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DTV antennas?

Discussion in 'Electronic Design' started by Joerg, Jul 11, 2008.

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  1. Joerg

    Joerg Guest

    Ok, guys, as predicted it's the pits. Digital TV cutting in and out at
    random because of multipath and other disturbances. Analog never does
    that, it simply performs better. But it's almost gone :-(

    Besides the fact that newer antennas should have their UHF section tuned
    to Ch51 instead of Ch69, what other caveats are there? I have seen some
    weird scenarios, like UHF-only yagis mounted vertically. AFAIK the
    transmitter antennas are supposed to be horizontal. Of course those
    simple FCC station lists say nothing about that, or much else for that
    matter.

    I wish the situation improves when DTV partially migrates to VHF. At
    least then we could see the news.
     
  2. mpm

    mpm Guest

    Bummer to hear about that.
    Not all DTV allocations replicate their analog coverages, but most do.

    As for the broadcast end of things, the FCC's website does provide
    antenna detail.
    Manuf, model, height and pattern. If you're good, you can pretty
    easily decode the horiz & vertical, # of bays, and gain of the
    antennas just by deciphering the antenna model number...

    For example, KFOX in El Paso, TX.
    http://www.fcc.gov/fcc-bin/tvq?list=0&facid=33716

    Send me your coordinates (or nearby street intersection) and the
    channel number you are having trouble with. I'll be happy to take a
    look at it for you. -mpm
     
  3. Joerg

    Joerg Guest

    Ok, yes, if you go into every filed record. That'll take forever. But
    not on a list.


    Thanks, but it's actually most of the digital channels. In bad weather
    nearly all collapse. Yesterday I saw something weird. Outside TV didn't
    have 29-1 programmed, so I did an "add channel search" upon which it
    promptly lost all memory of any DTV station. Great. But it found a cable
    leak signal that happened to be the channel we wanted to watch, in
    analog. Then the cabnle company must have lost the feed just like we do,
    audio cut out, blocks show up, blocks get bigger, the whole thing
    freezes into a huge Picasso. Oh boy, I foresee a great unhappiness
    boiling up with the masses.
     
  4. Joerg

    Joerg Guest

    Right :-(


    People who do antenna design told me. Many are tuned at the highest
    channel because that'll be the one with the largest path loss. But not
    for CATV head stations, there the antennas are often custom.

    A bow-tie would result in horrible DTV performance out here, too much
    multipath.

    Aha! Thanks.

    Nah. It has always puzzled me why the TV media in California are unable
    to provide a reliable and free directory like in other countries. After
    all, they should be after viewership. But they don't seem to get it.
     

  5. I am using bow-tie on the side (all terrestrial is vertical here) in
    the attic and it works great on long distances.
    Some multipath, no problems, but then again I am in Europe QAM 64.
    But the bow-tie is a nice antenna and at least has good screening for
    reflections from the back.
     
  6. mpm

    mpm Guest

    As I recall, (back in the late 80's), this DTV thing was sponsored by
    the broadcasters, NOT the FCC. They had just seen channels 70-83
    vanish to create cellular and you have to understand the mindset of TV
    networks: Their spectrum had been a sacred cow for years!!

    Channel 37 was allocated for deep space telemetry, now cellular.
    What's next?
    Let's offer digital programming - that way we can justify the sacred
    cow.

    Along the way, it morphed into an either-or situation. Somthing I'm
    still not too sure was ever in the mindset of the technology drivers
    at the time.....?

    I was involved in the DTV test in Charlotte (WCNC-6) and the DTV
    trials in Washington. In the early days, there was no (or very, very
    little outisde the top-20 markets) financial justification for DTV.
    Consumer sets were extremely expensive, very limited availability, and
    it wasn't even clear consumers would upgrade to "better" over-the-air
    TV. DVD's, satellite and digital cable were eroding over-the-air,
    besides which, just hanging a new, separate DTV broadcast antenna on
    the tower was somthing that generally overloaded the tower
    structurally. Couple that with a new DTV transmitter, licensing,
    land permits, construction, etc..., and you're into some very serious
    capital expense.

    But to answer Jeff's question - I strongly believe that TV-6 will be
    auctioned off next (most likely to the FM band). TV-6 is a terrible
    over-the-air TV/DTV channel anyway, especially given it's 10.7MHz mix
    with a mid-band FM channel. Propagation of DTV on 6, as well as 2, 3
    and 4 is also pretty crappy. (Though I think a handful of
    broadcasters eneded up with these low VHF assignments during DTV
    optimization rounds.) Very, very few full-powers are currently on
    TV-6, and even fewer elected TV-6 for DTV.

