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Author Topic: cliff affect  (Read 677 times)
KD5EJG
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Posts: 8




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« on: July 11, 2009, 04:41:15 PM »

how to solve the dignal cliff affect,pixellation on dtv ? any ideas
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W5RB
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Posts: 564




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« Reply #1 on: July 11, 2009, 04:44:27 PM »

Get more signal to the receiver.
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AC5UP
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Posts: 4413




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« Reply #2 on: July 11, 2009, 08:04:03 PM »

Nothing beats starting with a strong signal, but there is more to it than that... The signal needs to be stable and relatively free of multipath. I've been tinkering with the antenna setup and currently have a non-directional wire in the attic. Also have a large sweetgum (southern maple) tree in the front yard that overhangs that section of the roof.

On the three VHF-HI signals in my market the tree causes no problems. On the UHF side it does... On a windy day I can watch the level meter dance between 70% and 90% with occasional dropouts during the transitions. With analogue TV the signal level swings would be imperceptible, but either my receiver is dropping bits while the AGC does its thing -or- the tree is giving the wire an infinite number of multipath combinations while it dances in the breeze.

The effect is much less pronounced in the evenings after the wind goes nighty-night.

So... The next step will be to side-mount the wire vertically on the side of the house furthest from the trees. Might be a while before that happens as this is not a good time of year to be strolling in the attic.

BTW: The wire is 64" long with the feedpoint 16" from one end. Direct feed to 75 Ohm coax, I like the longer wire on the center pin and the shorter wire on the shield. That's a full wave on channel seven and a shade over a half wave on FM broadcast. Works much better than expected and I'm currently driving two DTV boxes plus an FM stereo receiver without any amplification. My best DTV station is consistently at 90% with a 32 dB S/N ratio and on the FM side it's good for 80 miles or so depending on condx.

I've found that small changes in the antenna alignment can make a big difference in the stability of a DTV signal... You want 50% or better on signal strength (~18 dB S/N) with as little fade and multipath as possible... You might even find that an antenna location which sacrifices a bit of signal level for improved signal purity works mo'better.
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AC5UP
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Posts: 4413




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« Reply #3 on: July 12, 2009, 12:40:39 PM »

UPDATE: Was feeling a bit too perky this morning and decided to make a quick run up in the attic before it got hotter than Satan's toaster oven... Glad this was a quick job.

Moved the wire exactly one rafter closer to the end of the house and flipped the wire. Instead of the longer wire on the left it's now on the right. This was not done because of any mystic permutation of circularly polarized RF, just a simple case of the new orientation giving me a bit more slack in the coax.

My worst DTV station is now running 30 dB S/N with 100% solid copy. If I was any happier I'd have to be twins.

FM performance might be a shade better as well, but we're talking a fraction of a nanoscoshe at most.

Two feet south and it's perfect.

Go Figure.
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W5ER
Member

Posts: 74




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« Reply #4 on: July 12, 2009, 08:59:43 PM »

Sounds like them darn green rf chokes (leaves) were messing with your tiny wavelength signals. I'd bet a lot of folks that used to get free tv are now out of luck.

73 Ed
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N5LRZ
Member

Posts: 0




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« Reply #5 on: July 13, 2009, 04:59:00 AM »

Digital Signals be it TV or digitial images on the amateur bands are a matter of you either get the adequate signal quality or you do not.  IF you do you get the picture.  IF you do not then you get absolutely nothing at all.

Welcome to all or none.
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AC5UP
Member

Posts: 4413




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« Reply #6 on: July 13, 2009, 05:06:15 AM »

Whilst channel surfing last night I was amused by what you'll be noticing more in the future... Visualize a movie made 10 years ago or better that is set at some point in the distant future.

Dramatic scene where a remote outpost is under attack by aliens or some such and the video feed to HQ starts to break up just as they're trying to say something really, really important. The picture fades in & out to snow.

10 years ago this was perfectly acceptable.

Today, you're thinking: "So... 200 years from now we're going back to analogue TV Huh"

Wink
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