Broadcast Intermod (?)

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Ed/KB1HYS:
I'm hearing numerous AM broadcast stations on 160, sometimes two at a time, on my R390A.  I'm also hearing it on another receiver (a KA1103) So I don't THINK it's the reciever.  Any Ideas about how to clean this up?  Is it the front end going south? or what?

Of course, they seem to land on the frequencies used for AM operations...

AB2EZ:
Hi!

One thing that can cause this is: actual mixing of two stations' RF signals... if their signals are both falling on a large metal object (not necessarily near you). The large metal object can be a tower, or the steel skeleton of a high rise building, or even a ship in the water.

Example:

Suppose 900kHz on your AM dial and 980 kHz on your AM dial are both transmitting strong signals that fall on a large metal object (e.g., the steel skeleton of a high rise building). These RF signals will cause currents to flow in the large metal object. If the large metal object has joints that are rusted or otherwise not perfect "ohmic contacts" (almost certainly to be the case)... then those joints will act as diode-mixers... and will produce additional currents in the large metal object that are at the sum and difference frequencies (in this case: 1880 kHz and 80 kHz). The current at the sum frequency (1880 kHz) will produce RF radiation from the large metal object... which will cause interference on 160 meters.

Stu

W4EWH:
Quote from: Ed/KB1HYS on February 23, 2008, 10:37:57 AM

I'm hearing numerous AM broadcast stations on 160, sometimes two at a time, on my R390A.  I'm also hearing it on another receiver (a KA1103) So I don't THINK it's the reciever.  Any Ideas about how to clean this up?  Is it the front end going south? or what?



Ed,
 If you get a list of local AM stations, you can find out when they're each off the air, and check for the intermod at that time. If your receiver's front end is the culprit, a notch filter may produce dramatic improvement: they're easy to cut & try, so it's usually my first step.Does varying the receiver RF Sensitivity change the intermod's level at a linear rate, or does it "drop off" toward the bottom of the range? If it stops when you reduce the gain, it's probably your front end or first IF.Can you hear the broadcasts clearly, or do they sound like mis-tuned ssb? The sum of two signals will be at a higher frequency than either alone, since the sidebands sum as well, and that means intermod instead of IF overload. If your front end is the mixing point, a high-pass filter may be in order: remember that even 6 db attenuation can make a big difference, since you'll be dropping both of the intermod sources at the same time.Can you move the receiver to different places and test it there? The idea is to stay equidistant from the broadcast station(s) involved (yes, I know you can't be equidistant from both unless you find the mirror), and find out if the interference remains or goes away. If it stays, odds are it's the receiver; if it disappears, it's probably local intermod.Is the intermod constant? If it's not, then you're looking for an active device controlled by someone else -
An older AM receiver with a leaky LO"Energy saver" lamps, etc.Some very old cordless phone base stations transmit in that range.
HTH. It's like looking for a (rusty) needle in a haystack, so keep eliminating possibilities until you find the source.

73, Bill W1AC

WA1GFZ:
Mt. Lincoln in Mass. is a great place for generating intermod. Years ago NPR radio had this tower with bad guy wire very rusty. When it got damp it was a great mixer.

AF9J:
Hi Ed,

I have the same type of BCI intermod problem you have.  In my case, it exists on both 160 & 75,  with the intermod by far, being the worst on 75m, between 3710, and 3850, about every 5 - 10 kHz.  My culprit is 4 AM broadcast stations.  2 of them are about 10 miles from me.  The other two are at the most, about 2 miles away from me (I can see the anti-collision lights from one of the close station's antennas, at night).  It's always cool to tune the SX-96 to the vicinity of 3725, just to find out the signal I'm tuning in on, is the "all sports" format, AM station.  Since I have so many intermod sources, I'm going to try to brute force it away with a high pass filter on my SX-96 (which is my AM receiver, and suffers the most from intermod).

73,
Ellen - AF9J

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