Boston TV Signals Disrupting Police Radio in NJ

jeffy124 writes "WCVB, a digital TV station in Boston, is disrupting police radio communications in South Jersey. It seems that under certain weather conditions, the signal reaches here travels 270 miles (it's normally 50) and blacks out the police frequencies, making communication between officers and from 911 call centers impossible. The article seems to suggest that as more TV stations go digital, more small-town police radio will be affected, as the digital signal is significantly stronger than analog. Insert Joisey-joke here."

Comments

It's called

[pa-guy]

tropospheric ducting.

Re:It's called

[mdechene]

I was actually at a presentation today where a RF Engineer from Nextel, I think, was talking about the same thing happening.

Apparently, when there is a layer of hot air above a layer of cold air (it's normally the opposite), the Snell's law can be satisfied for total internal reflection conditions. In other words, Nextel found that on certain days in the middle of the summer, they dropped like 60% of their calls on certain cells, as opposed to say 2% as is typical. Turned out that nearly all of the dropped calls were originating on the Michigan side of Lake Michigan, bouncing off these "inversion layer" ducts, and the phones were camped to Wisconsin base stations over 70 miles away.

Anyways, Nextel angled their antenna's down and decreased the power output somewhat, effectively minimizing the footprint, and the problem has been reduced (but not completely eliminated). Also interesting was that this thing seems to be much more likely to happen over a large body of water. They seemed to think in summer, the water cooled the air and then warm air blew across the cool layer. Apparently cool air has a higher refractive coefficient than warm air.....

Not the only occurence

[Papa Legba]

I work for a PBS station in VA, WHRO. We are currently being sued by a station WBOC-TV Salisbury, Md to stop the rollout of our Digital TV because it disrupts their signal on the Chesapeake side of the water.

From what I understand of the problem their were bad assumptions made by the FCC when it came to the digital signal.
1. That it would not bounce and doppler like analog signal does. Well it turns out it is even more prone to it than analog was due to the higher frequencies and watages involved.
2. That this would not affect a $Properly setup atena. Seems reasonable until you find out what the variable properly is. Apparently the FCC does not care about interference unless the atena is aligned directly towards the sending tower (that never happens and varies from channel to channel) and that it is not higher than 30 feet (one story home. Any deviation from that and it becomes your problem, not theirs.

This is also not the first case of this to happen. Their are previous cases in california and milwauke. Read more
here

This is going to crop up as more and more channels go digital. You will start seeing it reported more as stations start to battle each other. The sad part is that most likely the FCC will wash their hands initally and the airwaves will become as if the FCC does not exist.

Re:Not the only occurence

[Skuld-Chan]

That it would not bounce and doppler like analog signal does. Well it turns out it is even more prone to it than analog was due to the higher frequencies and watages involved.

Somewhere a ham radio operator is crying.

I think the term you are looking for is propagation (the way signals travel through the ether) :). In general doppler shift isn't that much of an issue unless the transmitter is moving very fast (like a low earth orbit satellite)

I have no idea what frequencies digital tv stations operate on, but in general on uhf tropospheric ducting is pretty rare - at least where I live.

Where analogue tv channel 2 (around 57 mhz) long range propagation is pretty common, but thats not tropospheric ducting - thats sporadic e layer propagation.

Past tv channel 7 tropospheric ducting is relatively common.

Past tv channel 13 tropospheric ducting still happens, but its not nearly as common.

I don't see why the mode would matter - I think digital television is a spread spectrum signal. If done right you should be able to operate other ss devices in the same frequency space. Narrow band radios recieve ss signals as low background noise typically - so that should be an issue. I wonder what the real interference problem is?

Gratuitous link

[Anonymous Coward]

Just in case you would like to know what tropospheric ducting is.

Re:Random related question

[borgboy]

hmmm....I dont know. I do know that Air Force One does have a large array of different comm systems in place, including UHF SatCom and I believe Iridium-style gear. But to call it "AF1's broadcast system" sounds a lot like you're saying it has only one communications system, which is certainly not the case. AF1 has to communicate with NORAD, the Pentagon, the White House, not to mention being capable of patching into the conventional phone network.

FCC regs...

[TheHawke]

Under FCC regulations, any interference with official bands (IE, police, fire, ambulance) by a TV station is considered illegal by law and MUST immediately stop of be fined per day per channel that experiences the interference.
This occurred with one cable TV station over in one town that i stayed in. They had brought up another channel into their lineup using AINCENT cable equipment that generated a harmonic with the City police's repeater and caused massive interference with their communications. The city immediately moved and filed a complaint with the FCC on this and the Gov't submitted a court order stating that the cable company shut down ALL services until this issue was cleared up. The channel was immediately shut down and the station was shifted to another channel that was more clearer and did not cause any further problems.

Pumping gas

[spineboy]

Why bother, I can have someone do it for me (in the freezing snow, rain, spiners, etc), it keeps my hands from smelling and it's cheaper than most of the country.

Do you cook your own hamburgers/steaks when you go out and eat? - no someone doe sit for you.

Minor corrections...

[KC7GR]

From the /. article header...

"The article seems to suggest that as more TV stations go digital, more small-town police radio will be affected, as the digital signal is significantly stronger than analog..."

