Thin-Air ATSC (and NTSC): Snowfall Windows And Mountain Shadows

Continued from ‘Thin-Air ATSC (and NTSC): Reception Irregularities And Hardware Retreats‘…

A few minutes later, KTVN abruptly came back on. I reported the positive development to Jack, along with asking him if abundant moisture in the air (which at that point was diminishing, complete with occasional sun appearances) might explain what had happened. Here’s his response (with slight grammar fixes by yours truly):

Moisture does have an effect, but not so much at TV frequencies, whether VHF or UHF. Vegetation has been found to have a more pronounced effect at UHF. Also, if it were simply moisture, whether in the air or absorbed into coax, it would tend to affect everything, not target just one channel.

Although, (and this just occurred to me), maybe a splice or junction in the coax is getting soaked. Then there could be an impedance mismatch, causing a reflection, which could then cause problems on one channel but not another. In TV transmitter work, coax line is big rigid stuff, basically two copper pipes, one inside the other. And it comes in 20′ lengths, except if you are on certain TV channels (channel 10 is one), when you instead use 19.5′ lengths. Why? Even though very small, each splice has a slight discontinuity, usually only a concern at higher powers, as even small problems at 20 kilowatts can cause fireworks. But in your case, maybe a remote possibility?

As far as your reception of us…No, nothing has changed here. Except maybe the air is drying out. Hmmmmmmmm…

Here’s my suspicion of what happened. Late Sunday afternoon, the falling snow started sticking…not only to the roads, driveway and deck, but also to the horizontally mounted antenna. I’d experienced plenty of snowfall accumulation on the antenna before, but never this wet. Monday morning, the sun came out, the snow melted away and fell off the antenna…and KTVN came back online.

Later on Monday, the rain and wind returned, and this time KOLO started flaking out. The signal would be fairly strong one moment and almost undetectable the next. As the storm strengthened, KOLO’s average signal got worse, but when the rain and wind diminished late in the afternoon, KOLO returned to its former solid condition. It reminded me of a past problem I’d had with KRXI, whose transmission coincidentally (or not) also comes from Peavine Mountain, and whose UHF reception also relies on my Antennas Direct ClearStream 2.

I realize that Jack Antonio said above that airborn precipitation effects are minimal-to-nonexistent. Jack’s comments jive with your past feedback as well as other Internet research results; take, for example, this discourse (again with my minor edits):

Atmospheric absorption from water vapor (humidity) and oxygen amounts to less than .01 dB in 80 miles at any frequency less than 1 GHz (any UHF frequency). In other words, it’s not the humidity. Even a hard rain (100 mm/H) will only account for about .002 dB /KM at UHF. Most of the fading experienced at UHF is due to one or more of the different types of atmospheric multipaths that can impact UHF.

If airborne rain and snow didn’t instigate the problem, then what was the root cause? The Antennas Direct Clearstream 2 was snow-free at the time. Wind-induced movement of either (or both) KRXI’s broadcast antenna at Peavine or the reception antenna on my house might explain the situation, but I’ve been in heavy windstorms before without noticing similar symptoms. Ideas, readers?

Finally, a few words on mountains as RF shadows. The two-part report that Paul Rako showcased in a blog post last week originally came to me per past dialogue with one of its authors, Lou Dorren; I subsequently forwarded it on to both Paul and to Ron Wilson given their Bay Area locations. I encourage you to read it (the bulk of which consists of reprints of various FCC memos), although I admittedly have some concerns about it (in line with my documented reasons for not passing CECB reception results on to readers of my recent feature article).

Any reception study is inherently limited by the scope of the equipment (antennas, tuners, demodulators, etc) used in it, both their capabilities and their limitations. And in spite of Dorren and Lewis’s claims that "neither one of us has an axe to grind regarding digital or analog television", I find plenty of evidence to suggest the contrary, beginning with their ‘pipe dream’ recommendation to substantially further delay the NTSC shut-off in order to create a "compatibility solution which will not obsolete analog TV receivers and give the time required to fix the DTV system."

