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Discussion Starter · #1 ·
After reading a post in the HDTV programming forum about the "failure" of HDTV in the marketplace, and the inevitable mention of the "C"-word, I thought I'd offer something here. A question for you RF guys out there:


How badly is a UHF signal affected by relections off a body of water? I've got two antennas on the roof of my house. One is near the back edge of a flat section of asphalt roof with a galvanized underlayment over the whole section (about 25' x 20'). The antenna (an indoor-type Silver Senser)is pointing towards the transmitter towers 15 miles away - across the flat (shielded) roof. Also thrown in is a harbor full of sailboats with aluminum masts, and an elm tree just 20' from my house. The antenna is about 40' above the water level. The antenna is un-amplified.


The other antenna is mounted about 10' higher. It's a medium-gain YAGI (a R/S UXR-75), fairly narrow beam, UHF only, with a 20db preamp. It's on the slant-roof side of the roof (no metal sheeting to worry about under the shingles), but it points across the harbor to a station 50 miles away. Also thrown into the mix - directly in line - are a bunch of steel storage tanks on the far side of the harbor.


Both antennas are coupled to a single coax using a Channel Master Jointenna. The first antenna is the all-channel antenna, and the second is for UHF 57 only. And, of course, they're diplexed together with a DBS dish antenna.


Here's the quiz: given all the pundits' complaints about the failings of the 8VSB modulation standard, do I have to worry about multipath problems in the above environment?


Anyone care to enlighten me as to how good or bad the DTV reception in my location should be? I'll let the pot stew for a while and see what happens, then tomorrow I'll followup with my actual experience.


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Jon Gauthier

"If I'm not sailing I'd rather be watching sailing in HD!" - Me
 

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Not a simple thing to answer.


You did not give enough info. What is the terrain like for the rest of the 15 or 50 miles???

You could have bad multi-path from the source.


RF travels well over water this is not a problem.


Your near terrain does not seem problematic. Except for the roof.


You could have reflections in your cabling. Look at an analog signal on a TV, are the reflections on the right or left. If they are on the right it is your cables.


The Silver sensor will not have problems from the boat masts ( I can not imagine a harbor that dense with boats) or water. It can get ground reflections off the roof and not work, or it could get a reflection and be shielded by the roof and work.





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Jim Burns
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Sounds like you should have good reception in both cases in normal weather.


Possible multipath problems in rough weather with the Silver sensor.


The other modulation would work well on your sailboat even in rough weather.
 

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my two cents. water acts more as an atennuater than a reflector at UHF frequencies ( think microwave ). nothing in your description screams multipath.
 

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Jon,

Could you respond to me vial e-mail, I would like to talk to you. I live in Tewksbury.


Pat


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Discussion Starter · #6 ·
Sorry 'bout the delay - been busy.


Few more details: the Silver Sensor points SW to the Boston/Needham towers, with only small hills between. Essentially a straight shot. It works indoors as well. DT stations there are WGBH-19, WCVB-20, WHUB-23, WBZ-30, WFXT-31, and WHDH-42. WHUB tower is actually a bit further away, a few degrees north. The only problem with the SS (besides it being an indoor antenna used outdoors) is if I place it within 18" of the roof (with its steel underlayment) the signal drops out completely. I've since mounted it on the same mast as the R/S antenna and I get rock solid 100% on the signal quality meter of my STB on all Boston channels.


The R/S U-75R pointed towards New Hampshire works reasonably well for ch. 57 (WENH), considering it only has about 9.5dBd gain @ ch. 30. I'm sure the gain is only about 8 dB or so at ch. 57. I seem to be getting some signal degredation when it's windy - I attribute that to the hundreds of 30-70' moving aluminum sailboat masts out in the harbor. Signal quality beats from 20-65. With clear skies and little wind, rock solid 65-68. My antenna mast doesn't budge in the wind - that's why I'm thinking multipath from the boats in the harbor.


Jim,


Terrain to the Boston towers is no problem - small hills, get a great signal with the Silver Sensor. I'm thinking of replacing the R/S with a higher gain antenna like the CM-4248 or the Winegard CA-9085. They both have narrower half-power roll-off than the R/S (34 & 32 deg. vs. 40). As far as reflections in the cabling - none noticeable from any of the Boston stations. For New Hampshire, I'll have to see what analog UHF channels are operating on the same tower as ch 57. WENH's analog xmitter on the tower is VHF ch. 11.


