Strange RFI problem

[K3MRK]

I've been an active ham for 45 years.
I've run into an RFI issue I have never experienced before.
I have a new modern receiver. ICOM IC-7300. 6 Different HF antennas.
Last Wed, for the first time, I noticed a very wide signal at 7250Khz. When I tuned to it, it sounded like AM broadcast. Now this is not unusual on 40m, at night. This was at about 10:00am. I switched the mode to AM and it is a local sports station, with another music station in the background. the signal is about S9+15db. As I tuned up and down the band, I found this signal every 100khz. 7150, 7050, 6950 and on and on. Some freqs were stronger and some played the music station louder with the sports station in the back ground. Then I checked 30m. 10030, 10130, 10230 and so on up the band. Every now and then it would stop, then come on in blips faster and faster then go full time again. It would continue this cycle.

By Friday, all it was was a weaker pulse every few seconds. Same over the weekend, but still at the same freqs with the same bandwidth. So this morning I start tuning 40m and there is is, stronger than ever, not cutting out at all and I can understand everything they are saying. The entire 40m CW band is wiped out.

During this process, I became concerned that it might be a filtering issue with my radio. So I turned on another transceiver I have it my desk and it was there. Went out to the mobile and it was there as well, just not as strong because of the ham stick on the trunk lid.

I have a neighbor over the hill who is a ham. He does not hear what I hear, but has been having major issues with a pulsing noise all over the HF Bands. May or may not be related. So my next step is to get in the car and drive around with the mobile. I need to do this during the day before the SW stations come on. It a little harder to do from the mobile because of the compromised antenna and lack of scope for visual verification.

Has anyone ever heard of anything like this? I'd understand it if I was using a 1940's vintage receiver. I've been in this location for 27 years, this was not there before.
Any ideas are welcomed.
Thanks
Mark
K3MRK

 

[WA8ZTZ]

At first glance, this sounds like a classic case of external mixing involving a strong local AM broadcast station.
However, since you mention that the same problem is present on different receivers and antennas, a problem
with your equipment can pretty much be ruled out. Beginning to smell like a spurious signal(s) generated by the AM station itself.
The fact that the problem had a sudden recent onset makes me wonder if the AM station may have done something recently that affects their
signal. Apparently some AM stations during the current national emergency have changed their power and/or pattern or have
turned off the digital sidebands or are broadcasting with wider bandwidth. This is merely speculation on my part so take it FWIW.

 

[Arkmood]

...it is a local sports station, with another music station in the background. ...some played the music station louder with the sports station in the back ground....
Any ideas are welcomed.
Thanks

Music station transmitting St.id - if not ...

 

[K3MRK]

At first glance, this sounds like a classic case of external mixing involving a strong local AM broadcast station.
However, since you mention that the same problem is present on different receivers and antennas, a problem
with your equipment can pretty much be ruled out. Beginning to smell like a spurious signal(s) generated by the AM station itself.
The fact that the problem had a sudden recent onset makes me wonder if the AM station may have done something recently that affects their
signal. Apparently some AM stations during the current national emergency have changed their power and/or pattern or have
turned off the digital sidebands or are broadcasting with wider bandwidth. This is merely speculation on my part so take it FWIW.

This was one of my first thoughts. Both transmitters, as far as I know, are at least 10 miles from me. I contacted 3 other hams within 5 miles of me and they are not hearing it. One is about a 1/4 mile from me. Some new info: I noticed it wasn't present at night. I turned on the radio at 7:30am this morning and the signal was not there. It came on at 7:55am. Played steady for a while, then went off and on for about 30 min. As I sit here now at 8:45am, it's going on and off in a random pattern.
It will be interesting to see if it disappears at 5:00pm. The closest business to me is a private school at about 400 yards. They are closed.
Still stumped

 

[WA8ZTZ]

Hmmm, 0755 is too late for local sunrise in eastern PA so it probably has nothing to do with the AM station changing power or pattern at sunrise, however, they may be on some kind of critical hours or pre-sunrise authorization... you will have to do some research regarding such
as this can get a bit involved and may be even more confused at this time of national emergency.
You also mention that other nearby hams don't hear what you are hearing. If an AM station were transmitting spurious signals
everybody should be hearing it. Makes me wonder if what you are hearing is being somehow locally generated. If you have a portable
SW receiver that tunes 40 meters, walk around your home and property with it tuned to the interference frequency and see what happens.

 

[AB4BF]

I'd first get a friend or another ham that has a 7300 to bring it over and try it, they are pretty good at signal rejection. Try that first. 6 antennas? try them all one at a time to see what changes if anything... could be if the antennas are close to each other there could be some kind of harmonic between them or among them associated with the frequency of the AM station. It may be as simple as moving the antennas towards each other or away, if they are parallel with each other.
If none of this works, contact the AM station's broadcast engineer. invite him over for a beer or a meal while he's listening to his station wreak havoc with your 7300. It might possibly be something wrong at his transmitter/antenna...