As someone who knows a number of people who live both in the Santa Cruz Mountains southwest of highway 280 and in areas to the northeast of the East Bay Hills (including the Diablo Range and the Berkeley Hills), I think that Dorren and Lewis have overstated the acceptability of the marginal analog television areas they showcase in yellow (versus unacceptable-red for digital equivalents) in their graphs. And I also continue to scratch my head at the reality basis for their upfront claim that the digital TV transition will disenfranchise "20 to 50 Million American Television Viewers With or Without Set Top Boxes" (revised later in the report to read "The DTV conversion slated for February 17, 2009 will disenfranchise 15 to 25 million TV viewers in the United States and obsolete more than 50 million TV, TV audio, and Reading Service For The Blind receivers"), in spite of Dorren’s subsequent explanation attempt to me:

The 20 to 50 million not only includes antenna TV viewers but over 25 to 50 million battery operated portable TVs and over 1 million reading service for the blind receivers that use the SAP channel to provide service. Also not taken into account are the millions of radios that have TV sound capabilities.

What I do find intriguing, however, is the RF shadow effects of Twin Peaks in San Francisco (specifically Mount Sutro, the home of Sutro Tower) and San Bruno Mountain to the south of it, especially in the context of one knowledgeable Bay Area reader’s response to a post of mine last week. I got a chance to see both mountains up close last Saturday on the way back home from my whirlwind road trip to Silicon Valley. You can get a sense of their respective interference impacts from the Google Maps links I’ve provided earlier in this paragraph, although for a more comprehensive understanding, I’d suggest you install Google Earth and give them both a more horizontal virtual perusal.

Mount Sutro (more accurately described by Wikipedia as "a hill") is fairly narrow and only 909 feet high, and the population density to the north of it is comparatively scant, so its attenuation impact is relatively muted. Conversely, with a 1,314′ peak and much broader overall footprint (Wikipedia reports that it "is topped by a four mile long ridge"), San Bruno Mountain casts a substantially more imposing RF shadow on the substantially higher population Bay Area Peninsula to the south of it. You can clearly see the destructive reception results of San Bruno Mountain’s presence in Dorren and Lewis’s graphs.

Several knowledgeable individuals have told me that as soon as the analog transmitters are removed from the top of Sutro Tower, the currently part-way up digital transmitters will replace them, thereby hopefully enabling Sutro Tower-originated ATSC signals to more effectively arc over San Bruno Mountain to receivers beyond it (either in a direct beam or in a single- or double-edge diffraction fashion). I can’t help but wonder, though, if broadcasters will eventually also need to install translators like those up here in the Sierras in order to blanket affected areas with redundant transmissions on other channels. Perhaps on Mission Peak, or joining their peers already on Monument Peak? Lou Dorren’s thoughts:

The FCC did not issue any experimental permits during 2008 for field testing of DTV on channel repeaters. In late November 2008 they issued a report and order to authorize untested on channel repeaters referred to as distributed transmission system ("DTS"). In the report and order it was stated (understated) "DTS may be a useful tool for stations to prevent some loss of service to existing analog viewers resulting from changes to the station’s service area in the transition to digital service."

As we explained in our report, this band-aid will create more propagation problems then not because it is impossible to predict sum and difference products over the DTV pass band after radiation. They rely on transmitter synchronization to contain the problem, however, the signals are only in sync until they radiate from the transmitting antennas, then all bets are off. Multipath from one transmitter due to environment is tough enough, but from multi on channel DTS transmitters is ridiculous.

Followup: I may have figured out the root cause of yesterday’s KOLO reception problem. Looking at the Antennas Direct ClearStream 2 a few minutes ago, I realized that the strong wind had slightly twisted it on its vertical clamp mounts, both towards due north (i.e. away from a direct northeast orientation to the summit of Peavine Mountain) and towards a large pine tree in my front yard, which was likely swaying in the wind during the storm.