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The only way to fully test an LNB is to fit it to a suitably aligned dish and connect a satellite receiver. Then check to make sure every channel is there. If no channels are missing and if it continues to work through a hot day and a cold night, the LNB is fine.
However, if some channels still appear to be missing (with a known good LNB), this could be the fault of the cable (bad connection, water ingress or kinked or wrong type), the receiver or the dish (distorted or misaligned) or local interference (DECT phone, car radar detector, etc.). So then you need to use a process of elimination by swapping the dish, the cable and the receiver (for a different make/model as some receivers won't work correctly with some LNBs). Note that some wall-plate connectors can also cause "channels missing" problems and some electronic devices (DECT 'phone base stations, "Snooper" type radar detectors and microwave ovens) can cause similar problems. I've even had it reported that a bad battery connection in a laptop computer wiped out some channels!
Can't I test it with a meter?
You can NOT test an LNB with a multimeter.
A cheap Satfinder meter is more useful but it simply looks at the average strength of all frequencies. It will not tell you if some frequencies are missing. An expensive meter looks at a specific frequency (sometimes more than one) but it will not tell you if some frequencies are missing.
Also, most meters rely on battery power. If the battery can not supply enough current to the LNB, it may give a false reading.
In short, you can not fully test an LNB with any meter but you can perform a partial test.
Faulty LNB? Could my LNB be damaged?
A "universal" LNB is designed to operate normally with an input voltage of up to 20 volts. At a somewhat higher voltage, you can expect the LNB to fail. The time taken to fail depends on the applied voltage and duration. It would take a highly unusual fault to cause a Digibox to provide more than 20 volts and any digibox that did so would almost certainly emit expensive smoke within seconds.
So it's simply not possible to "blow" an LNB with overvoltage. The receiver will always die first (and it's extremely rare).
The usual causes of "failure" are:-
1. Bad connection at the LNB. 2. Bad connection at the LNB. 3. Bad connection at the LNB. 4. Bad connection at the receiver.
The causes are:-
1. Incorrect F plug fitting.
2. Moisture ingress caused by incorrect weatherproofing and/or lack of silicone grease.
3. Using RG6, which has a steel core wire that has a very high resistance compared with copper AND is prone to rusting.
4. Using thin "shotgun" cable where the core wire is thin and doesn't create enough contact pressure.
There's also a possibility of cable damage, although it would usually be obvious that this was the cause (unless the installer is ignoring it and blaming the LNB for the fault, knowing that he'll make an LNB sale AND get called back).
The reason that LNB replacement often "cures" the fault is that it disturbs and cleans the bad connection, which returns a short time later as the connection goes bad again. So it's sensible to clean the connection and try again before replacing an LNB.
Is my LNB causing Grainy Pictures?
No.
Because the signals for digital channels are "multiplexed" together, the satellite TV receiver has to decode them and piece them back together. This means that neither the LNB nor the cable connecting it to the receiver can cause "noisy" or "grainy" pictures. Digital is an "all or nothing" system. You either see a perfect picture (as perfect as the original transmission) or else an extremely broken picture or no picture at all. There is no inbetween stage as there was with analogue. So, if you are seeing "grainy" pictures, the fault must lie in the receiver (rare) or the TV (rare) or in the cable connecting them together. It could be caused by interference in your house or it could simply be your imagination.
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در این مقاله قصد داریم به یکی از مهمترین خصوصیات ویژه ال ان سی و مالتی سوییچ, که آن رامعجزه اتصال بین دو دنیامی نامیم ,بپردازیم.
ال ان سی و انتن مرکزی الکتروپارت 9368663893
The graph shows a typical result of measuring the unwanted noise generated by two LNBs across their frequency range (horizontal axis). Low is good.
The red line shows a "0.2dB minimum" LNB and the blue line shows a "0.4dB minimum" plot.
Bearing in mind that the ideal situation is to have a minimum amount of noise across the entire frequency band, which LNB would you prefer?
Every LNB is different because of manufacturing tolerances. Measuring LNB noise is time-consuming and can't be done for LNBs selling for just a few pounds each. Instead, the manufacturer will make a few checks at specific frequencies (not across the whole band) on a few random samples.