 

[ridgescan]

Just throwing this in-does that MW station you hear have an FM simulcast? I know here in SFO I get some wideband warbling around 12mHz from KFOG which transmits off Sutro Tower 2.5 miles from me, on all my rigs including the R8600.

 

[K3MRK]

Thanks for all the great input. I ordered a CountyComm HF portable and I expect it by the weekend. A friend has one of these and it's amazing. I'll walk the neighborhood and see what i find. The first thing I'm going to do, when I'm able, is kill all the power to my house. I have battery back up on the the HF equipment, so this will at least tell me if it's being generated in my house. the other ham in the neighborhood lives on the other side of the hill from me. So his RF environment could be different. He's been working with me to figure this out. He has noticed some pulsing noise around the bands that wasnt there before. The hill between us is pretty high with an RF circus on top of a water tower at the peak. Antenna to antenna, that water tower is about 100 yards from me. I've been here a long time and never had this issue. Something changed somewhere.
FYI the problem was not very severe today compared to yesterday. More off than on and not as strong.
There may be a simulcast on the sports station. It is ESPN radio and is on 3 or 4 of the local broadcast stations AM and FM.
I've been pretty busy today and didn't check as often as other days, but at 4:40pm, it was gone. I'm looking forward to 7:55 tomorrow morning to see if it is there on a schedule.
Thanks again
Mark K3MRK

 

[krokus]

I am thinking it is intermodulation interference (IMI), which is caused by strong signals mixing on something functioning as a non-linear device. This could be corroded junction of metal, like links of a chain. The portable radio can help locate the source, if so.

 

[devicelab]

You need a HPF such as this: PAR BCST-HPF High Pass AM Broadcast Filter

Direct web site: SWL Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

Also, it's possible with all of your antennas you created a ground loop and thus your coax is acting like one giant antenna. The filter should help but you should also look at your grounding system.

Lastly you didn't say what antennas you were working with. Are any amplified loops? They're notorious for overloading receivers (indirectly) due to local AM stations. (Again the filter should help relieve that...)

FWIW, the smaller AM stations reduce transmitter power during the night hours (and sometimes on the weekends) to save on costs.

 

[RFI-EMI-GUY]

I have a local AM station that had energized my phone wires in the house even though they come in underground. My corless phone base was affected, I had to tune it out wwith chokes.

In your case I would look at wall warts and USB chargers to see if they are becoming a noisy mixer for your local AM station.

 

[prcguy]

The Icom IC-7300 is a great radio but it lacks in front end preselection and is vulnerable to out of band signals. Mine has a problem on HF when I run high power on 2m. I like to work with facts so before buying a filter or spending any $$ I like to identify the problem first. It could be rectification of a very strong AM broadcast signal by something on your property or nearby like a corroded fence or ?? It could be the 7300 getting too much signal on the AM broadcast band and it can't deal with it.

I would recommend borrowing a high pass filter that snuffs out the AM broadcast band and see what that does. If it cures your problem then its probably the radio getting overloaded and it might be worth picking up a filter. If the filters doesn't do anything then its a good chance you have some RF rectification creating IMD. That can take some time to figure out but a portable AM radio can sometimes locate it.

 

[K3MRK]

You need a HPF such as this: PAR BCST-HPF High Pass AM Broadcast Filter

Direct web site: SWL Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

Also, it's possible with all of your antennas you created a ground loop and thus your coax is acting like one giant antenna. The filter should help but you should also look at your grounding system.

Lastly you didn't say what antennas you were working with. Are any amplified loops? They're notorious for overloading receivers (indirectly) due to local AM stations. (Again the filter should help relieve that...)

FWIW, the smaller AM stations reduce transmitter power during the night hours (and sometimes on the weekends) to save on costs.

I have all separate antennas. 5 Dipoles and 1 Doublet. All have been in place for at least 1 yr, most have been there 4 or 5 yrs.
I received my new County Comm radio and will be sniffing around later this week. No loops, no pre-amps.

 

[K3MRK]

I am thinking it is intermodulation interference (IMI), which is caused by strong signals mixing on something functioning as a non-linear device. This could be corroded junction of metal, like links of a chain. The portable radio can help locate the source, if so.

This is the first thing I thought of and number 1 on my list. I was receiving this signal, same freqs on my mobile, just not as strong due to the mobile antenna being a hamstick on the trunk lid. This also a completely different radio. I had a day last week, that I could only hear the noise on my doublet at 75'. This leads me to believe it is not on my property. The big factor here is that the noise often goes off and on resembling a loose or corroded connection. I stepped back from this issue for a little while to focus my attention on some other things, but plan to go after it again later this week.

 

[K3MRK]

I have a local AM station that had energized my phone wires in the house even though they come in underground. My corless phone base was affected, I had to tune it out wwith chokes.

In your case I would look at wall warts and USB chargers to see if they are becoming a noisy mixer for your local AM station.

I'll be testing this with all power to the house cut this week. I've been putting this off because of all the issues it causes with clocks and my well system, but it should be done first before I start looking around the neighborhood.

