Broadcast Intermod (?)

[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]

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

[Rob K2CU]

If you think it is your receiver, first see if it shows up on another radio.  My 51J-4, which is similar to the R-390, easily overloads on strong local signals. Try putting a 10 dB pad on the input and see if the signals go down 10 dB or disappear. IF the intermod is being generated externally first verify that your own house is not responsible...i.e. gutters, guy wires, towers, etc. You could try and retune a small transistor AM radio to cover the 160 meter band., then use it as a sniffer.

If the intermod is coming from far away, are there any other guys in your area to help try and locate it? The little AM radio sniffer's loopstick antenna is pretty directional (a null) and you might be able to get bearings from it.

[K3ZS]

Worst case I experienced was when I was involved with the local club's 2M repeater activities.    A nearby installation was creating a signal on the input frequency of the repeater intermodulating with the repeater's transmitter signal.    This was proved with directional antennas and a spectrum analyzer.   We couldn't convince the other operator to do anything about it.   The club gave up and moved the repeater somewhere else.

[k4kyv]

I sometimes hear what sounds like FM broadcast on the HF bands.  I can just barely understand the audio at times and at other times just tell that it is speech or music.  The deviation seems much wider that the  bandwidth of my receiver, even at 16 kHz.  I seem to hear it most often on 40m, but I have heard it on 75/80 and maybe IIRC a time or two on 160.  At times it is very strong, and I pick the same thing up on multiple receivers.  I suppose it could be the same kind of intermod taking place between FM stations.  For example, 90.3 and 94.1 could leave a difference product at 3800 kHz.

[AF9J]

If you think it is your receiver, first see if it shows up on another radio.  My 51J-4, which is similar to the R-390, easily overloads on strong local signals. Try putting a 10 dB pad on the input and see if the signals go down 10 dB or disappear. IF the intermod is being generated externally first verify that your own house is not responsible...i.e. gutters, guy wires, towers, etc. You could try and retune a small transistor AM radio to cover the 160 meter band., then use it as a sniffer.

If the intermod is coming from far away, are there any other guys in your area to help try and locate it? The little AM radio sniffer's loopstick antenna is pretty directional (a null) and you might be able to get bearings from it.

Hi Rob,

All of my receivers suffer from intermod to a greater or lesser extent.  My FT-897D had problems with it (when I owned it).  My TS-820 also suffers on 75m (but hardly at all on 160).  The most intermod resistant receiver I have is in my IC-740, which only picks it up in a few spots on 75m. The worst is my SX-96.  There are a couple of spots on 160, and a whole bunch from about 3630 to about 3840 or 50 (it's hard to tell on the SX-96 bandspread dial - it isn't very frequency accurate) on 75m, that I have to deal with BCI intermod, on my SX-96.

73,
Ellen - AF9J

[Ed/KB1HYS]

Yep, I hear it on two different receivers.  When both are in the shack.  Seems worse in the daytime. I do hear more than one set of intermodulated stations.  Very clear, like listening to two radio's at the same time. I will have to start looking around for a possible source I guess.  This should be fun... 

[W2JBL]

make sure nothing in your antenna and feedline has anything like corrosion, solder everything, and generally be a good little boy scout with all connectors and cables all the way to your receiver. many times i have had BC intermod and junk generated in my own antenna system. i live 4 air miles with a direct path over water to thecombined WCBS/WNBC tower in the Bronx. the only receivers i can use on 160 without problems are R390 and R390A. NC300, SX101, SX73, 51J4, R388, SPR-4, R-4B all are next to useless for me on that band due to overload even using a very high Q link coupled tuner. no ricebox has ever been usable here on 160. if you are lucky you will nail the source of the mixing at your own site. in my situation one rusty bolt on the BC tower site is all it takes to wipe out everything from DC to 20 memetrs until they get it fixed.

[Ed-VA3ES]

I also suffer from more than moderate BC intermod on 80M.   I've traced it to corroded or oxidized connections at the feed-thrus I use to feed the balanced line through the basement wall.  I assume my outside connections are oxidized or suffer some corrosion.   We'll get to that in the spring, when I can attend to it.   I'll clean everything up and re-do 'em, and add some  battery compound or other anti-corrosion  compound to the joint, and add some caps to cover the nut-bolt-washer assemblies.

Winter is very hard on antenna systems up here.   My 10M, 5/8ths vertical has intermittents which I suspect is in the connector at the base of the antenna.    My dipole needs to be anchored properly. 

[AB2EZ]

Heres a simple way to determine whether the mixing/intermodulation is occurring:

a) outside your receiver (and thus the spurious signals are arriving at the input of your receiver; and are being generated via mixing in your own antenna system, or in a more distant source that is being picked up by your antenna)

or

b)inside your receiver:

1. Make a series-tuned circuit consisting of a broadcast variable capacitor (e.g., 365 pf x N sections) and an inductor... that resonates in the broadcast band

2. Place this series-tuned circuit across the r.f. input port of the receiver (in parallel with the antenna feed cable)

3. If the receiver can tune the broadcast band, then tune to a strong station; and verify that... when you adjust the series-tuned circuit you have placed across the input port of the receiver... you can null out (to some extent) that broadcast station. The series-tuned circuit is essentially a short circuit (or at least a low resistance) at the frequency of series resonance

4. Tune to a spot on 160 meters where you are hearing the spurious signal(s)

5. Adjust the series-tuned circuit you have placed across the r.f. input port of your receiver to see if you can null out (to some extent) the spurious signal

If the spurious signal is coming from outside your receiver (arriving at your antenna from somewhere else, or being generated in your own antenna system) then the series-tuned circuit will have little or no effect on how strong the spurious signal is. [i.e., a nulling out arriving signals in the broadcast band, before they enter the receiver, isn't going to affect signals arriving on 160 meters].

If the level of the spurious signal can be significantly changed (i.e., reduced) by adjusting the series-tuned circuit, then the spurious signal is being generated by mixing/intermodulation inside your receiver.

Any time a connection is made between dissimilar materials, a diode is formed at that junction. (The underlying physical mechanism that causes this is fairly complex.) For metal-to-metal connections (e.g., copper-to-aluminum), the diode effect is small enough that we don't notice it in our typical applications [except to the extent that corrosion forms, over time, at the junction]. For metal-corroded metal junctions, the effect is much larger.

Best regards
Stu