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| Thread ID: 66763 | 2006-03-06 21:29:00 | UHF aerial in Kohimarama | chrisath (9927) | PC World Chat |
| Post ID | Timestamp | Content | User | ||
| 436110 | 2006-03-08 04:52:00 | I guess we'll just have to agree to disagree on that one Graham, I accept that wide band designs are not as efficient as single channel jobbies, but that applies to any antenna tuned for a single frequency versus a wideband design. I speak from practical installation experience in applications for multi-channel antennas and measuring the comparative results with a signal strength meter. People won't pay if the on-screen results are not there, and I guess there are a fair number of DX TV enthusiasts around the world who would take issue with you as well. Cheers Billy 8-{) |
Billy T (70) | ||
| 436111 | 2006-03-08 06:30:00 | There's a lot of antenna theory around. The practice is what happens when you get into the real world. Any elements beyond "about" six are just extra windage and cost. They do nothing. load of bull. put a meter on it and you will see the difference. the difference between a 43 and a 91 is only around 2db for an aerial double in size, however that can make all the difference between poor pics and good pics. also large size size helps with signal shifts, thats where double/quad stacks are really handy. the old addage, the more metal in the air the better! frankly 6 elements dosn't pick up jack, you have to be line of sight to the transmitter and resonable close. frankly thats a BCL attitude....poor signal aeras don't exist therefore we can screw with the signal as much as we like :( |
tweak'e (69) | ||
| 436112 | 2006-03-08 23:46:00 | 2 dB for doubling the length of the antenna? . From 43 elements to 91? My point is made . An extra 2/38 ths of a dB per element . That's a lot less than 5/8 of not very much . A 3dB change is just noticeable . Your meter is probably good for =/- 10% . You could be right . . . "the more metal in the air the better" : most TV " Yagis" would probably get a better signal pointing at right angles to the the line to the transmitter . One of the NBS bulletins suggests a length tolerance for elements of . 003 of a wavelength, to be able to rely on gain calculations . That's at one frequency . The bandwidth required of these TV "Yagis" means that the range of "correct" element lengths is +/- 50% . There's not much place for design in that sort of thing . Obviouslly bits of metal have been bolted on until it became ridiculous . You mention stacks . A good idea, but they work properly only for one frequency . If you are [b]very[b] lucky, you might get 2 frequencies, harmonically related . The usual result is that you would get cancellation . ;) I always used to tell people that any bit of wire is an aerial . A bit of string will do, if it's wet . Wet with salt water is a bit better . I'm surprised people haven't used log periodics . They can be designed . |
Graham L (2) | ||
| 436113 | 2006-03-09 00:57:00 | Most TV " Yagis" would probably get a better signal pointing at right angles to the the line to the transmitter . We were almost on the same page until you made that comment Graham, but you blew any credibility you might have had right out the window with that nonsense, even allowing for an element of hyperbole . You mention stacks . A good idea, but they work properly only for one frequency . If you are very lucky, you might get 2 frequencies, harmonically related . The usual result is that you would get cancellation . ;) This one is in the same class, I begin to wonder if you actally have any practical experience with multi-channel antenna systems at all, or whether you just swallowed a book . I'm surprised people haven't used log periodics . They can be designed . Log periodics have been on the NZ market since the mid 70's and for directivity and gain in bands I & III they are hard to beat . AFAIK they are not sold here UHF TV work, but they are available in the UK ( . maxview . ltd . uk/trd_diglog . htm" target="_blank">www . maxview . ltd . uk) though, and if you keep your eyes open you will see them around for communications uses . Cheers Billy 8-{) |
Billy T (70) | ||
| 436114 | 2006-03-09 01:25:00 | 2 dB for doubling the length of the antenna? . From 43 elements to 91? My point is made . An extra 2/38 ths of a dB per element . That's a lot less than 5/8 of not very much . A 3dB change is just noticeable . Your meter is probably good for =/- 10% . 2db can be make a HUGE difference, its a fine line between crap and perfect . you cannot make up for that 2db loss, you can't amp whats not there . You mention stacks . A good idea, but they work properly only for one frequency . If you are [b]very[b] lucky, you might get 2 frequencies, harmonically related . The usual result is that you would get cancellation . ;) more crap . been useing muitichannel stacks for years (i even have one on my home) . naturally any multi channel is not as good as single channel . for a VHF(4-11) double stack about a 5 db gain is made . UHF stacks are a little tricky and not often done as you have to do cable combine rather than phasing bars, you loose a bit in cable and combiner losses . aerials is a "black art", leave the books at home . i've had things that proberly break the laws of phyics! unfortunatly its a dieing trade along with other electronic trades . |
tweak'e (69) | ||
| 436115 | 2006-03-09 01:34:00 | Billy, hyperbole or not, it's true . (And I did say "most') . I've seen TV aerials . Mostly they seem to rely on the fact that aluminium tubing is much the same as a wire . Or piece of wet string;) I have built enough Yagi antennae to know a bit about them . I'm not always convinced by the books . . . some bits I do believe are: the gain is always lower than the calculated values . The more elements, the narrower the bandwidth . They're sensitive to ground plane proximity . The feedpoint impedance is always (much) lower than the "theoretical" one . When I have done measurements, it's been with instruments; not "meters" . I have built and maintained large phased arrays . The vertical radiation pattern of stacked antennae at a fixed spacing depends on the frequency . If you have a 2 or 3 to 1 range of frequency, and you have chosen a spacing which will give a horizontal beam at, say, the centre frequency, you will get a horrible range of vertical angles as you go up and down in frequency . At some frequencies you are going to get complete cancellation of the "horizontal" beam you want . That's physics . That's just the way it is . It's not a matter of my credibility . Come back to my original point . Half of the furthest (20th) director has fallen off . I say it will have no effect on the performance . Am I wrong? I'm pleased to hear that L-Ps are available . I don't have much interest in TV technology . No improvement in signal strengths can improve the broadcast programmes . |
Graham L (2) | ||
| 436116 | 2006-03-09 01:44:00 | Come back to my original point. Half of the furthest (20th) director has fallen off. I say it will have no effect on the performance. Am I wrong? not at all. the loss is not even measurable. the other end are the critical ones. No improvement in signal strengths can improve the broadcast programmes. darn right! now with sat we have more channels of crap! |
tweak'e (69) | ||
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