Bob,


I get vitually zero multipath from the SS. Their website ( http://www.antiference.co.uk/sensor/ ) advertizes it as highly directional. I'm assuming this is due to it's log periodic design, which I thought was supposed to give you an even response across the spectrum. Heck, I'm a telecom engineer, not an RF one.


Prjoy - check your private messages...


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Jon Gauthier

"If I'm not sailing I'd rather be watching sailing in HD!" - Me
 

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John,


Try raising or even LOWERING your antenna height a foot or less at a time and see if things improve on the channel you're having trouble with.


In his "Digital TV" column in the latest print issue of TVTechnology magazine, RF expert Charles W. Rhodes states that nulls in the spectrum which arise from multipath distortion due to short delays of 90 feet or less can be affected by small changes in receive antenna height, up or DOWN. He suggests that moving the antenna up or down in small increments up to a few feet can effectively "move" such a null to a part of the spectrum away from the affected channel. It is possible that you'll simply move the problem to a different desired channel but hopefully not! This will only be effective if the problem is in fact due to a short delay.


As an aside, in the same column Mr. Rhodes also states that water vapor in the atmosphere and PARTICULARLY a nearby large body of water can be especially reflective to RF. He does not differentiate the relative reflectivity of VHF vs. UHF frequencies in the column, however.


Mr. Rhodes column, while in the print version of the magazine, does not appear to be included on their website as of today. You may care to check the site periodically in the future in case it eventually shows up there, or you may wish to subscribe and request that you be sent the current issue, which is Issue 19, Volume 20, dated October 3, 2001. Their website is www.tvtechnology.com .


If you try this I'd be curious about the results, as I have a neighbor who lives directly on the Intracoastal Waterway and seems to have a problem with a particular DTV channel (22) which I receive just fine only a few blocks way (and as many blocks away from the water.) I'm wondering if this might help his situation.


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HiDefDave




[This message has been edited by David McRoy (edited 10-12-2001).]
 

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Discussion Starter · #8 ·
Dave,


I'd thought of that, but I need to get better cooperation from my help (my wife)!


Anyway, I was checking the FCC site for the station I'm trying to receive (better! - I already can get it sometimes) and found this out: WENH-DT's broadcast antenna is 1233' above sea level, pumping out 589kW.


I have a topographical mapping program called TOPO!, but I don't have the maps for the NH region - my maps reach just into NH. So I can't pull up a profile to see if there's any hills in the way. Anyone know of a website with such info?


Anyway, with less than 600kW @ 50 miles, I probably need an antenna with more gain. I think I'll try the R/S U-100R - 'bout 3dBi more gain than my U-75R, with a narrower beam. If that don't cut it, I'm thinking of the Winegard CA-9085 from Stark Electronics. Seems to be a bit heavier-duty than the CM-4248 or 3023 with just a bit better gain and beam, and of course, twice the price. But $65 ain't bad...


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Jon Gauthier

"If I'm not sailing I'd rather be watching sailing in HD!" - Me
 

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John,


I've had good luck using a Radio Shack VU-210 with a Radio Shack 15-1113C indoor preamp, which provides up to 26 dB of gain (adjustable.) The antenna is 25 ft. up, but terrain here in South Florida is as flat as a pancake, save the condos!


The VU-210 is a monster, but it outperforms my Radio Shack 15-1262 on UHF, their biggest UHF-only rig. Plus, we have 3 VHF DTV allocations down here.


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HiDefDave


[This message has been edited by David McRoy (edited 10-12-2001).]
 

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I thought you would have problems with the mast and reflections off the water. That would be the extent of my knowledge with dynamic multipath in this case.

