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Had the IRS install today
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Had the IRS install today
"Doctor D" wrote in message
o.uk... "Paul Ratcliffe" wrote in message ... On Sat, 11 Sep 2010 12:30:44 +0100, Doctor D wrote: I understood that lengths of over 20m of cheap shotgun cable were lossy. This won't show up as a grainy picture but you may suffer more break up in the signal when it's raining heavily and the incoming signal to the dish is attenuated. You're talking about two different effects now. Cable loss is overcome by having enough gain in the LNB to account for it. Rain fade margin is defined by the noise figure of the LNB and the size of the dish. But, won't a long/poor cable run exacerbate the effect of rain fade? Surely a shorter/better cable run will help mask the effects of a reduction in signal levels produced at the LNB? Cable loss shouldn't have much effect on rain fade, the output of the LNB is an IF (intermediate frequency) output, too long and lossy a feeder will result is a weak signal under all conditions. To reduce rain fade use a larger and higher gain dish, for Astra 28e i try and use a Zone 2 dish if possible. Steve Terry -- "I would like to plead for my right to investigate natural phenomena without having guns pointed at me. I also ask for the right to be wrong without being hanged for it." - Wilhelm Reich, November 1947 |
Had the IRS install today
On Sep 11, 1:59*pm, Paul Ratcliffe
wrote: On Sat, 11 Sep 2010 12:30:44 +0100, Doctor D wrote: I understood that lengths of over 20m of cheap shotgun cable were lossy.. This won't show up as a grainy picture but you may suffer more break up in the signal when it's raining heavily and the incoming signal to the dish is attenuated. You're talking about two different effects now. Cable loss is overcome by having enough gain in the LNB to account for it. Rain fade margin is defined by the noise figure of the LNB and the size of the dish. The reduction in signal level (as opposed to s/n ratio) caused by rainfade is minute compared to the margin provided by the fact that the LNB output is many dBs higher than the signal level required at the receiver. Bill |
Had the IRS install today
On Sep 11, 3:47*pm, "Doctor D" wrote:
"Paul Ratcliffe" wrote in message ... On Sat, 11 Sep 2010 12:30:44 +0100, Doctor D wrote: I understood that lengths of over 20m of cheap shotgun cable were lossy. This won't show up as a grainy picture but you may suffer more break up in the signal when it's raining heavily and the incoming signal to the dish is attenuated. You're talking about two different effects now. Cable loss is overcome by having enough gain in the LNB to account for it. Rain fade margin is defined by the noise figure of the LNB and the size of the dish. But, won't a long/poor cable run exacerbate the effect of rain fade? Surely a shorter/better cable run will help mask the effects of a reduction in signal levels produced at the LNB? If the cable is so long that the levels presented to the receiver in good weather are, say, only 10dB above the receiver's threshold, then the 6dB signal reduction caused by (very severe and unusual) rainfade will start to make a difference. But with normal cable runs there's 20 or 25dB to play with, so it doesn't matter. If rain fade reduces the signal level by much more than 6dB reception from a Sky dish will suffer dramatically even if the downlead is only a foot long, so the rest is academic. Bill |
Had the IRS install today
On Sep 11, 7:39*pm, tony sayer wrote:
In article , Nick Le Lievre nicklelievr scribeth thus "Doctor D" wrote in message news:[email protected] co.uk... But, won't a long/poor cable run exacerbate the effect of rain fade? Surely a shorter/better cable run will help mask the effects of a reduction in signal levels produced at the LNB? I think the "shotgun" cable run is not that long, maybe up into the loft, I think there is a tap in the loft although I do not know what a tap is can someone explain Umm there'll prolly be a Multiswitch or amplifier up there which will make that irrelevant. Perhaps the Hon Member for Micklebring will comment?... -- Tony Sayer (All levels below are the stongest muxes in each of the four modes) Well yes, the switch would normally be set up so the output level of the sat muxes is about +17dBmV. This level is achieved by the use of preamps and equalisers where necessary. Since receiver threshold is about -10 to -15dBmV there's plenty of leeway to allow for cable and outlet loss (when the installers have fitted outlets that is). I like to see 0dBmV to 10dBmV at the receiver, but many installers regard that as excessive, and are happy with 0 to -10dBmV. Things are complicated by the fact that the muxes vary in level quite a bit, and receivers vary in their ability to cope with that, and that varying ability varies some more depending on the mean level, but in general those levels work OK. On a big system the noise added by pre-amps, line amps, and the amps built into the switches will add up, so it's neccessary to keep levels a little bit higher towards the end of the system. I don't like downlead to flats to be longer than 30dB; the need for equalisation becomes acute. Bill |
