Battling Multipath when transmitters are angularly dispersed - Canadian TV, Computing and Home Theatre Forums

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post #1 of 21 (permalink) Old 2011-02-03, 01:32 PM Thread Starter
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Question Battling Multipath when transmitters are angularly dispersed

Guys, I'd really appreciate it if you could make any suggestions on what to do for my multipath issue.

I have transmitters due east of me (90° about 6 miles away), at about 140° (just south of southeast about 80 miles away across Lake Ontario), and about 190° (just west of due South about 30 miles away). With my original CM4221, I point it at 140° transmitters and pick up the others on the side lobes.

This works reasonably well for my tv's tuner, but the tuner on my TiVo Premier is a real multipath hater, so I get fairly bad pixelation on the channels on the 90° and 190° transmitters (even with strong signals). Ideally, I'd like to get all 3 transmitters clean enough to keep the TiVo happy (Note: I've gotten the impression from TiVo it is what it is, so an antenna workaround will be necessary).

To complicate things, my original CM4221 is a little bent due to some mishaps while running some experiments over the years. Some questions:

1) Is it possible the slight damage to the antenna is causing enough multipath to push it over the edge? Maybe replacing it with a new 4221HD will resolve this??

2) Would there be a better performing antenna than the 4221/4221HD for my situation?

3) Is there any point in considering a multiple antenna solution or might this even increase the multipath? I was thinking about ganging two 4221HDs at about 60° apart or even right angles?

Thanks!

A-D DB4e & CS5, CM 4221 & 7778, TiVo Premiere & Roamio
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post #2 of 21 (permalink) Old 2011-02-03, 01:53 PM
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The multiple antenna solution may help if you try ganging two very directional antennas. I believe there is some discussion in this forum on ganging the more directional X91 type antennas. Those directional antennas could be in the $100 each range, so you'd have to decide if it's worth the investment and the hassles of ganging them together.

It would help DHC members to understand your problem if you post a link to your current tvfool report in this thread.

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post #3 of 21 (permalink) Old 2011-02-03, 02:22 PM Thread Starter
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Your request is my command! Below is my tvfool:

http://www.tvfool.com/?option=com_wr...a362a2d127abdf

I just realize my angles were even larger than I though - actually about 60° apart instead of 50°!

A-D DB4e & CS5, CM 4221 & 7778, TiVo Premiere & Roamio

Last edited by El Gran Chico; 2011-02-03 at 02:26 PM. Reason: Correction to angles
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post #4 of 21 (permalink) Old 2011-02-03, 02:23 PM
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Where do you record mostly from ? Two antennas with two downleads into an A-B switch may be a solution, but youll have to remember to switch antennas ahead of time.

My builds/plans (not the latest models) are located here.
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post #5 of 21 (permalink) Old 2011-02-03, 02:38 PM
 
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El Gran Chico: I have multipath issues. The way I got around it was to use a low gain antenna. I had to play around with different antenna with different gain. For me the trick was to get an antenna with enough gain to get good main signal but not too much to boost the multipath. I'm using a shorted bowtie (it has a gain of about 5 and low SWR which seems to help also). My NM are in the mid 50's.
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post #6 of 21 (permalink) Old 2011-02-03, 02:42 PM
 
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I plan to optimize a custom antenna for the GTA post-transitional channels,
and as they are coming to north west mississauga.
The situation is very close to yours with Toronto/Buffalo/Hamilton at angles -55/0/55 degrees.
I will have to deside between GH4n..GH10n and GH4n3..GH10n3,
and I'd like to have it built by the switch over date.
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post #7 of 21 (permalink) Old 2011-02-03, 03:50 PM
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Battling Multipath !

Battling Multipath is especially bad in the summer with stray tropo co-channeling coming out of the wilderness

This thread will be a good one for sharing various experiences, solution to specific locations etc. I have found pretty much the same solution as xauto has posted earlier.

My solution for my location was found to be the SH design with multdirectional coverage but low gain.

Here's my tvfool link:
http://www.tvfool.com/?option=com_wr...3cf47027d298ad

[Just want to note that all Canadian stations suddenly appear to be 4 NM higher than they used to be on my older tvfool reports. I can't explain this, as Canadian stations have done nothing yet to improve their ERP or digital contours that I am aware of]

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post #8 of 21 (permalink) Old 2011-02-03, 05:05 PM
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I would be temped to try two vertically stacked CM4221s 90° apart, one due east and one due south. Join them with a 75Ω splitter/coupler and two RG6's of equal length. The entire assembly could be rotated slightly to get the best results. I haven't tried this but it should work. If it doesn't, you can still use the stacked antennas with both facing in one direction or experiment with different angles. The only real problem I foresee is with co-channels in both directions.

At 20 I had a good mind. At 40 I had money. At 60 I've lost my mind and my money. Oh, to be 20 again. --Scary
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post #9 of 21 (permalink) Old 2011-02-03, 05:12 PM Thread Starter
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Quote:
Two antennas with two downleads into an A-B switch may be a solution, but youll have to remember to switch antennas ahead of time.
Unfortunately that sort of defeats the "set it and forget it" feature of the TiVo, so I'd prefer to just sacrifice the "190°" channels (actually 204° on my TVFool) which is what I'm doing right now. I may actually lose the one channel I ever watch from that direction (CHCH) anyway since it's reverting to VHF in 7 months - I'm doubting the 4221 will be able to pull it in. Still if there's a >0% chance I can get this to work post-Aug. 31, I'd like to try to find a feasible solution.

