Battling Multipath when transmitters are angularly dispersed - Page 2 - Canadian TV, Computing and Home Theatre Forums

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post #16 of 21 (permalink) Old 2011-02-06, 04:55 PM
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Good thread. Yes, seems to be a common problem for many on OTA.

Transmitters locally, or not too far, in 2, 3 or 4 different directions - some within 40, 50, 60 degrees of each other, and want to be able to receive all channels from all directions well, simultaneously, without the need for rotor.

How to design one simple antenna "system" to do that.

Find a general solution, to this ... and you've solved lots of OTA troubles.

-or- Find a cookbook method to solve this ... and you've got something very useful.

[I'm thinking about it ... and I'm playing with LOOP type antennas. They're simple and easy to build, medium gain, some directivity, can be made fairly wideband. They can be bi-directional, but can also be made one-directional with reflectors/reflector rods]

Just some ideas for thought.

Finding that sort of solution, will greatly increase OTA usage.

[ I'm thinking like multiple loop systems, aimed in different directions on a frame round a tower, with or without reflectors as needed. Combining signals as possible or necessary. Or not. Or scaling signals, combining and/or subtracting signals as necessary. Or if you can come up with single 3-direction antenna desigh, 4-direction antenna design, n-direction antenna design ]
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post #17 of 21 (permalink) Old 2011-02-06, 08:51 PM
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The thread's original question is about signals from 3 directions, 90, 140, and 190 degrees ... I think ... but I wanted to mention a possible solution for 2 directions - I don't know if that could be somehow incorporated into a solution for this 3-direction problem ...

Here it is ... for 2 directions:

I remember that Holl_Ands showed a method used to mount 2 antennas, horizontally separated by some distance, and combined in reverse, 180 deg out of phase, so that a NULL is created at the center aiming point, and two receive lobes are created at some separation angle.

Could that method, if set up properly, maybe give a combination of signals from 2 directions, ex. 50 deg apart, and help reject "Multipath" signals? Signals coming from "other directions".


link provided by Holl_ands:

http://www.hdtvprimer.com/ANTENNAS/ganging.html#TAT


Holl_ands' post:

Post # 2 in Thread:
Over-The-Air (OTA) Digital Television >Antenna Research & Devleopment > Antenna Techniques that Natively Filter/Trap Certain Channels?

Quote:
Originally Posted by Holl_ands
Two Horizontally Stacked Antennas can steer a null towards undesired direction:
http://www.hdtvprimer.com/ANTENNAS/ganging.html#TAT

{ so, I imagine the goal might be: try to put the nearby, 90 deg/6mi one to the East in a NULL to reduce received power off that one, and put the other two 140deg/80mi and 190deg/30mi ones, furthur away, in LOBEs to increase their received power ... COMPLEX}

But what Holl_ands posted earlier in this thread makes sense too ... gotta try and figure out what's actually going on before a real solution can be designed and tried. i.e. is it really multipath? which channels causing multipath? or is it adj or co-ch interference? etc. etc.
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post #18 of 21 (permalink) Old 2011-02-07, 12:31 AM Thread Starter
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Quote:
Originally Posted by holl_ands View Post
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....
holl_ands, have you seen my tvfool report in post 3? It's RF18 and 64 that are my big problems. I can't "prove" it's multipath but both are still a reasonable strong signal (in the 60s on my Aquos' tuner) and the TiVo has known multipath issues (if I can trust others on the internet). Only the TiVo has problems locking them from my favourite aim (directly at 14, 32, 33, 38, 39, 43). 3 other tuners get everything fine.

If I turns my antenna slightly to the east, RF64 becomes fine on the TiVo, but RF18 starts getting too weak for the other 3 tuners.

As I mentioned earlier RF 18 will revert to VHF 11 on Aug. 31. I'm now thinking that it might be worth waiting for this to happen, use the more easterly aim and use the VHF input on my 7778 pre-amp and hook up a VHF only antenna (if I get get or build something cheap).

Still it might be nice to figure something out that would work for me in the ensuing 6.5 months so I could get all these channels on all of my tuners.

A-D DB4e & CS5, CM 4221 & 7778, TiVo Premiere & Roamio
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post #19 of 21 (permalink) Old 2011-02-07, 04:34 PM
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Well it's tedious and inelegant, but I'd be inclined to make (not all that efficient, maybe tunable) notch filters as attenuators for the specific RF frequencies of those local channels you are having multipath issues from.... leave the antenna and everything else where it is. Might have to knock yourself down to 60% signal to drop the reflections below the detection threshold, but 60% should be rock solid, even if it bugs you not to see the magic 100.
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post #20 of 21 (permalink) Old 2011-02-07, 06:58 PM
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here's what my antenna is looking at when getting hit with multipath on uhf 38 WKBW, looking mostly west. So I figure it's bouncing off that hill, which has a nice size water tank on it.
Since it's just a homemade SBGH with no reflectors, it get's signal from the front off the hill/water tank, and directly from the rear where the transmitter is. This general direction is where CBLT and CHCH from Canada are optimal.
You can kinda see why (i.e between that small hill and the buildings of downtown Buffalo). Without getting too drastic, putting a few of the co-linear reflector rods back on might be enough to help, without losing the eastern edge channel, WPXJ. That's why I took 'em off in the first place. Got better overall results making it bi-directional. But at the time I didn't have a rotor up. Probably could also fold the elements inwards a bit more too.

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post #21 of 21 (permalink) Old 2011-02-20, 08:32 PM
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Well, was such a nice day today, figured I'd take down the SBGH and rebuild it.
You should see the wood from 1 1/2 - 2 years outside, started out as just an experiment, hehe.
Anyhow put in it's place a quick stealth hawk element I had thrown together a while ago, but never tested it.
Can do a 360 now and never lose a Buf station.
So far it depends which tuner I'm using.
All tuners lost CHCH 18. PC Tuner lost CHCH 18 and CBLT 20.
All tuners still get WPXJ 23. Can still get CBLT 20 on 2 tuners, will probably fade though.
Will keep an eye on it while I think of how I might rebuild the SBGH and get it
back up there in time for the next nice day, which around here could be quite a while.
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