Over-Modulated?

[K8PLW]

Hello everyone, looking to get some additional opinions on an interference issue I'm having with a local AM before I take any additional action.

Background:
I'm located 4 miles NNE of the 1090 WCAR TX site, smack dab in the middle of their major north-facing lobe. WCAR is a DA2 250W day / 500w night station. From my location, on modulation peaks, I'm able to hear them nearly 100 khz below, and 50-60 khz above their frequency. Needless to say it seriously impacts my ability to DX that portion of the band. Even other local AMs in the 990-1150 khz range are essentially unusable from here. The interference exists regardless of receiver-- car radio, portable, rigs here at home.

I've monitored them, and their audio sounds "Loud" to me, but not necessarily distorted. Doesnt seem right though, almost as if they're running it "Wide open", with no hard limiting, etc.

Lots of test equipment here, but nothing to directly evaluate their signal. Im not even sure what acceptable standards are for AM broadcast audio levels (I've been in radio for years. Both professionally and as a hobby, not broadcast radio however). Looking to get other opinions before I contact them with a complaint?

 

[davideduardo]

Hello everyone, looking to get some additional opinions on an interference issue I'm having with a local AM before I take any additional action.

Background:
I'm located 4 miles NNE of the 1090 WCAR TX site, smack dab in the middle of their major north-facing lobe. WCAR is a DA2 250W day / 500w night station. From my location, on modulation peaks, I'm able to hear them nearly 100 khz below, and 50-60 khz above their frequency. Needless to say it seriously impacts my ability to DX that portion of the band. Even other local AMs in the 990-1150 khz range are essentially unusable from here. The interference exists regardless of receiver-- car radio, portable, rigs here at home.

I've monitored them, and their audio sounds "Loud" to me, but not necessarily distorted. Doesnt seem right though, almost as if they're running it "Wide open", with no hard limiting, etc.

Lots of test equipment here, but nothing to directly evaluate their signal. Im not even sure what acceptable standards are for AM broadcast audio levels (I've been in radio for years. Both professionally and as a hobby, not broadcast radio however). Looking to get other opinions before I contact them with a complaint?

Were they overmodulated, generally you'd hear it with huge distortion and artefacts. Overmodulation on negative peaks causes carrier suppression.

AM stations of that power at the distance you are located should not splatter anywhere near that much. The first question is about what receiver you use. Some just can't deal with nearby signals.

On the other hand the station may not be conforming to NRSC standards:

Standards and Guidelines | National Radio Systems Committee | Setting Standards for the Future of Radio

www.nrscstandards.org

Or they may indeed have some kind of tuning issue with the transmitter, the phasor or the ATUs at the base of each tower.

Kelly? Cat? Frank?

 

[K8PLW]

Thanks David, its present regardless of which receiver I use. Seems to equally impact my vehicle radio, Sony ICF-2010 portables, Icom IC-756PROIII, IC-746PRO, etc. By contrast, I'm located about 16 miles from 50kw WJR, yet I'm easily able to listen to WABC on 770 without difficulty on my better receivers. No distortion on WCAR that I was able to perceive, just what appears to be an incredibly wide, splattering signal.

 

[davideduardo]

Thanks David, its present regardless of which receiver I use. Seems to equally impact my vehicle radio, Sony ICF-2010 portables, Icom IC-756PROIII, IC-746PRO, etc. By contrast, I'm located about 16 miles from 50kw WJR, yet I'm easily able to listen to WABC on 770 without difficulty on my better receivers. No distortion on WCAR that I was able to perceive, just what appears to be an incredibly wide, splattering signal.

More:

Over Modulation​

If the modulation depth rises above 100%, the transmission looks like this.

The carrier has completely gone for part of the audio cycle. This results in severe distortion. The transmission also splatters outside its usual bandwidth causing interference to stations on adjacent channels. This situation must be avoided.

From Amplitude Modulation
(A good and easy to understand description of the AM technology)

The key element is that negative peak overmod is severely distorted. Yes, it induces splattering but the audio sucks in proportion to the degree of carrier suppression.

 

[K8PLW]

Good info. I did look at their signal on my spectrum analyzer awhile back. They were significantly beyond 10 kc bandwidth from what I recall. I'll connect an antenna to the analyzer and check again tomorrow..

 

[frankberry]

There may be a high frequency oscillation in the audio stages of their transmitter. I had a similar situation with an RCA Ampliphase transmitter many years ago.
One of my friends was the CE at WCAR some time ago. I have asked him for any information that he can provide.

 

[davideduardo]

There may be a high frequency oscillation in the audio stages of their transmitter. I had a similar situation with an RCA Ampliphase transmitter many years ago.
One of my friends was the CE at WCAR some time ago. I have asked him for any information that he can provide.

I got a contract to work on an Amplifuzz at Radio Mambo, a 10 kw non-d in Guayaquil, Ecuador on 560. Another local station, owned by a friend, was also 10 kw on 540 and was frequently being drowned by interference. Another station up at 660 which was mine was noticing noise on our lower sideband. Traced to Mambo, we contacted the owner and they agreed to an inspection.

