How much RF does it take to get a clear NTSC picture? Usable? I know the local cable tower tried to get at least a -20 dBm signal (!!!) on analog TV inputs.
Most DTV converter units with published spec's need between -82 dBm and -86 dBm at the antenna jack on the back of the unit to capture a complete bit stream. I do *not* know how much SNR is required to get a complete bit stream. Personally, I'm not in the TV business but I crawl around on a lot of rooftops and 'nice' home-use TV towers. Most people's OTA setups contain at least one range-eating screw-up. These things will do a lot worse nasties to a digital signal than an analog: A> improperly assembled antenna. So many times I see the UHF 'bowtie' on store bought all-band antennas positioned incorrectly. The instruction sheet knows best. B> Unsealed coax connections. F connectors ain't waterproof, not even the 'snap-n-seal' ones. C> Most preamps I run into are at least 10 years old. Sure they might still work, but RF transistor technology has sure improved in the last few years. These amps are *not* going to pass DTV acceptably. In my rf/data experience otherwise, if the bitrate/frequency ratio is very high (DTV is doing 20 mbit out of 6 mhz of spectrum, that's a pretty damned high ratio) D> Height. If you want your antenna close to the ground, might I suggest getting a little round grey one that points to the Clark Belt, with a box attached that requires you to shove dollar bills in an envelope every month to continue working. You will be happier. Personally, I've always heard "rural TV requires one foot of height per one mile of distance in flatland conditions"... I think this holds true once you're >25 miles from the transmitter site (maybe less if the site isn't a typical 1950 ft tall tier-1 market TV tower). Check your location against topo maps. Just one small hill can screw up all your calculations (or a big river valley can pull signals in from ridiculous distances at low heights) E> Bad coax choices: Either incorrectly crimped, incorrectly handled, improperly spec'd (white indoor coax won't stay 'good' outdoors for more than a few months, maybe a year) etc coax used, then badly installed (not secured). There are very good 75 ohm coaxes available. RG-11 is pretty nice stuff. F> off-channel noise slaughtering your amplifier. Most TV amps are unfiltered (or contain a simple 88-108 FM trap), if you've got a big pager transmitter, repeater, etc (cellphone stuff doesn't tend to count since its fairly low power) nearby you could easily be 'hosing' the entire setup. Filters are your friends. I plan to do some experimentation with marketed-for-cable-company inline filters. Normally used to keep Basic Cable customers from seeing other channels, or cable-modem-only customers from watching TV, they sell these in all sorts of specialty frequencies for cheap. Can't beat cheap + published specifications. Here's how I'd get HDTV today, assuming I really wanted to watch OTA TV: Info: I'm about 175 miles from Houston's tower farm, 140 miles from DFW's tower farm, and there's a sprinkling of stations around Tyler and Lufkin... I have a 150' rohn 25 in my yard, about 185 ft from my house. There's currently 2 strands of multimode fiber going from my house to the tower, speaking Ethernet (100mbit, full duplex). I'd buy one of these: http://www.silicondust.com/products/hdhomerun ($180), place it in an outdoor-friendly metal enclosure with an Ethernet<->fiber converter/transceiver. I have isolated 120VAC running up my tower already, powering the gear isn't a problem. Add antennas. I'd most likely go with dedicated VHF and UHF antennas and tuned/filtered amplifiers on each, and use a nice 'ham radio' type rotator. Keep the coax short, use RG11, and seal it well. I'd buy a nice HTPC (Home Theater PC... nothing fancy, just a basic PC with TV-friendly outputs. If you've got a decent TV it'll take HDMI or DVI, which is even better). Microsoft's set-top/media center edition software is good enough to test with, at least. ($400-500ish total here) Dedicated duplex (2 strand) fiber run from the top of the tower to the top of my TV set... Of course, having the tools, fiber, ends and knowledge helps. And, after all this, I'd get to watch horrible reality shows, get my daily dose of propaganda (news), watch late-night TV guys make asses out of themselves, and the 3 decent shows PBS shows when they're begging for more money to run transmitters that they don't need (pbs.org should offer streaming...) Doesn't really seem worth the effort/wind load/money... :) JS -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Chuck Kelsey Sent: Friday, February 20, 2009 6:42 AM To: [email protected] Subject: Re: [Repeater-Builder] Somewhat OT - an interesting antenna design This is going to be a bigger problem than many stations may have anticipated. A friend of mine has an all-channel antenna on a rotor and can get 20 analog stations. He purchased a converter to see what he could get in digital. He can only get four, and three of those pixilate quite badly. Almost everything has gone to UHF here. My area is in the fringe of two markets. UHF doesn't work as well in the hills. I doubt translators will be implemented -- too much money. Chuck ----- Original Message ----- From: <[email protected]> To: <[email protected]> Sent: Thursday, February 19, 2009 11:38 PM Subject: Re: [Repeater-Builder] Somewhat OT - an interesting antenna design > At 2/19/2009 18:08, you wrote: >>Along these same lines.... >> >>Has anyone on this list built / put into practice a Gray-Hoverman panel >>antenna? How do they perform? Is it worth building one? Double or single? >>http://www.casano.com/projects/hoverman/index.html >> >>I'm about 70 miles from my target DTV market. (Pittsburgh, PA) NOBODY here >>can seem to get the Pittsburgh ABC affiliate with their current V/U >>antenna >>setups. (WTAE4) I have some people that I am helping get their converter >>boxes hooked up and working. They are VERY disappointed that they won't be >>able get their news from channel 4 once DTV has gone into full effect. >>(They >>DO have one of the areas BEST news teams - in my opinion.) > > The center of the map says it all IMO: A4 - D51. Would be nice if they > backfilled ch. 4 after the cutover, but I think the channel #s on the map > indicate the final assignment. > >>I'm wondering if a dedicated UHF panel and good UHF only preamp will help >>the situation. > > I think all you can do is go for max. gain @ ch. 51 & hope the problem > isn't multipath; if it is you're SOL unless you go WAY up in the air with > the antenna. Stacked Yagis might get the job done @ ch. 51, but then > you'd > need another antenna for the other channels. Don't know if a parabolic > would be broadband enough - depends on the type of feed & if the aperture > (diameter) is large enough to work reasonably at the lowest DTV channel. > > Bob NO6B > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > ------------------------------------ Yahoo! Groups Links
