief engineer, Archie Sitchel, would walk through the basement where the conduit for the antennae were routed, holding one one of a standard 40 watt flourescent bulb. It was fully illuminated by holding one end in his hand with ther other in the air, lit purely by RF. Many car radio speakers in the parking lot played all day with the voice coil being excited by RF.

If you're that close that you're in line of sight of the towers, I'm pretty confident the Sony's being overloaded.

 

[slim101]

Intermod (not receiver produced) is usually caused by mixing within the PA of one of the transmitters.

AMFMSW is right about the 1340 / 1450 mix.

1450 rf gets into the 1340 transmitter where it mixes with the 2nd harmonic of 1340 (harmonics present in the PA before filtering of the plate circuit). The product, 1230 gets through the output network, then gets transmitted by the 1340 antenna system.

If you do the math, it looks like the same situation in Tom Wells's case. (820 X 2 = 1640). Then, (1640 - 720 = 920).
Willow Springs 820 is only about 15 miles from WGN, so the intermod product is probably coming from the PA of the Willow Springs 820 transmitter.

On FM, its easy to spot the culprit (where the intermod is produced) , because the 2nd harmonic has double the modulation, so is twice as loud as the other signal.