 

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Jon,

Over the last couple of years of experimenting with antennas for UHF, DTV and it effect on 8VSB modulated signals. I have come to the following conclusions:


1) Narrow vertical beam width


Horizontal beam width is not as important as vertical beam width. Reason for this is that most but not all multi-path is ground reflected. This is true, I believe for distances of 5 to 30 miles out from the transmitter. This problem gets worse with higher transmitter heights. Reflection angels as small at 1 degree can cause real multipath issues. Transmitters at higher elevations will increas that reflection angle. Receivers actually deal with this reasonably well for static multipath, but add a moving car, something metallic that moves or windy day with tree limbs moving all around and you have enough change in the signal that it disrupts the demodulator in the receiver. Antennas that have parallel groups of elements or stacked antennas will improve the vertical beam width and it doesn't take much help.


2) Many active antenna elements


The reason for this is two fold, a) Very flat response over the entire UHF band. b)Very low VSWR producing consistent internal impedance (this can act just like multipath if it is inconsistent). Clean is the word on this front, to many bumps in the VSWR over the bandwidth of the antenna and you may be able to receive some stations but others will be not be receivable even though the signals originate from the same location. Also it gives a larger apparent reception area that provides a broad area of reception for signals fronts that are moving around because of dynamic mulipath-path conditions.


3)Keep the installation simple and clean.


Avoid using different antennas coupled together and pointing in different directions. Use a single antenna with a rotor. Use a UHF only antenna if that can be done in your area. Use high quality coax and connectors. Make sure that they are water proofed in some way. Water will ruin your day. Only use pre-amps if you are 30 miles or more from the transmitter site. Keep the coax feed as short as possible. Avoid cable splitters and diplexers if possible. Keep it simple.


Most antennas of traditional design do not take any of these factors into consideration. I don't think many of the manufactures have any idea what it take to receive a digital signal. Cable companies that offer digital service learned and sometimes the hard way that attention to detail to clean up their systems was the only way to get digital running reliably. It is the same with OTA.


Just my 2 cents...


Pat




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There are much worse environments than yours which still produce perfect DTV reception. The boat masts and tanks will cause some multipath, but not as bad as a metropolitan cityscape or mountain ranges.


Take a close look at your splitter/combiner/amp/diplexer arrangement. I suspect that this could cause more problems than the terrain. How is the preamp powered? If it is powered over the coax, the splitter/combiner/diplexer will break that connection. Try an experiment: Remove all that stuff and connect the UHF antenna directly to your receiver with one coax run. I bet you'll see a difference.


Before you go for a higher gain antenna, try to raise its height. Even a few feet can make a huge difference with a long path.


I envy you your view. Sounds beautiful. Why are you wasting time watching TV?

 

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One more point. The reputed multipath problems with 8VSB are vastly overrated. In my recent experience an analog NTSC picture can be unwatchable due to multipath, while an HDTV signal from the same location comes in perfectly.
 

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Discussion Starter · #14 ·
OK, I finally had time to work on things.


I was going to get a Wingard UHF antenna, but decided to not wait and went to Radio Shack and found the U-120R, a 100 inch "monster" corner reflector Yagi.


I took down the Silver Sensor and replaced it with the R/S U-70R pointed at the Boston transmitters. It's connected to the all-channels port of the Jointenna. No preamp. Pulls in all the Boston Digital stations at nailed at 100 on the signal quality meter (WCVB-20 wavers a bit, 94-100, but no visible or audible artifacts). Nice going so far...


Since WENH-DT doesn't transmit until 7:00 PM, I used WMUR-DT in Mansfield, NH to line up the new U-120R. Now, WMUR is on ch. 59, which is attenuated on the Jointenna cut for 57, but what the heck. Without the Jointenna in place and no preamp, I got a 65. And this station is just over 50 miles away. So I went ahead and ran the U-120R thru the Jointenna - signal quality dropped to about 20. Since WMUR is an ABC affiliate, I'm not worried - I'll just watch WCVB.


Once 7:00PM rolled around, WENH-DT came in at around 65. I only saw a couple of hits all night. Bottom line is I'm satisfied. I think I'll try one of the Channel Master preamps to nail it and make WMUR-DT more watchable.


Bottom line is that two UHF antennas on a common mast coupled with a CM Jointenna do work in my situation. The higher gain and the narrower beam width of the R/S U-120R (manufactured by Antennacraft, I believe) works well.
 

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Jon,

Looks like you are making progress and that the DTV channels have stabilized. I will curious to know how you make out with the preamp on ENH.


Pat
 
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