Without a full graph showing the actual measurements of the LNB that you buy, it's impossible to know what you've got. The manufacturer may state a "typical" or "minimum" noise figure but that's based on measurements of only a few random samples at a specific frequency.
In the end, you have to rely on results. If a lot of people say (for example) "the zzzzzz quad-output LNB works for me in a fringe reception area with a dish made by xxxx and a receiver made by yyyy" then that's probably the best information you are going to get.
(If you are not in a "fringe reception area", just buy the cheapest. It won't make ANY difference whatsoever to your picture quality or rain margin. The dish size is MUCH more important. If in doubt, buy a "Sky zone 2 minidish" or larger.)
The "0.1dB" specification is utterly meaningless. Seriously, once the noise figure is below 0.6dB right across both bands (Hi and Lo), then lowering it further (even if that were technically possible, which it isn't) would make no discernible difference to reception.
Even if we paid £100 for an LNB, the manufacturer couldn't guarantee 0.1dB max. noise across both Hi and Lo bands in real world conditions.
Of course, that doesn't stop crafty sellers from sticking a label on and relying on the fact that nobody has the £50,000 equipment to prove them wrong.
You should also bear in mind that there are other important parameters:-
GAIN
This is the amplification factor and, again, it varies across the frequency range, just like the noise factor does. The Gain determines how strong the signal is when it is sent down the cable. Ideally you want a high gain and a low noise factor for a long cable but the two are mutually exclusive so you won't get it. For a short cable run you don't want a high gain because there's a risk of overloading the tuner inside the receiver - especially with a large dish.
CROSS POLAR REJECTION
Satellite signals from the Astra satellites (and most others viewable in the UK) are transmitted in either vertical or horizontal polarisation. A "universal" LNB can switch between the two and this switching is controlled by the LNB voltage supply: 13v gets you vertical and 18v gets you horizontal polarisation. But signals of different polarisation can interfere with each other. The ability of an LNB to prevent this is called its "cross-polar rejection" (CPR) and is measured in deciBells.
CPR is important if you are in an area of fringe reception where one polarisation is much stronger than the other. If the LNB can't reject the unwanted signal, it will swamp the weaker one resulting in "No satellite signal". In general, CPR isn't going to matter with a dish less than 1.8m in diameter.
BAND
A universal LNB can receive signals from two separate frequency bands - high and low. The associated satellite receiver "tells" the LNB to use High Band by sending a 22kHz tone up the cable. Without the tone, the LNB will revert to Low Band.
POLARISATION/SKEW
The satellite transmits signals which are horizontally polarised and vertically polarised, which minimises interference between adjacent frequencies. The receiver "tells" the LNB which polarisation to "look at" by supplying it with 13 volts (V) or 18 volts (H). In addition, the LNB "skew" (rotational position) has to be set to the correct angle so that horizontal and vertical signals are received with equal strength. The best guide to the correct "skew" position on a Sky Digibox is the "signal quality" indicator. Rotate the LNB for the highest reading.
SPLITTING THE FEED
Because the LNB is looking for a steady 13 volt or 18 volt supply and a steady 22kHz tone, you can't "split the feed" to drive two receivers unless you appreciate that one receiver will always "win". The receiver supplying 18 volts and 22kHz tone will win over the other. That's fine if both programmes happen to be on (for example) High Band, Horizontal Polarisation but otherwise you'll get a conflict. The answer is to use an LNB with multiple outputs - usually a "quad-output" LNB.
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+ نوشته شده در چهارشنبه یازدهم اردیبهشت ۱۳۹۸ساعت 17:57 توسط EPC
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انجمن الکتروپارت زیرمجموعه گروه فنی مهندسی ماهان تکنیک .طراحی , تولید و توسعه دهنده انواع محصولات الکترونیکی های تک ویژن مدیا , میکروپروسسورهای زئوتک وعرضه کننده انواع مالتی سوییچ ودیواسهای انتن مرکزی, سیسیکم و .. فروش انواع محصولات آنتن مرکزی توسط مجموعه الکتروپارت برای کسب اطلاعات بروز , بمنظور آشنایی و خرید محصولات الکترونیکی , پکیج های آنتن مرکزی و دوربین های مدار بسته میتوانید . از ما بپرسید . مشاوره بگیرید و تهیه بفرمایید.