 

[K3MRK]

You need a HPF such as this: PAR BCST-HPF High Pass AM Broadcast Filter

Direct web site: SWL Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

Also, it's possible with all of your antennas you created a ground loop and thus your coax is acting like one giant antenna. The filter should help but you should also look at your grounding system.

Lastly you didn't say what antennas you were working with. Are any amplified loops? They're notorious for overloading receivers (indirectly) due to local AM stations. (Again the filter should help relieve that...)

FWIW, the smaller AM stations reduce transmitter power during the night hours (and sometimes on the weekends) to save on costs.

I can whip one up pretty quick. I dont think it's going to help since all my receivers are picking this up on the exact same frequencies, but the cost and time to build one is minimal. Thanks

 

[wb6uqa]

You need an portable receiver. An small loop ant. and an attenuater

 

[dlwtrunked]

I've been an active ham for 45 years.
I've run into an RFI issue I have never experienced before.
I have a new modern receiver. ICOM IC-7300. 6 Different HF antennas.
Last Wed, for the first time, I noticed a very wide signal at 7250Khz. When I tuned to it, it sounded like AM broadcast. Now this is not unusual on 40m, at night. This was at about 10:00am. I switched the mode to AM and it is a local sports station, with another music station in the background. the signal is about S9+15db. As I tuned up and down the band, I found this signal every 100khz. 7150, 7050, 6950 and on and on. ...
Mark
K3MRK

Important fact you did not tell us that also others should have found out by a little research... You live in Lancaster, PA which has broadcast station on 1390 and 1490 kHz. (Side note: The "k" is not captalized in in "kHz, but the "M" is in "MHz".) The stations are 100 kHz apart and that points at the problem. When I lived in Bridgeport, CT, I had a similar problem with two AM broadcast stations there. Even though they were a couple miles apart, the signal from one was being received by the other at times, and in the case, the intermod was being generated in the second transmitter and broadcast at various places on HF. I talked to them, and from that conversation, I suspect they knew they had a problem as they told me what must be happening , and eventually it soon went away. Otherwise, the intermod is from these two stations and being generated by something like loose connections or metal contacting metal on something near your antennas.
And note this from Wikipedia:
"On January 7, 2010, WLAN informed the FCC that it was about to lose its long-time transmitter site.[3][4] Franklin & Marshall College, which owned the site, decided not to renew the lease.[4] WLAN chose to apply to move its transmitter to the WLPA (AM) transmitter site and diplex its signal into WLPA's non-directional antenna.[4] The change from a directional antenna array at the former site to a non-directional antenna resulted in WLAN having to reduce power from 5,000 watts day/1,000 watts night to 1,100 watts day/18 watts night,[4] with a corresponding downgrade from Class B to Class D. The FCC granted WLAN a new license with the updated facilities on August 23, 2010.[5] "

It is strongly like the problem is theirs.

 

[WA8ZTZ]

Important fact you did not tell us that also others should have found out by a little research... You live in Lancaster, PA which has broadcast station on 1390 and 1490 kHz. (Side note: The "k" is not captalized in in "kHz, but the "M" is in "MHz".) The stations are 100 kHz apart and that points at the problem. When I lived in Bridgeport, CT, I had a similar problem with two AM broadcast stations there. Even though they were a couple miles apart, the signal from one was being received by the other at times, and in the case, the intermod was being generated in the second transmitter and broadcast at various places on HF. I talked to them, and from that conversation, I suspect they knew they had a problem as they told me what must be happening , and eventually it soon went away. Otherwise, the intermod is from these two stations and being generated by something like loose connections or metal contacting metal on something near your antennas.
And note this from Wikipedia:
"On January 7, 2010, WLAN informed the FCC that it was about to lose its long-time transmitter site.[3][4] Franklin & Marshall College, which owned the site, decided not to renew the lease.[4] WLAN chose to apply to move its transmitter to the WLPA (AM) transmitter site and diplex its signal into WLPA's non-directional antenna.[4] The change from a directional antenna array at the former site to a non-directional antenna resulted in WLAN having to reduce power from 5,000 watts day/1,000 watts night to 1,100 watts day/18 watts night,[4] with a corresponding downgrade from Class B to Class D. The FCC granted WLAN a new license with the updated facilities on August 23, 2010.[5] "

It is strongly like the problem is theirs.

Good detective work.
The OP never got back regarding this.
My suggestions were that he make sure things were OK at his site but my suspicion was that the local AM station (stations as it turns out in this case) may be generating spurious signals.
Anyway, looks like you may have nailed the problem.

 

[dlwtrunked]

Good detective work.
The OP never got back regarding this.
My suggestions were that he make sure things were OK at his site but my suspicion was that the local AM station (stations as it turns out in this case) may be generating spurious signals.
Anyway, looks like you may have nailed the problem.

Having been through a similar situation, it was not too hard to suspect items to research. While he did not indicate others saw the same problem, my experience is often they will say "No" but they did not really look. He lives about 1.5 to 2 miles from that AM transmitter site so also others farther away may not hear it. I suspect something at the transmitter site was/is periodically causing the problem.