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Update: DTV antenna on VHF
On Fri, 12 Jun 2009 23:25:40 +0200, Gordon wrote:
AS expected, local channels 8, 10 and 12 moved their digital broadcasts back to their VHF assignments last night. They had been temporarily broadcasting digital in the UHF band. I was expecting to have problems with my dual bowtie antenna. But this morning I rescaned the channels (and verified that the move had taken place). Then checked reception on the affected channels. It was great. No problems. All that worrying for nothing. Thanks for listening. One situation that cropped up in my area (Cincinnati) is that channel 9, which was broadcasting digital on channel 10 VHF, had to wait until just before the conversion to raise their DTV antenna from 100 ft down to the top of their tower. We're also using a dual bowtie, and their change made a difference. Maybe not THE factor in your case, but glad things worked out for you. -- Rich W2RG |
#2
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Update: DTV antenna on VHF
On Fri, 12 Jun 2009 17:33:55 -0500, Rich Griffiths wrote:
On Fri, 12 Jun 2009 23:25:40 +0200, Gordon wrote: AS expected, local channels 8, 10 and 12 moved their digital broadcasts back to their VHF assignments last night. They had been temporarily broadcasting digital in the UHF band. I was expecting to have problems with my dual bowtie antenna. But this morning I rescaned the channels (and verified that the move had taken place). Then checked reception on the affected channels. It was great. No problems. All that worrying for nothing. Thanks for listening. One situation that cropped up in my area (Cincinnati) is that channel 9, which was broadcasting digital on channel 10 VHF, had to wait until just before the conversion to raise their DTV antenna from 100 ft down to the top of their tower. We're also using a dual bowtie, and their change made a difference. Maybe not THE factor in your case, but glad things worked out for you. I should have mentioned: what especially interested me about this is how relatively unimportant antenna height has seemed to be in my microwave work. Until this past year (when rotator cuff surgery took me out), I had been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz). I was often impressed by how far over the horizon it would work with an antenna only about 5 ft off the ground and about 1 W of power. Granted we were working with MUCH lower signal quality requirements than the TV stations, but I still am surprised by how poor our reception is of DTV channel 12 (and earlier, ch9), which is transmitting MANY kW from a multihundred-ft tower only about 16 km away. Engineers may argue otherwise, but it still seems to have a certain element of black magic to me :-) -- Rich W2RG |
#3
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Update: DTV antenna on VHF
Rich Griffiths wrote:
On Fri, 12 Jun 2009 17:33:55 -0500, Rich Griffiths wrote: On Fri, 12 Jun 2009 23:25:40 +0200, Gordon wrote: AS expected, local channels 8, 10 and 12 moved their digital broadcasts back to their VHF assignments last night. They had been temporarily broadcasting digital in the UHF band. I was expecting to have problems with my dual bowtie antenna. But this morning I rescaned the channels (and verified that the move had taken place). Then checked reception on the affected channels. It was great. No problems. All that worrying for nothing. Thanks for listening. One situation that cropped up in my area (Cincinnati) is that channel 9, which was broadcasting digital on channel 10 VHF, had to wait until just before the conversion to raise their DTV antenna from 100 ft down to the top of their tower. We're also using a dual bowtie, and their change made a difference. Maybe not THE factor in your case, but glad things worked out for you. I should have mentioned: what especially interested me about this is how relatively unimportant antenna height has seemed to be in my microwave work. Until this past year (when rotator cuff surgery took me out), I had been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz). I was often impressed by how far over the horizon it would work with an antenna only about 5 ft off the ground and about 1 W of power. Granted we were working with MUCH lower signal quality requirements than the TV stations, but I still am surprised by how poor our reception is of DTV channel 12 (and earlier, ch9), which is transmitting MANY kW from a multihundred-ft tower only about 16 km away. Engineers may argue otherwise, but it still seems to have a certain element of black magic to me :-) I dare say you weren't transmitting digital data at 20 Mbps in a 6MHz wide channel as a rover. CW or SSB with a bandwidth of a few hundred Hz or maybe a couple kHz at a SNR of 0dB vs 6MHz BW and a SNR of 20dB 30-40 dB of increased noise bandwidth 20dB more signal power relative to that noise for decoding There's your 100kW right there (100kW = 50dB over 1W) And, transmit end of the link has lower gain than you probably use as a microwave rover, because it's omni (in the horizontal plane, at least). A 1-2 degree beamwidth works out to about 30dB in gain This kind of thing is why working the world with 10W on PSK31 is pretty straightforward, compared to SSB. |