Actually, the type of modulation (digital or analog) has little to do with the signal "strength" (which is a function of transmitter power output, transmission line losses, and antenna design and orientation).

Now, with that said, digital modulation, being much closer to a square wave than an analog voice signal, is much richer in HARMONICS than said analog signal.

I've lost count of how many times I've heard interference from digital paging transmitters bleeding into ham radio repeaters. The harmonics from the digital modulation mix with the transmitter's carrier, and that of whatever other transmitters happen to be on the same hilltop, and close to the same frequency range. It sounds awful, and it looks even worse on a spectrum analyzer screen.

The problem may be correctable through (as others have pointed out) better receiver design, in terms of filtering, and good installation practices being followed where the transmitter and antenna system are concerned. Good filtering and modulation techniques at the transmitter end won't do any harm either.

Re:Minor corrections...

[mesocyclone]

Now, with that said, digital modulation, being much closer to a square wave than an analog voice signal, is much richer in HARMONICS than said analog signal.

This is utter and total nonsense. The modulated RF signal is harmonic and spectrally in accordance by regulation. Digital modulation doesn't cause carrier harmonics (the two are unrelated) and the harmonics from the square wave digital (which cause sidebands or wider bandwidth depending on mode) are of course attenuated at base band before modulation is applied and/or filtered out.

After all, any energy wasted in "harmonics" is energy that is not available to carry information to the receiver. Digital modem designers (RF or otherwise) have known about this for approximately forever.

I've lost count of how many times I've heard interference from digital paging transmitters bleeding into ham radio repeaters. The harmonics from the digital modulation mix with the transmitter's carrier, and that of whatever other transmitters happen to be on the same hilltop, and close to the same frequency range. It sounds awful, and it looks even worse on a spectrum analyzer screen.

Again, this has nothing to do with the DIGITAL nature of the transmitters. Intermodulation is usually not a result of harmonics from the transmitter, but rather harmonics generated in your own receiver - totally independent of modulation mode. And yes, it does look ugly on a spectrum analyzer - whether it is digital or analog.

Re:Same frequencies?

[mobets]

Yes, our 'friends' at the FCC have been selling Digital TV channel 20 to police stations. The city bought those frequencies in '98. or maybe they trurned a band used for police into channel 20. Either way, they should have seen this comming. My question is what they were planning to do when Digital TV becomes more common.

Allocations

[rfg]

A lot of innacurate information has been passed around here.

UHF communication frequencies generally go from 450-470 mHz and were fully populated years ago. What the FCC did is to allocate certain UHF TV channels to communications, in the 470-512 range; ie TV channels (not cable channels) 14-20 for communications use in certain areas. The areas in question are laid out in a plan, so that in some areas a certain channel is used for TV and and in some areas that same channel is used for communications.

This came about because in a given area you cannot have adjacent TV channels used by TV or they interfere with each other. Also, UHF TV was never really popular with broadcasters and many channels were loped off on the upper end (ch 70-88 as I recall).

Thus it is perfectly in accordance with the FCC plan to have Channel 20 allocated to TV in Boston and to communications in Southern New Jersy. Up to now, however, channel 20 was never used in Boston, it was empty and now has been allocated to digital TV.

Analog TV stations must convert to digital by a certain date (2006, but keeps slipping....). During the interim period, the station may transmit Analog on its present channel, and digital on the new channel. This is precisely what WCVB is doing. Eventually the station will be strictly digital on Channel 20 and the Analog VHF transmission will terminate.

East coast atlantic tropospheric ducting is common and radio hams and others are well aware of it; I am surprised the FCC did not take this into account when they allocated the channels. If I had to speculate, I would say that the FCC will require WCVB to reduce power, use a directional antenna or change channels - which may be tricky. This will be fertile ground for hordes of lawyers.

Re:Just desserts

[r2ravens]

Yes, without New Jersey we wouldn't have...uhhh...hmmm...

A place to put our toxic waste?

But seriously folks...

Jon Bon Jovi? Paul Simon? Allen Ginsburg? Jack Nicholson? Joe Piscopo? Kevin Spacey? Frank Sinatra? Meryl Streep? Ray Liotta? Michael Douglas? John Travolta? Elizabeth Shue (hubba hubba)? Jerry Lewis (ok, we could do without him...)

Thomas Edison? Irving Langmuir (Incandescent lamp)? Edmund Germer (Flourescent Lamp)? Lloyd Conover (Tetracycline)? James Hillier (Electron Microscope)? Donald Fletcher Holmes (Polyurethane)? Roy Plunkett (Teflon)? Lewis Sarett (Cortisone)? Vladimir Kosma Zworykin (Cathode-Ray Tube)?

First college football game (Rutger v. Princeton, 1869)? First organized baseball game? First pro basketball game?

Campbell's soup? Cranberry Sauce? Salt water taffy?

Electric guitar (Les Paul, 1940)? First submarine (1878)? First ferry (between Hoboken and Manhattan, 1811)? First brewery (Hoboken, 1642)?

No, I am not a Jerseyite. I live in Arizona and have never been to New Jersey. (Ain't the web wonderful? It's always good to learn new things - especially at my advanced age.) Of course, none of these things is enough to motivate me to vist. :)