Had the IRS install today
" wrote in message
... On Sep 11, 3:47 pm, "Doctor D" wrote: "Paul Ratcliffe" wrote in message ... On Sat, 11 Sep 2010 12:30:44 +0100, Doctor D wrote: I understood that lengths of over 20m of cheap shotgun cable were lossy. This won't show up as a grainy picture but you may suffer more break up in the signal when it's raining heavily and the incoming signal to the dish is attenuated. You're talking about two different effects now. Cable loss is overcome by having enough gain in the LNB to account for it. Rain fade margin is defined by the noise figure of the LNB and the size of the dish. But, won't a long/poor cable run exacerbate the effect of rain fade? Surely a shorter/better cable run will help mask the effects of a reduction in signal levels produced at the LNB? If the cable is so long that the levels presented to the receiver in good weather are, say, only 10dB above the receiver's threshold, then the 6dB signal reduction caused by (very severe and unusual) rainfade will start to make a difference. But with normal cable runs there's 20 or 25dB to play with, so it doesn't matter. What has the microwave RF detected signal by the LNB got to do with the level of UHF IF signal down the feeder? Steve Terry -- "I would like to plead for my right to investigate natural phenomena without having guns pointed at me. I also ask for the right to be wrong without being hanged for it." - Wilhelm Reich, November 1947 |
Had the IRS install today
On Sep 12, 1:02*am, "
wrote: On Sep 11, 7:39*pm, tony sayer wrote: I don't like downlead to flats to be longer than 30dB; the need for equalisation becomes acute. OK I'll put my hand up. I was a bit tired, I meant 30m. And downleadS. Bill |
Had the IRS install today
On Sep 12, 3:12*am, "Steve Terry" wrote:
" wrote in message ... On Sep 11, 3:47 pm, "Doctor D" wrote: "Paul Ratcliffe" wrote in message ... On Sat, 11 Sep 2010 12:30:44 +0100, Doctor D wrote: I understood that lengths of over 20m of cheap shotgun cable were lossy. This won't show up as a grainy picture but you may suffer more break up in the signal when it's raining heavily and the incoming signal to the dish is attenuated. You're talking about two different effects now. Cable loss is overcome by having enough gain in the LNB to account for it. Rain fade margin is defined by the noise figure of the LNB and the size of the dish. But, won't a long/poor cable run exacerbate the effect of rain fade? Surely a shorter/better cable run will help mask the effects of a reduction in signal levels produced at the LNB? If the cable is so long that the levels presented to the receiver in good weather are, say, only 10dB above the receiver's threshold, then the 6dB signal reduction caused by (very severe and unusual) rainfade will start to make a difference. But with normal cable runs there's 20 or 25dB to play with, so it doesn't matter. What has the microwave RF detected signal by the LNB got to do with the level of UHF IF signal down the feeder? It determines it. The LNB amplifies by a fixed amount and mixes the band with the LO. Therefore the amplitude of the IF signal is always directly proportional to the amplitude of the coresponding 11-12GHz signal. (Alas, when it leaves the LNB it is accompanied by LO and amplifier noise.) I agree with your implication, which I assume is that we could do with LNBs that have AGC on a mux by mux basis. That would be super. Care to invent one? Bill |
Had the IRS install today
On Sat, 11 Sep 2010 15:47:55 +0100, Doctor D wrote:
I understood that lengths of over 20m of cheap shotgun cable were lossy. This won't show up as a grainy picture but you may suffer more break up in the signal when it's raining heavily and the incoming signal to the dish is attenuated. You're talking about two different effects now. Cable loss is overcome by having enough gain in the LNB to account for it. Rain fade margin is defined by the noise figure of the LNB and the size of the dish. But, won't a long/poor cable run exacerbate the effect of rain fade? Not really, unless it is right on the edge, which is unlikely. Surely a shorter/better cable run will help mask the effects of a reduction in signal levels produced at the LNB? No, if you've lost it at the front end, you can't put it back downstream. |
Had the IRS install today
On Sat, 11 Sep 2010 16:04:15 +0100, Nick Le Lievre
wrote: I think the "shotgun" cable run is not that long, maybe up into the loft, I think there is a tap in the loft although I do not know what a tap is can someone explain That's your hot water system. You don't have taps on satellite systems. |
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