#4
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Update: DTV antenna on VHF
On Fri, 12 Jun 2009 16:13:40 -0700, Jim Lux wrote:
Rich Griffiths wrote: On Fri, 12 Jun 2009 17:33:55 -0500, Rich Griffiths wrote: On Fri, 12 Jun 2009 23:25:40 +0200, Gordon wrote: AS expected, local channels 8, 10 and 12 moved their digital broadcasts back to their VHF assignments last night. They had been temporarily broadcasting digital in the UHF band. I was expecting to have problems with my dual bowtie antenna. But this morning I rescaned the channels (and verified that the move had taken place). Then checked reception on the affected channels. It was great. No problems. All that worrying for nothing. Thanks for listening. One situation that cropped up in my area (Cincinnati) is that channel 9, which was broadcasting digital on channel 10 VHF, had to wait until just before the conversion to raise their DTV antenna from 100 ft down to the top of their tower. We're also using a dual bowtie, and their change made a difference. Maybe not THE factor in your case, but glad things worked out for you. I should have mentioned: what especially interested me about this is how relatively unimportant antenna height has seemed to be in my microwave work. Until this past year (when rotator cuff surgery took me out), I had been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz). I was often impressed by how far over the horizon it would work with an antenna only about 5 ft off the ground and about 1 W of power. Granted we were working with MUCH lower signal quality requirements than the TV stations, but I still am surprised by how poor our reception is of DTV channel 12 (and earlier, ch9), which is transmitting MANY kW from a multihundred-ft tower only about 16 km away. Engineers may argue otherwise, but it still seems to have a certain element of black magic to me :-) I dare say you weren't transmitting digital data at 20 Mbps in a 6MHz wide channel as a rover. Nope CW or SSB with a bandwidth of a few hundred Hz or maybe a couple kHz at a SNR of 0dB vs 6MHz BW and a SNR of 20dB Yup 30-40 dB of increased noise bandwidth 20dB more signal power relative to that noise for decoding Yup. Which is why I acknowledged that "we were working with MUCH lower signal quality requirements than the TV stations, but ..." There's your 100kW right there (100kW = 50dB over 1W) And, transmit end of the link has lower gain than you probably use as a microwave rover, because it's omni (in the horizontal plane, at least). A 1-2 degree beamwidth works out to about 30dB in gain Yup. 1 W to a 60-cm (2-ft) dish at 10 GHz yields about 2 kW ERP with a 3.5 degree beamwidth. This kind of thing is why working the world with 10W on PSK31 is pretty straightforward, compared to SSB. I know how to do the system and path loss calculations. Yet I'm still impressed when I can work another rover at 300+ km, sometimes with S9 signals and then go home and see the choppy signals I sometimes get between ch.12 at 300 m HAAT and my antenna at more than 10 m HAAT. Amateur microwave work is pretty cool stuff :-) BTW, using roughly 2-m dishes and only 100 mW, amateurs have transmitted 802.11b (WiFi) and higher-rate signals around 300 km several times. This allows digital video and audio. I think the current record is 389 km. -- Rich W2RG |
#5
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Update: DTV antenna on VHF
"Rich Griffiths" wrote in message communications... Until this past year (when rotator cuff surgery took me out), I had been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz). I was often impressed by how far over the horizon it would work with an antenna only about 5 ft off the ground and about 1 W of power. Antenna gain on both ends explains most of that. Granted we were working with MUCH lower signal quality requirements than the TV stations, but I still am surprised by how poor our reception is of DTV channel 12 (and earlier, ch9), which is transmitting MANY kW from a multihundred-ft tower only about 16 km away. Shannon's equations provide most of the answers: http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html Some of that is hard to follow, but the net effect is that you need a certain (minimum) amount of power to send a complex signal in a confined bandwidth. With ATSC, they put about 20 Mbps into a 6 MHz channel. To get a decent SNR (16 dB or better), they need MANY KW. I did a little mickey-wave engineering, myself. Point-to-point is easier than broadcast! "Sal" |
