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HDTV antenna
Are there any omnidirectional HDTV antennas?
My LPDA is too directional. Also, is TVI from ham transmitters reduced with HDTV? -- 73, Cecil http://www.w5dxp.com |
#2
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HDTV antenna
In article ,
Cecil Moore wrote: Are there any omnidirectional HDTV antennas? My LPDA is too directional. Well, considering that there really is no such thing as an HDTV antenna per se, yes. HDTV in the U.S. (ATSC) uses a subset of the same frequencies used by ATSC television, and so all the same considerations apply. The commonest commercial solution to your need seems to take the form of a saucer-shaped radome, containing an amplified near-omnidirectional UHF/VHF antenna. One source is http://www.starkelectronic.com/allant.htm Internally, I believe that these consist of a standard or folded dipole, with the elements "swept" into an S shape. They're probably not terribly good antennas, as far as matching or sensitivity or pattern is concerned, which is likely why an onboard amplifier is included. If you wanted to tune up only one (or a few closely-spaced) HDTV stations with an omni, you could probably build yourself a turnstile antenna... a crossed pair of bow-tie elements, and a 90-degree phasing harness cut for the middle of the band would probably work. I doubt that'll work as a general-purpose solution for the full band, though. The specific sort of antenna design you might need to work well in your area, will depend a lot on which frequencies are going to be in use for DTV in your area after The Big Day Next Year. In many areas of the country, all TV signals will be in the UHF band - this simplifies matters for residents of those areas, as only a UHF antenna will be needed. In more crowded areas of the country (and in some locations where the broadcasters don't want to re-fit their main transmitters) DTV will include stations operating in the VHF high band (channels 7-13) and you'll need an antenna which works on these frequencies as well. In a very few areas, there will still be stations operating DTV in the VHF low band (channels 2-6), and residents of those areas will still need full-coverage antennas. Also, is TVI from ham transmitters reduced with HDTV? It depends. The worst ham TVI problems seem to have affected Channel 2, due to its proximity to the 6-meter band and its second-harmonic relationship to the 10-meter band. There will be *very* few TV stations using this frequency after The Day, and this may reduce TVI complaints for hams operating on 2 and 6 meters. Strong-signal overload/desense problems of TVs (or antenna/CATV distribution amplifiers) will probably still exist, especially for TV stations operating in the VHF low and high bands. I imagine that high-power ham transmissions in the 70 cm band may cause overload problems for UHF TV stations in some cases. The actual behavior of the TV sets, in the face of ham TVI, will definitely be different. No more herringbones in the video, no more desense-related snow. Instead, the picture and audio will be unaffected, until the TVI is severe enough that the DTV tuner and demodulator can no longer get enough of a signal to allow the digital error-correction process to work - at that point, the image will start freezing, breaking up into rectangular "macroblock" artifacts, or disappearing entirely. TVI is likely to present as much more of an "all-or-nothing" thing that it did in the past. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#3
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HDTV antenna
"Dave Platt" wrote in message ... In article , Cecil Moore wrote: .................................................. ........ The worst ham TVI problems seem to have affected Channel 2, due to its proximity to the 6-meter band and its second-harmonic relationship to the 10-meter band. There will be *very* few TV stations using this frequency after The Day, and this may reduce TVI complaints for hams operating on 2 and 6 meters. By my count there will only be 7 stations in the whole country on CH2 after the switcover: Flagstaff, Grand Junction, Bangor, Nort Platte NE, Las Vegas, Rapid City SD, and Jackson WY. Tam/WB2TT -- ................................................ Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#5
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HDTV antenna
In article ,
wrote: Well, considering that there really is no such thing as an HDTV antenna per se, yes. HDTV in the U.S. (ATSC) uses a subset of the same frequencies used by ATSC television, and so all the same considerations apply. With 2 to 4 stations on the same frequency allotment as the old analog in some instances. At any rate some squeeze 4 in the space that used to be used by one. They do this at the expense of bandwidth (IE resolution) They're actually multiplexing two or more digital data streams into a single RF transmission, coming from a single transmitter and tower. No matter whether they're sending one HDTV program, or four independent SDTV digital signals, it's all just one digital bitstream being sent to the modulator, and sent out in a single 6 MHz slice of the spectrum (the same slice of spectrum that a single NTSC transmission would have used). You're correct, the extra programs are sent at the expense of the main program... it must reduce its bit-rate, which means either switching down to a lower resolution, or having more motion artifacts, or sticking to programs which have relatively little motion on-screen and are thus easier to compress via MPEG. If you wanted to tune up only one (or a few closely-spaced) HDTV stations with an omni, you could probably build yourself a turnstile antenna... a crossed pair of bow-tie elements, and a 90-degree phasing harness cut for the middle of the band would probably work. I doubt that'll work as a general-purpose solution for the full band, though. It does here at least out to about 40 or 50 miles from 40 feet. Very good to know - thanks! BTW these antennas are about 30 feet below a pair of 12L, vertically polarized yagis and about 12 feet above the tops of a pair of Diamond repeater antennas. The tribander is about 8 to 10 feet above the TV antennas and the 7L 6-meter yagi is about 15 feet above the tribander. I run the legal limit to the tribander and a 1000 to 1200 PEP on 6-meters. So far no problems. My complements to your filters and amplifiers and your care in setting up the whole system! Only problem I ever had was one neighbor having a foot ball party out in his garage using rabbit ears for an antenna about 120 feet from the base of my tower. "Hello Test" with 50 watts to the 12L pair at 130 feet would give him a blank screen on Ch 12. I gave him a directional TV antenna and from then on I was invited over to the parties and free beer. Now *that* is the kind of QRM-resolution situation we should all strive for! -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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