Quote:
The way I got around it was to use a low gain antenna
I'm worried that a low gain antenna might end up with me losing the channels at roughly 140° (namely the 3 major network stations in South Buffalo). Or does my tvfool report indicate I should be fine?

If it comes down to deciding between living with multipath vs. reducing multipath while losing channels, or course living with multipath will win out.

A-D DB4e & CS5, CM 4221 & 7778, TiVo Premiere & Roamio
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post #10 of 21 (permalink) Old 2011-02-03, 06:09 PM
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Kinda in the same boat:
Check out this link to this post the other day, sure looks like multipath to me.
What do you guys think?
http://www.digitalhome.ca/forum/show...2&postcount=46
I'm still thinkin about it...like what would be around me that would cause that, etc.
I do see a water tank on a hill not too far away (1/2 mile maybe) in that general direction.
Pretty sure that's what a multipath degraded ATSC signal looks like though.
Be nice to see some other examples to be sure.

I would use a rotor, and just rotate my way out of it. To me, your more likely to be able to find a sweet spot where
everyone's happy rotating the antenna from the comfort of ur living room, than running up & down a ladder.
A diseqc like interface for rotors would be a nice addition to the OTA world, but the market would probably be slim.

Since TV tuners aren't setup to accomodate diversity reception, I'm not sure ya can do much more than traditional stacking, and antenna location experiments, to overcome it. In my example it always degraded at around the same azimuth, long before I ever threw a VHF antenna and pre-amp in the mix. So I don't think those components play much of a role, but we'll get back to testing again in the spring

Maybe it's possible to overcome with some fancier antenna designs? But I doubt it, or else we'd have already seen that somewhere, like in the wireless phone world.
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post #11 of 21 (permalink) Old 2011-02-03, 06:31 PM
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Quote:
Originally Posted by ota_canuck
The multiple antenna solution may help if you try ganging two very directional antennas.
Yep, the standard prescription from the analogue OTA era for multipath troubles is to use an extremely directional antenna such as a corner reflector yagi (in this case 2 of them). Add to that the new standard prescription from the digital era, which is to use tuners with Generation 6 ATSC chipsets (they were specifically created to battle multipath better than previous generations).

In your case, EGC, the Tivo tuner situation cannot be helped, but the directional antenna(s) idea would, such as using a 91XG or similar. The SH's omni-like pattern is worth a shot too.



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post #12 of 21 (permalink) Old 2011-02-03, 07:19 PM
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RE: majortom's 'link' to his WKBW problems.

I wouldn't rule out multi-path causing those WKBW problems.

I don't have a Spectrum Analyser, though after looking at your WKBW situation, you may have the same issues that I have occaisionally/intermittantly with certain channels. I have narrowed my issues down to ignition noise from, lawnmovers, dirt bikes, snowmoblies, etc. When these poorly tuned machines are running nearby my antenna [within 100ft] my analog channels show slight horizontal lines across the picture. That problem used to be insignificant with analog viewing, but it's enough to ruin digital viewing.

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post #13 of 21 (permalink) Old 2011-02-03, 08:22 PM
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Yeah, electrical noise is different though, with different symptoms. It won't affect the shape of the signal your looking at, from what I've seen it'll show up on the analyzer as noise spikes throughout the span your looking at. And will affect many channels simultaneously (typically dc to daylight).
A matter of which channels have the least margin to begin with as to which ones you'll actually notice degraded / dropping out, etc. But they're pretty much all affected by electrical noise. Whereas multipath should typically affect a specific channel (a signal from a specific transmitter that's bouncing around off various objects, etc).
I'm thinkin with Digital signals, because ya can't actually see it like analog, short of trial and error experimenting, recognising it's signature on a spectrum analyzer is the only way we'd know for certain multipath is the cause of a given issue.
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post #14 of 21 (permalink) Old 2011-02-04, 12:49 PM Thread Starter
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Thumbs up Thanks for your input so far

Thanks for your comments. As I dread to spend a lot of money on this without any guarantee of results any better than I have now, I'm considering this strategy (one the weather warms up a bit ):

- buy a CM4221HD. I understand it has a little less gain than my original 4221. Perhaps that and the fact it will be in pristine condition will make enough of a difference for the TiVo to maintain a lock

- If that is not fruitful, build a SH and try that

- If still no luck, buy another CM4221HD and try to vertically gang them (using identically lengthed RG6 to the combiner - the feed points to be separated more than 25 inches) with 90° separation first, and a few other angles

I don't think I want to go as far as ganging X91 type antennas, but if there is any other useful and not too expensive (both in $ and time terms) things I might try, I'd appreciate any other suggestions.

A-D DB4e & CS5, CM 4221 & 7778, TiVo Premiere & Roamio
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post #15 of 21 (permalink) Old 2011-02-04, 01:07 PM
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It would help if you could tell us WHICH channels you are having "multipath" problems,
and which channels you MUST HAVE.... And how do you KNOW it's "multipath" and not
something else.....such as co-channel or adjacent channel interference....

Then we can talk about a second antenna and some single channel filter(s) to allow JUST
the desired channel(s) through and filter out "multipath" on all the other channels....
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