#6
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Update: DTV antenna on VHF
On Sun, 14 Jun 2009 00:46:50 -0700, Sal M. Onella wrote:
"Rich Griffiths" wrote in message communications... Until this past year (when rotator cuff surgery took me out), I had been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz). I was often impressed by how far over the horizon it would work with an antenna only about 5 ft off the ground and about 1 W of power. Antenna gain on both ends explains most of that. To me, it's a partial explanation, but not most of it. Microwavers often resort to quasi-scientific explanations like "troposcattering", as do VHF- ers. And sometimes the explanations get even fuzzier, like "enhancement". They (and ducting) are somewhat predictable, but as a rover the operating mantra was always just 'Let's try it". Granted we were working with MUCH lower signal quality requirements than the TV stations, but I still am surprised by how poor our reception is of DTV channel 12 (and earlier, ch9), which is transmitting MANY kW from a multihundred-ft tower only about 16 km away. Shannon's equations provide most of the answers: http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html Shannon's equations don't actually tell you much that's useful, as a Ham. And even in a lot of commercial situations, it can be hard to be sure what assumptions to make about noise and signal strengths, for example. Some of that is hard to follow, but the net effect is that you need a certain (minimum) amount of power to send a complex signal in a confined bandwidth. With ATSC, they put about 20 Mbps into a 6 MHz channel. To get a decent SNR (16 dB or better), they need MANY KW. I did a little mickey-wave engineering, myself. Point-to-point is easier than broadcast! "Sal" As an amateur rover, I generally found point-to-point to be more difficult, because of simple real-world Ham issues that don't have a lot to do with heavy theory. At 10 GHz, a 60-cm dish has a beamwidth of about 3.5 degrees. The rovers can cope with that with some care, but it seems many fixed stations are using rotators that don't provide that kind of resolution. So it could be hard lining up. In contrast, over-the- horizon signals smear out a lot horizontally. I once estimated another rover who was 180 km away as having about a 45-degree signal width, and found 20+ degrees to be common. Besides that, 10 GHz, and sometimes 5.7 GHz, will occasionally do downright freaky/cool things. Rainscatter is one fun example. And I once worked a station (S9, SSB) over a 50+ km path with a 90-degree bend in it, because the "line-of-sight" path was heavily obstructed by trees etc. We told each other we were bouncing off downtown Cincinnati, which was not visible over the horizon. I guess I'd like to end this sales pitch with the note that the Ham microwave bands can be great fun. The advantages, and Ham capabilities, of playing around with "real radio" and the small antennas that involves can be substantial. 50 MHz and up ... Everything else is DC. -- Rich |
#7
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Update: DTV antenna on VHF
On Sun, 14 Jun 2009 10:17:20 -0500, Rich Griffiths
wrote: Shannon's equations provide most of the answers: http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html Shannon's equations don't actually tell you much that's useful, as a Ham. This would presume that the Ham is rather indifferent or incapable. Shannon's work is exceedingly useful, and at the core, quite simple to perceive which further illuminates those mediocre qualities of your Ham. It is unfortunate that the link above offers no graphs by which Shannon's points would become startling apparent. For instance: http://www.aero.org/publications/cro...ages/04_04.gif shows how signal to noise ratio has a vast effect over bit error in digital transmission. In the face of equal powers (noise and bit level), you would run the odds of 1 bit in 10 being mistaken (pretty good odds, actually). If you were to raise the power in the bit by 10dB, that would fall to 1 bit in a million being mistaken. In the same graph, Shannon reveals how, if you code your bits (I will leave it to the student to discover the meaning of that), you could achieve the same 1:1000000 advantage with the addition of less than 1 dB of power boost. 73's Richard Clark, KB7QHC |
#8
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Update: DTV antenna on VHF
On Jun 14, 10:03*am, Richard Clark wrote:
In the same graph, Shannon reveals how, if you code your bits (I will leave it to the student to discover the meaning of that), you could achieve the same 1:1000000 advantage with the addition of less than 1 dB of power boost. 73's Richard Clark, KB7QHC Until fairly recently, hams didn't do much coding, for a variety of reasons. Computational horsepower is probably a big reason. Coding's easy, decoding not so easy, at least in a "parts readily available from Radio Shack" sort of sense. Obviously, today, one could do all sorts of coding on a laptop PC, particularly at low bit rates, but you'd still need to have an unusual convergence of someone who knows how to implement the coding algorithms who's also interested in amateur microwave operating. It's not anything like a turnkey thing, or even a "go get gnuradio" thing. Where you see coding in common ham use, it's buried in an application (PSK31, JT65, and the like) The other problem is the frequency control issue. If you want low rates and ragged edge of Shannon, you need good frequency stability and control (and to a lesser extent, good phase noise). Until recently (with GPS disciplined oscillators and surplus Rb sources) this was a real challenge. As Rich commented with respect to antenna pointing, you also have to be right on for frequency, and that's hard, especially in a field situation. Tuning to 10Hz accuracy at 10GHz implies 1E-9 frequency accuracy, which is challenging. To a certain extent, processing power in a PC helps (get close, do parallel demodulation, find the signal), but just like for coding, it requires finding a person (or small group) who can deal with building low phase noise stable oscillators AND with developing software that is somewhat complex, compared to the usual "whack it out in a weekend of coding" stuff. I suspect there ARE hams experimenting with this, but it's a long way from critical mass wide acceptance. You need something that you can write an article in QST, and offer $100 widgets to make that happen. There's not much cheap surplus gear either, since commercial equipment these days tends to be more specialized and isn't as amenable to hackery. There are also proprietary rights issues with some coding techniques (e.g. Turbo) but I suspect that legal issues aren't what's holding hams back. For things like LDPC, there are published software implementations that are free to use. I haven't looked but I imagine that various convolutional codes and decoders are also publicly available, along with Viterbi soft-decision decoders. |
#9
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Update: DTV antenna on VHF
On Sun, 14 Jun 2009 11:08:36 -0700 (PDT), Jim Lux
wrote: Until fairly recently, hams didn't do much coding, for a variety of reasons. This neither negates the specific issues of signal to noise in their relationship, a matter that is quite in the power of the Ham to control to some extent; nor does it invalidate the simplicity of that relationship revealed through one graphic that serves to reduce the obscurity of a lot of math. As for the variety of reasons, computation power would seem to be in abundance (the first mythical Cray is a door stop today). That as an excuse is a croak. It's not anything like a turnkey thing, Like I said, an indifferent or incapable individual in the guise of "Ham." I am amazed how that Lid is raised on a pedestal. or even a "go get gnuradio" thing. Where you see coding in common ham use, it's buried in an application (PSK31, JT65, and the like) So, let me get this straight, because it is available (a seeming contradiction from the tenor of your response), it is not accessible? Or it is not useful? Or it is not understood? Or Shannon has been rendered obsolete? Your objections are answered with your own solutions and yet the sense of what you say is shove Shannon out the window and whine about the noise. 73's Richard Clark, KB7QHC |
#10
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Update: DTV antenna on VHF
On Sun, 14 Jun 2009 10:03:29 -0700, Richard Clark wrote:
On Sun, 14 Jun 2009 10:03:29 -0700, Richard Clark wrote: On Sun, 14 Jun 2009 10:17:20 -0500, Rich Griffiths wrote: Shannon's equations don't actually tell you much that's useful, as a Ham. This would presume that the Ham is rather indifferent or incapable. Shannon's work is exceedingly useful snip Well, I do admit that I went overboard there! Probably shouldn't speak too quickly about moonbounce, meteor scatter, etc. I do think, however, that for many Hams (me, at least :-) ) the applicable word is "indifferent" rather than "incapable". If you can genuinely find useful application to what microwave rovers do -- or most microwave operators, for that matter -- that would be a special contribution. I expect that would be even less likely for most HF and VHF operations. I'd characterize Shannon's work as more of academic interest (for most Ham radio) than practical interest. In most of what most Hams do, there are just too many other issues to deal with. Please note the use of the word "most". I'm sure there are exceptions, but I expect they're a small part of hamdom (as is microwaving, sigh). -- Rich W2RG |
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