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Rain fade

Discussion in 'DIRECTV HD DVR/Receiver Discussion' started by GirkMonster, Mar 30, 2007.

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  1. GirkMonster

    GirkMonster Godfather

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    Mar 19, 2007
    Anyone experience more frequent rain fade with the Slimline dish compared with the older triple-LNB dish?

    Related question, why would the HR20 demonstrate something similar to rain fade using an attic-mounted OTA during a significant storm? It was much more brief than the Sat channels, but surprising to me nonetheless.
     
  2. gabe23

    gabe23 Legend

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    Mar 6, 2007
    In my experience, the Slimline dish is much better at avoiding rain fade than the older dish. Also, my OTA is sometimes affected by strong storms on any tuner (H20, TV tuners), so I don't think that's an HR20 problem.
     
  3. GirkMonster

    GirkMonster Godfather

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    Mar 19, 2007
    Thanks. I don't think it lasted very long and the storm was fairly severe. I went to the OTA because a local station has 24-hour weather on one of its digital multiplex channels.
     
  4. Tom Robertson

    Tom Robertson Lifetime Achiever DBSTalk Club

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    Nov 15, 2005
    Wow, that was a gully washer and raining cats and dogs! I've had zero rainfade with my AT9 since it was installed (and I forced the installer to align it correctly). We don't get many rainstorms, but they can be intense.

    Next cloudy/rainy day compare your signal strenghts on 99° and 103°. The spot beam for your locale should drop a little, but still be above 75. Everything else might fall off completely (I only have one of all 13 transponders that is higher then 50. And its 95 most of the time.)

    Cheers,
    Tom
     
  5. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Dec 9, 2006
    After "cats & dogs", it becomes "lions and tigers and bears, oh my".
    I get them here too.:)
     
  6. Tom Robertson

    Tom Robertson Lifetime Achiever DBSTalk Club

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    Nov 15, 2005
    The two desert and near-desert locations I've lived, Phoenix and Salt Lake, have produced some of the more intense storms I've seen. And in Phoenix it was a yearly event. Here in SLC, not quite so often.

    Cheers,
    Tom
     
  7. bonscott87

    bonscott87 Cutting Edge: ECHELON '07

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    Jan 21, 2003
    I have had OTA issues before in very intense storms. It's usually the lightning between me and the towers. Most of the time is really, really bad lightning at the towers, not at my house.

    This has happened twice in the past 3 weeks and both times there was nothing going on at my house or for 20 miles around. But the location where the towers were was getting hammered.
     
  8. 94SupraTT

    94SupraTT AllStar

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    Nov 18, 2005
    I'm having this very issue with my Slimeline dish. I live in tornado alley and the 3LNB dish rarely experienced rainfade whereas I'm almost guaranteed to lose signal for moderate storm with the Slimline.
     
  9. Hansen

    Hansen Hall Of Fame

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    Jan 1, 2006
    Come to Texas where the intensity of the storms are on par with the size of Texas. .
     
  10. glennb

    glennb Hall Of Fame

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    The OTA transmission is digital just like the digital satellite transmission.

    The OTA will be affected by heavy rain or wet snowstorms.
     
  11. GTS

    GTS Member

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    Mar 4, 2007
    This problem is becoming more and more prevalent. The installers are cutting corners by not performing the dither alignment and thus not truly peaking the dish for the Ka birds. If they don't have a meter capable of seeing the 99 and 103 satellite signals they must perform the dither or the dish simply is not peaked. Thre's no reason a properly installed and aligned dish can't achieve a signal level in the 90s on the 99 and 103 birds. I've seen a lot of "so long as the signal is above 70 it's fine", but this is not so. The biggest problem caused by this misconception is "premature fade".
     
