I seem to recall that you're on the left coast and don't get thunderstorms and the like, but I have noticed that here in the Mid-Atlantic, that very wet weather (not necessarily thunderstorms) can render even the strongest local DTV channels unwatchable. With the exact same antenna on analog stations there was no noticeable degredation (except for nearby lightning) at all in the same weather conditions.
My pet theory is some sort of multipath from wet tree leaves but I have no scientific evidence at all!
Digital television is typically operated only a few dB above threshold, producing a "perfect" image. However, if the signal level drops only 2-5 dB, the signal can be below thresholds for extended period of times and the picture is useless.
If the normal signal to noise ratio is say 50 dB, you would hardly notice, if it drops to 45-48 dB.
At least in Europe, the digital transmitter ERP power was initially
10-20 dB below the analog TV synch tip ERP power, however, in order to maintain sufficient fade margin, the -20 dB settings are now out of the question.
The refractive index for vacuum is by definition n=1 and for air n=1.0003xx. In propagation calculations, the relative refractive index N is often used and it is the number of ppms above 1.0. At sea level N=300 is quite common.
The relative refractive index depends on the "dry term", which depends of air pressure, temperature and height and the "wet term" depending on temperature and amount of humidity.
The relative reflective index in general drops at higher altitudes and reaching N=0 in space. The globally annual average gradient is -40/km, while at some places it can be -30 /km and others up to -150/km (in desert areas in particular). For a particular place and a particular altitude layer, the instant gradient can vary even more than this during the year.
The reflective gradient is responsible for slightly bending the radio waves towards the ground and hence, the radio horizon is slightly further away than the optical horizon. For N=-40/km, the radio horizon is about 15 % further away than the optical horizon. For N=-157/km, the radio waves exactly follow the curvature of the earth and the signals could travel thousands of kilometers as long as the gradient remains stable.
In practice any layer of warm/cold and/or wet/dry masses at some altitude along the signal path will alter the gradient, sometimes bending the signal slightly upwards above the receiver and into the space. Sometimes it is bent slightly more downwards than normally and strong signals are available just behind the horizon.
With a strong gradient, the transmitter signal hits the ground in front of the receiver and is reflected from ground up to the space and is lost. With very strong gradients (N
Note that many DTV channels are a) on different frequencies b) from different transmitter sites/antennas with c) different power levels than with NTSC.
That said, in DC "7" and "9" DTV are on RF 7 & 9. But there's been a major effort to understand why VHF freq's have delivered so much worse ASTC propagation than was there under NTSC. Multiple stations have gotten temporary FCC authority to raise power levels. One suspect is the cheap CECB {couponed converters} suffer from front-end desensitization by FM broadcasters.
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Sorry, must have confued you with another living in the hills with ATSC reception problems and an ugly HOA.
Kind of wierd to have AM/PM type fading in VHF and UHF bands. The multiple antennas would point in very different directions. Not single channel path diversity but per channel group separated antennas pointed at the best signal (usually right at the transmit antenna but not always).
And Internet TV has total tracking (no privacy) and many times the ads.
Ok, the next step would be to buy a motorhome and drive 100ft farther, see if it works there. Then back up again when the clouds have passed :-)
There comes a point when the effort to obtain a signal just ain't worth it anymore.
Kids are pretty smart these days. They can tune out any ad they want and if tracking would ever be a concern there's proxy servers, anonymizers and all that. But I don't need that stuff.
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In long tunnels there are often AM and FM repeeater systems, as well a repeater systems for local emergency responder channels. Simple underpass are not so equipped.
Oops, forgot to mention, while rebroadcast systems are possible for XM and Sirius; State agencies often refrain from providing them because of the propritary modulation and fee for service aspects. They may allow the statellite providers to install and maintain their own systems. In case you are intersted they operate from 2160 to 2180 MHz.
There are differences, but they may be "in the mud" for most people. If you are paying attention to what you are listening to WinMedia has a small sound quality versus bitrate advantage, on the other hand MS is proprietary and MP3 is more generally usable.
This is a typical flat (broadband) fading case caused by varying refractive index.
Never heard of AM/PM fading, what is that ?
While a few meters or some hundred MHz might be sufficient diversity distance against frequency selective multipath problems, in order to combat flat fading, the separation would have to be tens or hundreds kilometers.
Oldsters remember when bandwidth was expensive, like i "dimed up" and downloaded the the packet drivers for early Ethernet cards from Clarkson Uni to the Lost Angles area ar 2400 baud over a few nights, cost me about $100 long distance for a little less than a MB. Now that would be a few seconds and included (and would dissapear) in my monthly. Today, a sloppy webpage will eat up a MB or more, and an overnight DL would be about 5 GB; over 5 thousand times the data volume. Just about 20 years difference.
How about this news? A well known AM radio station lost all three towers a few days ago. It was a well know country music station, with its Wheeling Jamboree.
In those cases I'd rather send them a SASE envelope, a blank diskette and $20 for the effoert to copy and the walk by the mail room. Then use the remaining $80 for a nice dinner with the wife.
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Regards, Joerg
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Only because the Obama administration and local tree huggers will oppose the reconstruction.
Amusing... when I lived in WV I didn't listen to WWVA, I listened to the rock-n-roll stations in Huntington.
As a married student at MIT, early every morning I drove my wife to work at Honeywell Datamatic and played WWVA in the car... very loud to keep awake :-) ...Jim Thompson
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