Tips 4 catching nearby (groundwave) AM with its boxe--er IBOC off briefly?

[pianoplayer88key]

Hi al…

I was recently wanting to do my own A/B test of a station's reception quality with its IBOC off vs on. I've seen rbrucecarter5 mention that coverage suffers with IBOC on, and I have yet to find that to be the case.

A few weeks ago while doing a video bandscan on my Tecsun PL-398mp one early afternoon, as I approached 1070 (KNX) I noticed the indicated signals on the adjacents were MUCH lower than normal - around 3-5 dBu or so. 1070 itself was still clocking in at about 47 dBu, its normal strength here. Several minutes later, as I went by 1070 again, I noticed the sidebands were again around 22 dBu, indicating their IBOC was back on. I didn't notice any difference in reception quality, especiallly since I was using the 1 kHz audio (2 kHz RF) DSP filter. (KNX is about 111 miles from here.)
I wonder if my radio's filters being tighter than Bruce's radios could have something to do with the difference in our results?

My gluteus maximus is STILL(!) sore from me kicking myself after carelessly deleting that video.

So, I'm hoping to catch someone with their IBOC off for a few minutes or an hour or so, preferably around noon on a weekday It doesn't have to be 1070 - could be 570 KLAC, 600 KOGO, 640 KFI, 740 KBRT, maybe 980 KFWB or 1110 KDIS. If possible i'd want to know a few days ahead of time, so if necessary I could go somewhere to either weaken KOGO's signal (it's noise-free local at my house) or strengthen KFWB (analog barely above noise) or KDIS (has a blowtorch 2nd-adjacent here) and maybe KLAC (strength between 980 and 1070 - too weak to hear IBOC hash but strong enough for analog to be listenable).

One I'm thinking is 600 KOGO - I wonder if they sometimes shut IBOC off when they broadcast live sports events? If so maybe some day they could have a mid-day Aztecs game on a weekday with their IBOC off? (I'd need to go farther from their TX as it's local grade here though.)
If 1070 could repeat their IBOC outage it'd be nice too. Or would 740 be better, even though on 730 my DSP radios have internal noise and there's a strong local on 760? Or what about 640 KFI, or would they be likely operating lower power into the shorter antenna if their IBOC is out?

73, Stephen

 

[rbrucecarter5]

Hi al…

I was recently wanting to do my own A/B test of a station's reception quality with its IBOC off vs on. I've seen rbrucecarter5 mention that coverage suffers with IBOC on, and I have yet to find that to be the case.

One of the examples I often cite is WBAP 820 Ft. Worth / Dallas - one of the few 50kW clears that actually acknowledges and appreciates their DX audience both in the daytime and at night. Their daytime footprint is legendary - I've heard them out to Roswell, NM in the daytime on one of the old Delcos with a big whip antenna. Of course nighttime they are well known across the country. When they discovered that HD was sapping their coverage, they quickly abandoned it to get their coverage back.

I notice WOAI 1200 San Antonio - right in iBiquity's back yard, and long a poster child for IBOC, has abandoned it. The reason is pretty clear, they are close enough to Austin to be a player in the ratings there, and Austin's quick population growth makes it too large an audience to pass up. With more power after they abandoned HD, they are significantly stronger there.

Of course, both stations penetrate buildings better in their primary city.

I have no idea why you are getting different results. It is pretty conclusive here. All of my radios are modified for narrow IF response - one of my tricks to squeeze every last mile of DX out of them while getting rid of local garbage on adjacent frequencies - like my nemesis KEYH 850 which splatters high frequency hash all over 860. But they still detect the difference in signal strength when a station drops HD. WOAI went from a pitiful, weak, static filled signal to almost a local when they dropped HD. Static free on most radios now.

I notice the same effect on FM, with KGLK having HD off, then on, then off for at least a year. I drove back and forth from Dallas, and the reception line moved from Centerville to Huntsville and back depending on HD. Not a big deal for KGLK if they miss those little towns out there, but the implications for signal strength in buildings and on the usual cheap radios people have are enormous. And, like WBAP, whether it is FM or AM - if you increase the radius of your signal by 40%, the ring created has vastly more area than the original area of coverage. If you add several dozen of those little towns of 3000 people, it adds up to a significant population. Since most radio ads now days seem to be Geico and other national brands and services, it is a game of numbers. You add 100,000 to your audience over the competition, it is a viable marketing point. Especially in medium and small markets - places like Midland, where people in those remote communities look to Midland as "the big city" and their "local" stations. Since Midland is under 100,000, and even with Odessa under 200,000 - you can bet they want as many of those little towns scattered over West Texas and Eastern NM as they can get. You get several dozen of them out there, and you double the audience (and advertising revenue) I doubt a single station out there runs HD.

