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Guest Message by DevFuse


Anyway to work around Multipathing?

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5 replies to this topic

#1 OFFLINE   tonyp56



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Posted 05 February 2005 - 01:32 AM

Does anyone know a way to get around multipathing? I bought a RCA ANT1250 amplified antenna, that has 45db gain on VHF and 35db gain on UHF. It has separate controls for VHF gain and UHF gain and it is plugged into my Dish 811 receiver. Here is some back ground, I live about 40 miles west of Tulsa OK, and about 35 miles east and around 60 miles north of OKC, I receive with 80% plus signal strength CBS, NBC, PBS, PAX, FOX, WB, and a independent channel 52, all out of OKC, Fox 23, PBS 11, and PAX 44 are out of Tulsa, and they come in at any where from 61% to 78%. (I get better signal at night than that) anyways, out of the 20 or so channels between Tulsa and OKC two are VHF, and the rest are UHF. So what I've been doing to get the best signal strength is turning off the VHF gain, and retracting the VHF antennas. (which gives me 5-6% more strength on my UHF channels)

I believe that I am experiencing multipathing on the two VHF channels (7 and 10) which are the ABC affiliates out of Tulsa (10) and OKC (7). (yes I turn the VHF gain back on and extend the antennas when I try to tune them) Both stay at 49%, might briefly bounce past 60% and then fall back down. Beyond these channels I don't receive the Tulsa UPN, CBS, and NBC channels, and all of which show 49% signal strength with jumping above 60%, but never lock in. And there are a few other OKC channels that I don't care to have that I can't get. But mostly I'd like to receive the Tulsa ABC and UPN channels to an addition to what I already get, but I don't know what I need to do to get them.

I don't know if it is because the channels have weak broadcast signals, or because of multipathing for sure. But It sounds like what people talk about when they discuss mu litpathing.

Why is it I can receive OKC channels better than I can Tulsa channels, when I am almost twice as far from OKC? What is multipathing? Is it, for instance were one channel sits in between two other channels, i.e. channels 6, 7, & 8, channel seven would be the one open to multipathing?

Thanks for any info.
Dish 811
SW: P380
Hardware ID: LADD-N
Boot Strap Version: 1014
Two Dish 301s
RCA/Dish Network 40" Widescreen HDTV
RCA RT2600 DD5.1, DTS, DPL 1-2 Surround Sound System

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#2 OFFLINE   boylehome


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Posted 05 February 2005 - 07:56 AM

The antenna is too directional, especially for indoor VHF. Multipath is ghosting on analog signals. An indoor antenna suffers from signals bouncing off various reflective surfaces unlike a good antenna on a rooftop. Using that antenna, you may need to be more creative in placing it where multipath is least.

#3 OFFLINE   KKlare



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Posted 07 February 2005 - 01:44 PM

You are pushing the limits on indoors antennas.

Channel 2 is the longest wavelength and in the group up to 6 -- low VHF. Channels 7 to 13 are about 1/3 the wavelength, high VHF, and you might do well with a partially extended rabbit ears, which would be easier to maneuver. The low VHF is much less directional but no rabbit ears are.

I found the Phillips model with 45 dB to have no gain. A bad example? The RCA model did no better than a Phillips dish rim antenna. I now have an ancient long combo outdoors antenna that I could hang from the bottom of my deck. It faces the 49 mile away towers, barely viewed with binoculars. They all have problems with the low VHF, 2 4 5 for some reason -- line of sight is close to a 2 mile distant water tower or between houses?

Moral: get an outdoor antenna, get a good view, have luck.

921 DVI #2/2 (#1 7/9/04 lightning nearby) 120B/F051/L211HECD-N
811 Component #1/1 (1/04)
501 RF/S-vid #2/4 machine (#1 few months, #2 flakey, #3 1 day, #2 came back, ran #2 + #4 until kept quieter, running for 2 years)
Versions: current software
Monitor: Hitachi 46H83
Dish: 500/500 Legacy SW64. Power inserter on 811.
Orbits: 119, 110, 148
Underdeck old 78" or more antenna. Still in 49 mi line of sight. It is diplexed at SW64 on 811 line. Signal strength 100 to 123 except KOB-NBC in 80s.

#4 OFFLINE   Jim5506


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Posted 07 February 2005 - 10:40 PM

I set up my HD tuner and my Silver Sensor in the living room for the Super Bowl and suddenly I was experiencing pixillation on a strong signal. Never had that problem in my office. I could walk around tha antenna and stand in certain places and cause pixillation. A little experimenting showed me that I was reflecting the UHF signal off my body at certain angles to the antenna, I rotated the antenna from horizonal to vertical polarization (vertical element orientation) and viola! no more pixillation. The antenna was evidently much more directional mounted vertically. Wached 7 hours in HD no dropouts at all.

Samsung SIR-T351
Dishnetwork PVR 510
2 X Dishnetwork 301's
TiVo Series 2 Stand Alone
Toshiba 32" SDTV
Sanyo PLC-SP20N (Contrary to Projector Central - it will do HDTV - very well thank you).
RCA RT2350

#5 OFFLINE   KKlare



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Posted 08 February 2005 - 02:26 PM

VHF business/commercial signals are generally vertical. TV signals are generally horizontally polarized. It is also possible to send circularly polarized signals, which would work on either but reduce the signal strength by 30%. Any hams to verify? V for DXing?

A vertical antenna would have little directionality so maybe you just were not pointed correctly and they luckily were circularly polarized.

#6 OFFLINE   kenglish



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Posted 18 May 2005 - 08:39 AM

Amplifiers will often cause problems. Try a small outdoor antenna (mounted outdoors), preferably without an amplifier. Anything that amplifies 45 dB is going to add LOTS of noise, and probably distortion. That is the real killer of digital.....the DTV standards allow for correction of a good bit of multipath, but noise and distortion "bury" the desired signals where the receivers can't find the data.

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