I commute from Aloha to Hollywood (NE Portland) daily and prefer to have some type of mind-challenging entertainmnet so I listen to AM talk a lot. I've noticed over the last many months that every station I can think of that I listen to will randomly drop out for a couple of seconds, where I only here distortion and vague sounds of other stations - then come back. While it's not a huge problem, it's very distracting and it seems to happen to every station I listen to, across all ownerships. What's going on? Seems very unprofessional. Is this HD Radio related? I've never noticed this in the past. Just curious.

Would this be about the times of sunrise or sunset?

Possibly, it's on my commute either to or from work. Hadn't really thought about sunrise / sunset.

From The FCC site, the official sunrise/sunset times that stations in our area must use for day/night power/pattern changes. Transmitters are usually turned off for a few seconds while the change is made. This may be what you are hearing. Some stations do not have to change.

http://www.fcc.gov/fcc-bin/srsstime?dlat=45&mlat=25&slat=20.00&dlon=122&mlon=33& slon=57.00&tzone=D

That is so weird! That may very well be what's going on, I had never noticed before. Sometimes it seems to happen more than once in an evening but I may be mistaken - the sunrise / sunset theory sounds much more likely.

And also if you are listening to a directional AM station, you often will drive through a minima in their directional pattern. If its an inline tower array, which most of them are, you might be on the 'backside' of the array and in a minimal signal area. Other skywave stations or noise may come in at that time. Some AM stations have several mimimas and maximas. Those AM stations really sound strange especially when you are just a few miles from the transmitter and all this happens.

I have literally driven down Edison street here in Kennewick, looking across the Columbia river over to west Pasco at the KALE/960 AM transmitter site 3 tower array at nightime power of 1KW and couldnt hear the station over CFAC in Calgary, Alberta ! KALE's minima is South, SE E. and NE and Maxima is pronounced and large to the West-NW towards Richland, its city of license. You can hear KALE like gang busters on I-5 between Woodland, Wa and Olympia and out to Westport, Aberdeen and Hoquiam like it was a local station ! KALE protects 960/CFAC, 960 in K-FALLs ( used to be KAGO ) and a 960 in Oakland , CA ( used to be KNEW ). When KALE went directional ( long after 1950 ) the pattern was tailored to put a strong signal into the Hanford Nuclear Reservation, while protecting all the other stations especially Calgary.

So, the times are not random and the bottom line is that the power/pattern change happens twice a day, and the times change every month. You probably won't hear a change in the summer, as the times are way early and way late.

I think every AM station in Portland except 1330 & 1450 make some kind of change. If it's only a power level change the signal probably won't get inerrrupted (800, 1640, etc.).

All this info, including maps of coverage, are at http://www.radio-locator.com

Years ago, there was a commercial that started out with, "This is the sound of Ampex tape", followed by complete silence and then the words, "Didn't hear anything, did you? That's because Ampex tape...". On this particular morning, they decided to run the spot at the exact instant of the pattern change. I believe it was KGW. Here is what was heard: "This is the sound of Ampex tape. Kerplunk! Didn't hear anything did you?" I was recently reminded of this when hearing a similar commercial and turning a corner, bringing up some wheel noise at just the right moment.

Legally, sunrise and sunset pattern changes have to be made on the closest quarter hour to sunrise or sunset on the 15th of the given month. What I don't know is whether it is based on City of License (I think so) or transmitter location.

Coolest pattern change I ever heard was at 9:00 p.m. one summer evening in Vancouver (Canada) in 1971. I was listening to CHQM Vancouver on 1320 KHz on a car radio. Effective Radiated Power would be about 100,000 watts at me because they had to protect South. Even on the crappy car radio, I heard a perfect "KCPX Salt Lake City" jingle. Nothing before it or after it. And none of the jingle cut off.

I worked at CHQM at the time as an Operator, so it got awfully tempting to air a KCPX jingle around sunset one night. But, of course, the logger tapes were a studio feed, not off air, so I'd get caught.

I'm not sure if any of you remember KRED Eureaka on 1480, but they were off the air for something like 90 seconds at sunset for their pattern change all through the 1960s. It was so long that they actually had to run a fairly long recorded explanation.

On the other hand, I thought I was watching the DJ at 1590 Tillamook change patterns. He just adjusted some dials by hand. And the station never went off the air at all. All I remember for sure was that he was running 8000 watts instead of the assigned 5000.

Well Monday 1190 did it at 5:29 PM then Tuesday 970 did it at 5:29 PM so it does sound like some sort of sundown switching. My bad :-) It's not so annoying now that I know what's going on.

On the way into work a couple of nights ago, around 5:30 PM when KEX switched to their night pattern the IBOC or HD (whatever you want to call it) had a weird echo effect right before the signal went off and right after it came back on. I don't know if that is normal or a problem.

isn't HDdigital supposed to eliminate problems with signal dropping out???thats what the announcement on KEX implys!!!!!

You are confusing the issue. When the pattern changes, for a second, there is no carrier. Too much digital information is missing for your receiver to compensate. I'm sure the announcement on KEX is about reception in general, and they are really stretching the truth because ...

When boundary conditions are not met, there will be "dropouts" in the signal. It doesn't matter what kind of modulation is riding on the carrier. Sorry to be the bearer of bad news, but HD doesn't change the core physics of RF propagation. You can't demodulate the signal if it's not there. What digital modulation (spread spectrum) does is minimize other anomalies and presents the modulation many times within the signal envelope so that the receiver has multiple chances of recovery. There will be a reduction in multipath effects, but it's not a bullet proof solution or guarantee.

but, does propagation change the chacteristics of the modulation such that its not reallya valid signal anymore(phse shifting and/or frequency/amplitude shifting??

Excellent question. The answer is yes.

The propagation, or geometric spreading happens because the wavefront radiated signal energy expands like a big column as a function of the distance from the transmitter.

• Signal power is diminished by geometric spreading of the wavefront, commonly known as free space loss
• Signal power is attenuated as the wave passes through solid objects such as trees, walls, window and the floors of buildings
• The signal is scattered and can interfere with itself if there are objects in the beam of the transmit antenna even if these objects are not on the direct path between the transmitter and the receiver.

If the wave is damaged, then the modulation may be damaged, too. This is why a strong multipath signal may sometimes be intelligible and sometimes not.
Here's where digital spread spectrum modulation may outperform a sister signal of the analog variety.

Some spots around had townhave/had abig hum on AM signals caused by power linesI believe somehow.what happens to HD signals in those hum places???

In most noisy locations that I have observed, HD AM provides noise free reception... power line static is simply just not heard! If the power line noise is very high (and/or HD reception is weak) the HD signal will be disrupted, and the radio will revert to analog (with plenty of noise).

Thank you 2009

JP

PS: I know and can give you all a grasshopper blunder in thunder writen wise... But I just wanted to thank the HD bunny lovers out their and be me JP... As I love the air Mac now! How in no copper wires will it work in the jungle when everything is down or not going to be stuck in the hole as the wireless is dead because of no wires...