 

[JohnnyElectron]

rBruce = as a 'local' could you convince WBAP to turn their CQUAM stereo back on? Could hear them in stereo about half the time in Ohio.

As far as KBRT; Cris Alexander is pretty good about helping out engineers and might let you know when their new operation will be HD free for a couple of minutes or hours as they bring their new 50KW transmitter online in SoCal after moving operations over from Catalina Island.

 

[Geographer]

Some posters on this board have said the AM stations that run IBOC lose coverage. I travel frequently and am very familiar with the range of many of the 50 KW stations. Although I am not a supporter of IBOC for AM, I do not see any evidence of it reducing the coverage of the analog signal. I will concede, however, that stations running IBOC sound softer, or not as robustly modulated. Perhaps a station that runs IBOC can switch in on and off at 1 minute intervals and announce it here in advance. I do not see any risk to the stations profits since nobody is probably listening the the digital signal anyways. Any listeners will likely see it as a typical dropout. I'm not denying that it can sap coverage, but am highly skeptical. Perhaps it can affect skywave coverage due to some cancellation resulting from the varying of the signal strength, but as far as groundwave coverage is concerned, I just can't see any evidence of reduced coverage. A simple on and off test would answer the question.

 

[stacker]

Here is what I hear locally. When listening to the local AM 50 kW station running IBOC (by local, I mean listening within the city of license), I hear an inrush of noise from the digital sidebands whenever I travel near a re-radiating structure. That means power lines at intersections, some buildings, overpasses, etc. I suppose this is due to the digital sidebands reaching the receiver out of phase and not cancelling as they should. The result is that the host analog signal is wrecked. The same thing happens when listening to this same 50 kW station when it is on it's night pattern. If you are on the back side of the pattern, the noise goes way up. If you drive through one of the broad nulls, the signal is almost un-listenable. I suspect this phenomenon gets worse as distance from the transmitter increases.

As a result, the analog signal from an IBOC station is inherently less durable.

Plus, consider that the energy to create the IBOC signal has to come from somewhere. Watts are watts, no matter how distributed.

 

[rbrucecarter5]

Again I will cite WBAP - "Today, according to an independent study, WBAP has the greatest daytime coverage of any radio station in America, and as much coverage at night as any other U.S. radio station." -from http://www.wbap.com/page.php?page_id=121

They are proud of their coverage, and have a long history of reaching out to rural listeners. Being an ABC owned station, they had HD forced on them. If there were any benefit, if it bought them anything at all, or even if it were totally neutral in terms of coverage, they would still be running HD. But their coverage suffered, I am sure they had complaints from listeners. So they opted, instead, to put their signal on a full class C FM as well. Albeit a rim shot from far NW, a signal that doesn't gain them much in the Eastern part of the DFW metroplex. Still, a full class C FM has a large footprint over the areas from DFW to Wichita Falls, I can get them in the car easily out to Benjamin, TX, with occasional car reception to Guthrie, so they are roughly covering the same area their AM does.

I can only report repeatable observations, especially from KGLK which was on again / off again HD for over a year. If I wasn't getting them by Centerville, I knew they were running HD again and wouldn't be there until Huntsville. I got them in Centerville, HD was off. I now use satellite from Huntsville to Fairfield when I make the drive, because FW stations used to make it to Centerville, where Houston stations would take over. Now, South of Fairfield to North of Huntsville, there is virtually no FM coverage from DFW or Houston. The fault - IBOC.

 

[audioguy]

Listening to the three low dial 50 kW AM stations at our summer cottage in SW Michigan at a distance of ~130 miles from the transmitters, WBBM with its IBOC hiss on is significantly softer and less robust while driving around in the daytime than either WSCR or WGN. At night, they all get killed by the IBOC sidebands from NYC on adjacent channels. And I mean, BURIED. I read on here that WOR might be shutting off their IBOC soon, and that would certainly be a boon to WGN reception at night in Michigan.

 

[MarioMania]

How is HD sapping coverage can you explain it??

 

[KB1OKL]

WBAP was a semi regular up here in the winter in Ma when it was a clear channel. WBZ is 40 miles from me and comes in significantly worse since it's had the hash maker on.

 

[Geographer]

Again I will cite WBAP - "Today, according to an independent study, WBAP has the greatest daytime coverage of any radio station in America, and as much coverage at night as any other U.S. radio station." -from http://www.wbap.com/page.php?page_id=121

As good as WBAP's coverage is, I don't believe they have the greatest daytime coverage of any station in America. Did that study refer to ALL stations or just Clear Channel stations? For instance, the low on the dial 5 KW stations in the Central States such as WNAX, KWTO, KFRM, and KWMT have superior daytime signals due to the low dial position, excellent ground conductivity and lack of interfering stations. While traveling I noticed that these stations had great daytime coverage, and the maps on radio-locator.com confirmed my observation of the superior coverage to the 50 KW stations I have listened to. If you count Canada, CBK also has superior coverage.