    Even TV-5 might see the day where its 6 MHz are converted to FM.
    The conversion of TV 5 & 6 to FM will mostly affect the low power TV
    stations (LPTV), with some notable exceptions, of course. There is an
    active, open preceeding at the FCC to reallocate these channels last
    time I checked. And it's not a new idea by any means, having been
    considered at least twice before. As they say: "Stay Tuned!"

    -mpm
     
  7. mpm

    mpm Guest

    Are you out in the boonies or something??
    In most cases DTV replicates the former analog coverages.
    Are you sure you don't have a bad cable, connectors, or set??

    Sounds fishy to me. My two cents.

    BTW: You only have to look up the stations of interest in your area.
    Just pick one that's not coming in, and review the 41dBu countour.

    For instance, the KFOX-DT example provided previously.
    Here's its 41dBu contour:

    http://www.fcc.gov/fcc-bin/FMTV-service-area?x=DT1092992.html
    If you lived in El Paso, would your residence be contained within the
    blue line or not?

    Actually, this is a really good example. KFOX (from memory) is on
    Ranger Peak. From there, at about 250 feet AGL or so, you could have
    skimming line-of-sight propagation into Las Cruces, NM - but I'll bet
    DTV coverage there isn't as great as advertised on the graph!! (?)

    I believe these countours are calculated using Longley-Rice
    propagation models, which would consider terrain. But maybe KFOX is
    higher than I thought - though none of those towers on Ranger Peak are
    all that tall.. Hummm....

    Anyway, I do think that poking around on a few channel contours would
    help you confirm whether you "should" be receiving signal or not. Or
    call the local station engineer. Most I know are very willing to
    help. It's actually part of their job description. Good luck.

    -mpm
     
  8. mpm

    mpm Guest


    You can also try:

    http://www.v-soft.com/ZipSignal/zip_answer.asp

    This is only good for AM & FM. (From Doug over at VSoft)
    I do not know of a similar online resource for DTV, but there might be
    one out there.
    Navigate up a page to understand the dBu levels (signal intensity), if
    they are unfamiliar to you.

    -mpm
     
  9. Joerg

    Joerg Guest

    That's the problem. They think that oversimplified SNR calcs are good
    enough. With ATSC it ain't, it falls apart the minute you hit multipath.

    Or just hang up on TV ;-)
     
  10. mpm

    mpm Guest

    A Narda meter will read field strength in terms of E or H field, not
    power density.
    Though I supose if you know enough about the probe, you can do the
    conversions...

    Also, be advised that for FM at least, the FCC uses a modified
    Bullington propagation model which will not consider obstructions (or
    water) closer than 3km, and no further away than 16km. So for FM
    stations along the front range in Colorado for example, the FCC
    patterns will deviate significantly from real world performance. (FCC
    plots have more to do with short-spacing and licensing issues, not
    coverage.)

    The null you're seeing in the FCC pattern (KFOX) is terrain
    obstruction. Not an antenna pattern null.
    This pattern is typical of all stations on Ranger Peak.

    Still, for the purpose of this thread (i.e. poor or no DTV reception),
    field is not the issue. Power density is.
    The numbers & plots provided on the FCC public website are completely
    adequate for determining whether or not you "should" be receiving a
    DTV signal at your location.
     
  11. mpm

    mpm Guest

    That same topography (all things being equal) should have been a
    problem for analog as well then.
    Though, with analog, you can still get a picture even with some
    multipath and/or knife-edge diffraction going on.
    Much less so with DTV, though the receivers are getting better....
     
  12. mpm

    mpm Guest

    For FM, there was another one: rronline.com (I think).
    Was formerly run by "Radio and Records" outift.

    Nice thinig about that one was you could get the Arbitron ratings for
    the prior quarter.
     
  13. That's my recommendation. No TV for me for almost 20 years and I'm
    not at all inclined to change it. And I live in a house where my wife
    _does_ watch. I'm never even tempted to sit by her at those times and
    watch. Other things perhaps, while she is occupied, yes. But not
    watch the TV.

    Jon
     
  14. On a sunny day (Sat, 12 Jul 2008 06:47:22 -0700) it happened Joerg
    <MB2ek.15198$>:

    Actually I wonder about all those DTV problems.
    I would have though that you, Joerg, with all your RF experience
    and radio amateur experience?, would have dealt with that issue long
    time ago.
    I mean, with all respect, one can measure field strength, shield reflections,
    find a suitable antenna etc.
    If it then is clear there is a true problem and reception is not possible,
    write to your representative...