  12. TomCat

    TomCat Broadcast Engineer

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    Aug 31, 2002
    It rarely rains where I live, but my numbers went up an average of about 15 points when I replaced my Phase III with a Slimline. That leads me to a conclusion that the Slimline is hotter, since I vetted precise alignment myself on both dishes (and I used the HR20 to align both, so no, it's not a HR2x issue).

    It may be as simple as a poor aiming job. The Slimline is much more critical for the Ka channels (most of MPEG-4 HD is on Ka). IOW, you can be aligned half-assed for Ku and still get good numbers on those transponders, but it takes precise alignment for the Ka transponders because since the downlink frequency is higher, the focal points are more focused, and therefore about 1/3 the size of a Ku focal point. Also, wind-loading is an issue with a larger, heavier dish, which means bracing struts are commonly used. Make sure your Slimline has those, and install them if you don't have them.

    The OTA interference problem is similar because the attenuating mechanism is similar (objects, such as rain drops, getting in the line of sight and absorbing or reflecting the signal). Both UHF/VHF and microwave (sat) have rain-fade issues.
     
  13. TomCat

    TomCat Broadcast Engineer

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    Aug 31, 2002
    Well, not exactly, although I agree with your suspicion that installers are doing a rush job on Ku and leaving it at that. For them, doing a precise alignment on a dish they don't own is like getting an oil change in a rental car. Why should they care?

    By "not exactly" I mean that Slimline alignment is done on the Ku sats, not on the Ka sats (or at least not if following the instructions). If done precisely to 110/119, the Ka sats fall directly in line. You don't need a Birdog or digital SLM to do this accurately, although that helps speed the process. I had the advantage of being able to stand on my balcony and adjust the dish while watching my 60" Sony through a picture window 8 feet away, so I guess it seemed simple, but if you really want to get accurate, schlep a TV and HR2x (and power) out to your dish.

    Download the Slimline manual (Google is our friend) and simply follow the directions, and remember that dithering precisely is the key, and dithering precisely means using the "outside" of each matching number, not the "outside" of one and the "inside" of the other (once you get the concept, that will make sense, trust me). I had mine done in half an hour, and when the installer (finally) arrived he could not improve the numbers using his meter, though he seemed motivated to prove he could, not even at all.

    Bottom line, whether you have a meter or not, dither, dither, dither. And once you get there on 119/110, repeak on the Ka sats (even though that isn't really in the instructions or even necessary if done properly to Ku in the first place) as that verifies max rain fade on HD.
     
  14. Kansas Zephyr

    Kansas Zephyr Hall Of Fame

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    Digital vs. analog isn't the issue here.

    The higher frequency (satellite) will suffer more from attenuation by rainfall/snowfall, versus the lower (OTA) frequencies.

    In a case where you have the same signal strength to start with sat/OTA, you will always lose the sat signal first, and get it back last.
     
  15. TomCat

    TomCat Broadcast Engineer

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    Aug 31, 2002
    Comparing signal strength for sat and OTA is an "apples vs. oranges" issue. Whether you get OTA back first or not depends on the comparitive amount of rain fade margin, which will be quite different depending on how far away the OTA towers are from any particular receive antenna. Different transmit and receive systems have different rain fade margins designed into them, and these systems are very different.

    That said, there is a very simple reason why rain is more destructive, all else held equal, to sat downlink signals vs. OTA signals, which is that Ku and Ka wavelengths are very similar in size to raindrops, while UHF wavelengths are significantly larger than raindrops. Raindrops are like little "roach motels" to sat signals--signals go in, but they don't come out. Since the raindrop is the same size as the wavelength, the sat signal causes the raindrop to vibrate sympathetically at frequency, and instead of passing through and refracting, much of the signal is converted to heat, thus attenuating the aggregate signal level. That doesn't happen with OTA signals.

    What that means is that raindrops can both absorb and refract the sat signal, but those same-sized raindrops can not significantly absorb OTA signals, they only can refract them. The corresponding attenuation is therefore much higher at Ka/Ku frequencies. It's simple physics.
     
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