With respect to IBOC sapping the coverage of AM stations, if you are referring to adjacent IBOC stations hissing on another station, i.e. WABC vs WJR, WOR vs WLW, then I'll concede that IBOC saps the coverage. But I do not see any evidence of the a stations own digital signal interfering with the coverage of their analog signal other than degrading the sound quality and hissing on its own signal. For instance, WLW could be received during the day in Milwaukee and Davenport, IA, with or without IBOC.

 

[Savage]

IBOC degrades coverage of the host AM station because most analog receivers are wideband enough to get some of the unwanted adjacent-channel steady-state digital noise (hence the background hiss.) The always-on digital carriers (at least 25 in the passband of each adjacent channel, high and low) are very high amplitude which is one factor which makes the interference so obnoxious. Your analog receiver's AGC confuses the high-power digital noise with desired analog and throttles the sensitivity back. Once you're outside the very strong local-contour coverage, the digital sidebands effectively become more powerful relative to the desired analog modulation, so the regular AM starts to get buried in QRM and QRN - digital and otherwise. Turn off the hash, the AGC opens up and the desired signals start coming through again.

IBOC-AM: the gift that keeps on giving. Fragile and dropout-prone from every "pop" from a light switch or distant lightning crackle. Sounds crappy in digital and forces quality compromises to the main analog channel. Costs a ton and screws up live sports and remotes with the unavoidable decoding delay. And steps on the only coverage that generates revenue for an AM. Total genius! :

 

[DaveBayArea]

How is HD sapping coverage can you explain it??

With AM HD you must do two things to the Analog signal:

1. Narrow the bandwidth so the high frequency portion of the analog signal doesn't fall within the HD carriers' spectrum.

2. Reduce the modulation level so that it does not exceed some number on negative peaks - I've heard everywhere from 92 to 95%. So this means the analog signal is not as "loud" and therefore more susceptible to noise.

The other issue of phase cancellation - where the digital carriers are much stronger than the analog signal at some locations - has been brought up already in this thread.

Dave B.

 

[rbrucecarter5]

IBOC degrades coverage of the host AM station because most analog receivers are wideband enough to get some of the unwanted adjacent-channel steady-state digital noise (hence the background hiss.)

I think the key point here is that the decrease in coverage is receiver based. The iBiquity people drove around with spectrum analyzers, and declared no signal degradation. They forgot about the second part of the transmission system - the receivers. Radio receivers are NOT spectrum analyzers. They will interpret the sidebands as noise / adjacent stations, and adjust their AGC accordingly. This explains the coverage loss for FM as well - the receivers do what they were designed to do - ramp down the gain in response to interfering signals. It makes perfect sense when you think about it.

 

[Ai4i]

...superior daytime signals...if you count Canada...

If you count Canadian coverage of a US station, KFYR is #1.

 

[rbrucecarter5]

Again I will cite WBAP - "Today, according to an independent study, WBAP has the greatest daytime coverage of any radio station in America, and as much coverage at night as any other U.S. radio station." -from http://www.wbap.com/page.php?page_id=121

As good as WBAP's coverage is, I don't believe they have the greatest daytime coverage of any station in America. Did that study refer to ALL stations or just Clear Channel stations? For instance, the low on the dial 5 KW stations in the Central States such as WNAX, KWTO, KFRM, and KWMT have superior daytime signals due to the low dial position, excellent ground conductivity and lack of interfering stations. While traveling I noticed that these stations had great daytime coverage, and the maps on radio-locator.com confirmed my observation of the superior coverage to the 50 KW stations I have listened to. If you count Canada, CBK also has superior coverage.

With respect to IBOC sapping the coverage of AM stations, if you are referring to adjacent IBOC stations hissing on another station, i.e. WABC vs WJR, WOR vs WLW, then I'll concede that IBOC saps the coverage. But I do not see any evidence of the a stations own digital signal interfering with the coverage of their analog signal other than degrading the sound quality and hissing on its own signal. For instance, WLW could be received during the day in Milwaukee and Davenport, IA, with or without IBOC.

WBAP goes all the way to Roswell, NM in the daytime. Although the coverage actually goes away as you descend the caprock just East of town. If you draw a circle of that radius, and I think it is valid, and add potential coverage into the Gulf, you are talking about a lot of square miles. But this thread could easily disintegrate into "my station is bigger than yours" thread. If I had to venture a guess, I would think XEWA would have the largest North American footprint if you don't consider water path. I suspect they are the 540 I am getting in Houston, and not KDFT. But I don't speak foreign so I don't know.