    As to the search and find no more channels, that is why I use an USB DVB-T
    receiver and the PC (disk) for recording,
    It is easy to scan, and of course I made a backup of the best scan list
    I got,
    Also easy to port that list to an other PC, and easy to stream over
    the net around the house, or around the world wherever you are, so you
    do not have to miss you pet programs,
    Of course this did not all occur to me on the first day I received
    a digital signal.
    But I can even steer the satellite dish from anywhere in the world.
    It is nice, this afternoon ITV1 had one of those old Cliff Richard
    movies, 'Summer Holiday', just recorded it remotely and streamed it
    via TCP/IP.

    Maybe take these remarks as hints, also for the others, complaining
    sucks, solutions are much better,
     
  15. Joerg

    Joerg Guest

    Analog degrades very gracefully. But we had situations where analog was
    nearly perfect yet almost all digital channels croaked.
     
  16. Joerg

    Joerg Guest


    BTDT. You can get Ch40 to come in perfectly but then you lose Ch13. Get
    Ch13 perfect and you lose Ch29. And so on. With analog that was not the
    case.

    I'll wait until Feb 2009 when all the frequencies are re-assigned and
    (hopefully) the DTV signal of the main stations scoot over to VHF. If
    it's still the pits I will sure write.

    The missus won't like a PC in the living room :)
     
  17. | Ok, guys, as predicted it's the pits. Digital TV cutting in and out at
    | random because of multipath and other disturbances. Analog never does
    | that, it simply performs better. But it's almost gone :-(
    |
    | Besides the fact that newer antennas should have their UHF section tuned
    | to Ch51 instead of Ch69, what other caveats are there? I have seen some
    | weird scenarios, like UHF-only yagis mounted vertically. AFAIK the
    | transmitter antennas are supposed to be horizontal. Of course those
    | simple FCC station lists say nothing about that, or much else for that
    | matter.
    |
    | I wish the situation improves when DTV partially migrates to VHF. At
    | least then we could see the news.
    |
    | --
    | Regards, Joerg
    |
    | http://www.analogconsultants.com/
    |
    | "gmail" domain blocked because of excessive spam.
    | Use another domain or send PM.

    What is your boom length Jorge?

    Cheers
     
  18. mpm

    mpm Guest

    Jeff,
    I've spent a career creating, reviewing, and analyzing RF propagation
    maps.
    I agree the one you posted (of the amateur station) is pretty hard to
    read.

    As for the single contour, you can't say with certainty that past the
    line there will no reception.
    In fact, there will be. And, inside the lines there will also be
    places with no coverage.
    But 41dBu of signal is sufficient for adequate coverage - absent any
    weird stuff.

    Also, the dBu requirement changes slightly by frequency. Keep that in
    mind.

    What you might need is a full study, using the tighest topo data and
    calculation grids you can live with - but even that won't give you
    indoor reception. For that, you'd need extensive ray-tracing models,
    etc..., and frankly by that point, you're more likely to invest in an
    outdoor antenna on a pole, or cable / satellite, etc...

    For cell sites, remember they are more concerned about adjacent
    channel re-use (and interference with CDMA systems, etc..) , not
    maximum RF coverage. So generally, cell site plots much more
    resolution than broadcast to be of any practical use.
     
  19. mpm

    mpm Guest

    Another piece of potential fall-out from all this DTV switchover:

    I wonder how much attention is being given to hurricane prone
    coastlines and the need to receive news and weather updates while the
    power is out??

    How many of us own portable DTV's? Probably well below 1%.
    If the power goes out, (and you dont' have a generator), the cable,
    satellite and DTV converter boxes all stop working.
    After 2009, turn on your portable TV and you'll find it doesn't work
    either.

    I spoke to a Chief Engineer at a Miami station a month or so ago, and
    he claims that after 2009, they won't even have the exciter equipment
    available to switch back to analog during a hurricane disaster
    (assuming the FCC would permit it, which they probably would?) It all
    boils down to economics. They don't want to pay extra tower antenna
    space for anything related to analog after they turn it off. So the
    transmitter goes, the antenna and transmission line goes, etc...

    And presumably, they will have stopped upconversion by then and be
    digital end-to-end, though I seriously doubt it!
     
  20. Rich Webb

    Rich Webb Guest

    Exactly. When (not if) cable and power go away for a week or so, it's
    all-the-way with OTA. I guess it's back to radio days.
    Amazon lists a couple but they're "currently unavailable."
     
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