The result was a need for rather frequent adjustment of the mid-60's RCA. One improvement was air conditioning the transmitter room and recirculating air; the site was in a tidal flat and the salt air... maybe good for taffy... was condensing inside the transmitter and creating all kinds of bizarre thing besides making the rig unstable.

Any instability in the circuit that splits the carrier and rotates one of them will cause everything beyond that point to go crazy in interesting and profitable (for me) ways.

Oh, and a "tidal flat" is actually a marsh married to a swamp. They don't smell nice, either.

Oh, and "Radio Mambo" was not named for the archaic dance rhythm; "Mambo" was the dominant brand of laundry soap bars back when we washed with a stone and a washboard behind the house. (Not kidding)

 

[frankberry]

I know Radio Mambo well. Their old 10kW Ampliphase was made in Italy. I was there in 1976 to install a 50kW Ampliphase. At that time, the transmitter room was not air conditioned. I told them that the 50G Ampliphase would be nothing but trouble if they did not air condition the new transmitter room. I doubt that they ever followed up on my suggestion. These are two photos of the 50G sitting in Guayaquil.
Now ... back to the original subject, WCAR.

 

[ncradioeng]

Hearing it 60 khz above and below the carrier frequency is a clue - PDM transmitters use a 60 - 70 kHz pulse frequency in the modulator section. The cure is to replace the PDM filter capacitors (the big blue ones in the Harris SX and Gates series).

 

[frankberry]

I understand tha WCAR is using an old MW-1 transmitter which has not been well maintained through the years. Their main SX-1 transmitter is down. Apparently, it's been down for quite sometime.

 

[K8PLW]

Nice pics of their transmitter. I believe @frankberry may be on to something regarding the high frequency oscillation on their audio. From what I've read, the MW-1 transmitter was the original from when the station went on the air c1961. The SX-1 came from sister station WBRB 1430 in Mount Clemens when it went silent.

I can confirm the station has had ongoing issues-- They were off the air for several weeks, back on, then a ground loop-ish buzz appeared and was present on their audio for months, then off the air again. The current issue has been going on for several years now. I just haven't followed up on it, but its incredibly annoying. Makes any signal within the ~990-1150 khz portion of the band all but unlistenable.

I did connect my spectrum analyzer today, and their occupied bw looked pretty much in line with what it should be and appears similar to other local signals, difficult to see anything useful. The issue only occurs on modulation peaks. Im closeby and directly within their lobe, their signal was -18dbm today on the analyzer with a random wire connected, they're easily the strongest local here to me.

I really appreciate everyone's help and input. Based on this I'll be reaching out to them for assistance resolving it.

 

[davideduardo]

I know Radio Mambo well. Their old 10kW Ampliphase was made in Italy. I was there in 1976 to install a 50kW Ampliphase. At that time, the transmitter room was not air conditioned. I told them that the 50G Ampliphase would be nothing but trouble if they did not air condition the new transmitter room. I doubt that they ever followed up on my suggestion. These are two photos of the 50G sitting in Guayaquil.
Now ... back to the original subject, WCAR.

Small world!

It appears that they either built a new transmitter building or removed the AC I had asked them to install. In fact, their solution was to mount 4 larger capacity room AC units in the wall facing the transmitter. I doubt that those units lasted the 8 years between my visits and yours.

Did you notice that the old 10 kw manual was in Italian? Actually, an easy read for Spanish speakers. RCA back then had a very convoluted international sales organization, which is why I never bought and RCA gear.

 

[platecap]

My former client is Sima Birach the present WCAR 1090 owner. I was at the site about 2 years ago, and it's ,mostly a disaster. During the 1990s, under a different owner (when I was CE) we were inspected by the FCC and were given a clean bill of health -- including some of the critical nighttime Monitor Points. Under Birach's ownership, however, the station has fallen into disrepair. The former back-up transmitter is now the Main -- a barely functioning MW-1. The station has operated illegally for years using their Night power/pattern 24/7. This is because their ATU shacks leaked, blowing out the solenoids in the Contactors. At times, the vegetation overgrowth have been seen growing up the towers across the base insulators. It is unlikely the Night pattern is "in" because it needed constant attention. In about 1996, the last WCAR Partial Proof was done by me under the direction of the late Bernie Segal, when we replaced Night tower N-4. At that time, the SX-1 transmitter was working well within tolerance as we did the required yearly Spectrum Analysis. If Mr. Birach would only pay any part of my outstanding invoice, I would give him some guidance, but he only wants free advice. If the interference persists, I would encourage someone to file a complaint with the FCC. They should also take a look at the interior perimeter fencing around the towers. As I said, Birach is my former client. The designated CE is his son, Sima Birach, Jr.

 

[frankberry]

Small world!