Prior to IBOC, I could get WLW in the daytime in Houston before the local 700 signed on. And I could get KOA in Dallas daytime on occasion before an 850 signed on there. This was car radio reception, not a DX setup. After IBOC - nothing like that. I have to go North of Amarillo to get much on KOA now.

 

[audioguy]

I would like to offer another idea about why IBOC degrades AM reception even in the daytime... because I have noticed this frequently on WBBM AM over in SW Michigan while driving around. I think that over a longer path where you have some selective fading (not uncommon even in the daytime), the two noise sidebands do NOT reliably cancel like they are supposed to, thus adding additional noise to the AM signal. In effect what you have is self interference caused by the digital sidebands to the analog host signal.

 

[rbrucecarter5]

I would like to offer another idea about why IBOC degrades AM reception even in the daytime... because I have noticed this frequently on WBBM AM over in SW Michigan while driving around. I think that over a longer path where you have some selective fading (not uncommon even in the daytime), the two noise sidebands do NOT reliably cancel like they are supposed to, thus adding additional noise to the AM signal. In effect what you have is self interference caused by the digital sidebands to the analog host signal.

Add to that - it is virtually impossible to tune exactly on center frequency, which is what is required for the cancellation to occur. I was lucky one day to find a station broadcasting HD with no audio on either analog or sidebands. It was amazing just how even minor adjustments to the tuning made the sidebands come rushing into audibility. And most digitally tuned radios are not perfectly on frequency, they program an analog tuning voltage to get very close, and a small degree of AFC is supposed to do the rest. But it is debatable how effective the scheme really is.

Another aspect that I found interesting. When I did get the radio tuned perfectly on center frequency, a peculiar warbling sound became apparent. I don't have video of all this, but I do have the audio tape as I tuned back and forth.

 

[Savage]

Was the warbling effect present on all HD-AM stations, rbruce?

 

[rbrucecarter5]

Was the warbling effect present on all HD-AM stations, rbruce?

I only had the one that was broadcasting dead air with the HD on. I've often wondered myself if the warbling was due to:

(1) Asymmetrical HD sidebands.

(2) Asymmetrical IF response of the radio.

(3) Phase shift at the transmitter towers.

I suspect a combination of all three.

Incidentally - I posted here about daytime WLW reception in Houston and daytime KOA reception in Dallas prior to locals signing on and HD. I need to clarify that this was seasonal, peaking in the winter - as opposed to reliable year round like the 450 mile reception of WBAP, which is a constant.

Daytime reception beyond 500 miles requires a large loop - four to five feet, a low noise / high ground conductivity listening location, and patience because the stations are subject to deep fades lasting as much as an hour or two. Like the reception noted above, it does have a seasonal component although I have done it at various times of the year. Definitely not your run of the mill type of reception.

What was surprisingly constant was the persistence of HD sidebands long after the carrier and analog portions of the station were gone, hundreds of miles further. You can read my archived posts to find the details. Suffice it to say, I became very excited about the prospects of full digital AM - I am thinking AM stations could achieve coverage unheard of today. At some point in the future, if AM HD survives, the band is thinned by 90% or more, and all AM is digital - I think 1000 mile coverage by 50 kW clears is definitely in the realm of possibility. Sadly, the failure in the marketplace, proliferation of jammers, and general lack of interest in AM by broadcasters will likely kill the prospect before it can be realized.

 

[Geographer]

WBAP goes all the way to Roswell, NM in the daytime. Although the coverage actually goes away as you descend the caprock just East of town. If you draw a circle of that radius, and I think it is valid, and add potential coverage into the Gulf, you are talking about a lot of square miles. But this thread could easily disintegrate into "my station is bigger than yours" thread. If I had to venture a guess, I would think XEWA would have the largest North American footprint if you don't consider water path. I suspect they are the 540 I am getting in Houston, and not KDFT. But I don't speak foreign so I don't know.

Prior to IBOC, I could get WLW in the daytime in Houston before the local 700 signed on. And I could get KOA in Dallas daytime on occasion before an 850 signed on there. This was car radio reception, not a DX setup. After IBOC - nothing like that. I have to go North of Amarillo to get much on KOA now.

I know the daytime footprint of WBAP was impressive when I heard them sound like a local on a cheap radio in a hotel room in Wichita Falls. When you heard WLW in Houston during the day, was that midday skip, or was the reception reliable? I know WLW has an impressive daytime footprint, but about 400 miles was the limit for me.