Thin-Air ATSC (And NTSC): The Friday FCC Follow-Up

August 1, 2008

I ran Wednesday’s two-part writeup on over-the-air television reception (and not) at my mountain-surrounded residence by analog technical editor Paul Rako and overall tech whiz Ron Wilson, and got some interesting feedback. Here are Paul’s thoughts:

For transmitters it depends on what the FCC will allow them. A "clear channel" is one that is allowed to broadcast at full power 24/7 in a 360 degree pattern. The FCC might make a station have a cardioid or other pattern in order to reduce interference at another city. The FCC also will make the stations turn down the power in the evening so that the signal does not skip off the ionosphere and cause interference in other cities. As to what those particular stations are allowed, I would not know where to look, perhaps the FCC has the data or the engineer at the station will know for sure.

Ron followed up Paul’s comments:

I think Paul is on the right track. Instead of looking at signal strength, look at SNR on a particular digital channel. I believe the reason the experts say a head-end LNA [editor note: low-noise amplifier] won’t help much is that usually in urban areas, the problem for the digital broadcasts isn’t the signal strength, it’s the noise level. As Paul points out, even though the FCC has tried to legislate overlapping antenna patterns out of existence, there are still lots of noise sources out there. And if one of them happens to be even a little bit correlated every once in a while (like a coincidence involving a lower-frequency signal aliasing up, for instance), there is just not enough redundancy in those little digital channels for all the information to survive.

Ron’s ATSC-centric observations might explain why I was previously unable to reliably tune in Fox 40’s digital stream back in Sacramento (followup) but it doesn’t necessarily explain (for example) my poor-to-nonexistent reception of KOLO and KREN’s NTSC (analog) signals, which originate from a mountain ridge top that’s 20 miles away and line-of-sight visible to my eyeballs and antenna. So I’m going to take Paul’s FCC research suggestion to heart, with your (hoped-for) assistance.

Below you’ll find direct links to the FCC database entries for the stations mentioned in Wednesday’s writeup (note that below, I list only the primary NTSC channel, not the translator channel that in many cases is what I’m tuning into…see Wednesday’s report for translator data):

You’ll find data on, for example, transmission tower locations, broadcast power, antenna type(s) and, if one or multiple antenna are directional, the broadcast ‘footprint’. Please take a look at the information, correlate it to my observations documented earlier this week, and see if you can figure out a pattern to my per-channel reception presence-or-absence. I’m particularly interested in figuring out why I can’t tune in the signals originating from Slide Mountain. Conversely, I can easily rationalize why I’m not able to tune in broadcasts coming from towers deep down in the Reno-area Washoe Valley, on the other side of the ~10,000 foot (3,000 meter) Mt. Rose, Verdi and Bald Mountain Ranges.

As much time as I spent around antennas back at Magnavox, I admittedly didn’t soak up much knowledge about their particulars. Therefore, this ‘binary guy’ thanks you RF experts in advance for your insights! I’ll also attempt to contact the stations’ engineers; if I hear back anything interesting, I’ll pass it along here at Brian’s Brain.

Posted by Brian Dipert on August 1, 2008 | Comments (4)

August 13, 2008

In response to: Thin-Air ATSC (And NTSC): The Friday FCC Follow-Up
Interactive_ace commented:

Jed is right for the most part. But ducting can last for hours in the summer time along the Eastern shore. stations from hundreds of miles away would roll in in the summer and cause problems for the local VHF reception.

August 13, 2008

In response to: Thin-Air ATSC (And NTSC): The Friday FCC Follow-Up
Martin Rowe commented:

Ahhh... here come the complaints about DTV. They will only get worse as the switchover date (2/17/09) approaches and passes. There will be an uproar, which I predicted on 2/16/06. www.tmworld.com/article/CA6308148.html

August 3, 2008

In response to: Thin-Air ATSC (And NTSC): The Friday FCC Follow-Up
jed martin commented:

actually there are many different mechanisms for signals to get propagated much further than normal. during sunspot maximum (still a few years away) ionospheric propagation can take place up into the 70Mhz range. during thunderstorm season, signals between 50 and 250Mhz can get "ducted" along a violent weather front several hundred miles with very little loss of signal strength. this actually can happen during winter storms, but is much more common with thunderstorm squall lines. i have experienced both effects multiple times with NTSC. the ionospheric sporadic E propagation usually fades in, remains stable for up to an hour or so, then begins fading out. the ducting effect is usually of short duration, rarely beyond 10 minutes, and is very unstable, accompanied by rapid "picket-fencing" of the signal, and rapid cross-fades between a local signal and a distant one. i could see how these effects could play absolute havoc with ATSC signals, since the receiver would have very confusing data streams.

August 1, 2008

In response to: Thin-Air ATSC (And NTSC): The Friday FCC Follow-Up
Larry M commented:

Multipath reception which caused only ghost images in NTSC is death on ATSC. I'm only 22 miles from the antenna farm of 4 major stations, but there is a granite outcropping across the street from me that's as high as my rooftop antenna. I get a bounced signal from a high-rise about a mile away that's probably as strong as the direct signal. (Old timers in the neighborhood cite coming of the high-rise as the time all the NTSC ghosts started.) For ATSC it's less important to point the antenna at the farm than it is to point the antenna's null at the high-rise. Could you be plagued with multipath reception from mountains?

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