It appears that they either built a new transmitter building or removed the AC I had asked them to install. In fact, their solution was to mount 4 larger capacity room AC units in the wall facing the transmitter. I doubt that those units lasted the 8 years between my visits and yours.

Did you notice that the old 10 kw manual was in Italian? Actually, an easy read for Spanish speakers. RCA back then had a very convoluted international sales organization, which is why I never bought and RCA gear.

When I was there, the 10kW transmitter was located in a room attached to the caretaker's house. They added another, bigger room and high voltage vault to the building for the 50kW transmitter. there was no air conditioning anywhere in sight.

 

[Kelly A]

Thanks David, its present regardless of which receiver I use. Seems to equally impact my vehicle radio, Sony ICF-2010 portables, Icom IC-756PROIII, IC-746PRO, etc. By contrast, I'm located about 16 miles from 50kw WJR, yet I'm easily able to listen to WABC on 770 without difficulty on my better receivers. No distortion on WCAR that I was able to perceive, just what appears to be an incredibly wide, splattering signal.

As David mentioned, if they were overmodulating to the point of creating interference, it would sound really distorted because the modulation envelope would be closing once the station got past 99% negative.
I suspect the front end of your receivers are just being overloaded.
Over the years I've dealt with many Ham's or DX'er's complaining about (usually) my AM stations for the similar symptoms. Used to dread those phone calls: "I just wanted you to know that you're causing interference to the bla, bla, band." I'd have to spend several minutes of my day explaining how the high field of the station was overloading their receivers, or that harmonics were being created inside their tuner(s). One guy kept calling, threatening to call the Commission. I finally drove over to his home with a spectrum analyzer and FIM 21, and showed him, that the harmonics he claimed the station was producing, existed on in his radios. Ultimately he stopped bothering me, but those were hours of my time wasted dealing with some know it all.

 

[frankberry]

Another thought ... if WCAR has an MW-1A, with a modulation enhancer (clipper) they may be clipping too heavily. If they are not using the required 10kHz brick wall filter, the transmitter could certainly produce some wide sidebands.
I'm still leaning towards some kind of oscillation in the transmitter or, perhaps, their processor. It seems unlikely that a 250 or 500 watt station could produce a serious amount of receiver overload unless the receiver was very close to the transmitter site.

 

[Kelly A]

It seems unlikely that a 250 or 500 watt station could produce a serious amount of receiver overload unless the receiver was very close to the transmitter site.

In the complaining DX'er cases I dealt with, they usually were using long wire antennas well inside the 50mVm field of the AM station. When they switched to a loop antenna, the harmonics disappeared.

 

[K8PLW]

In the complaining DX'er cases I dealt with, they usually were using long wire antennas well inside the 50mVm field of the AM station. When they switched to a loop antenna, the harmonics disappeared.

I'd agree with you, but this occurs on all my receivers. I was in the parking lot at work yesterday evening, a good 8 statute miles from their site, and WCAR was obliterating WHO on 1040 khz. A 500w AM splattering a station 50 kc below it from 8 miles away??

 

[K8PLW]

Another thought ... if WCAR has an MW-1A, with a modulation enhancer (clipper) they may be clipping too heavily. If they are not using the required 10kHz brick wall filter, the transmitter could certainly produce some wide sidebands.
I'm still leaning towards some kind of oscillation in the transmitter or, perhaps, their processor. It seems unlikely that a 250 or 500 watt station could produce a serious amount of receiver overload unless the receiver was very close to the transmitter site.

WCAR's audio, listening by ear, sounds louder, and appears to have greater range and more high frequency components when compared to other local signals. Which is why I suspect @frankberry may be correct. Whatever audio processing is needed to limit occupied bandwidth is missing or malfuncting.

 

[K8PLW]

As David mentioned, if they were overmodulating to the point of creating interference, it would sound really distorted because the modulation envelope would be closing once the station got past 99% negative.
I suspect the front end of your receivers are just being overloaded.
Over the years I've dealt with many Ham's or DX'er's complaining about (usually) my AM stations for the similar symptoms. Used to dread those phone calls: "I just wanted you to know that you're causing interference to the bla, bla, band." I'd have to spend several minutes of my day explaining how the high field of the station was overloading their receivers, or that harmonics were being created inside their tuner(s). One guy kept calling, threatening to call the Commission. I finally drove over to his home with a spectrum analyzer and FIM 21, and showed him, that the harmonics he claimed the station was producing, existed on in his radios. Ultimately he stopped bothering me, but those were hours of my time wasted dealing with some know it all.

I'm certainly not a "Know it all" (Which is why I reached out for assistance), and I realize that hams come with varying competency levels. Ive never worked in broadcast radio, but I've been in the commercial wireless/ two-way industry for many years. So I'm quite familiar with receiver overload, images, intermodulation, harmonics, etc. What i'm not familiar with is broadcast audio processing and bandwidth standards in a broadcast radio environment. I assure you this isn't a "Dumb ham" scenario.