SatCure, PO Box 12, Sandbach, CW11 1XA, England Return to SatCure index page LNB mysteries explained Low Noise Block-downconvertor (so called because it converts a whole band or "block" of frequencies to a lower band). ============================ Is there actually a different LNB for prime focus dishes + offset dishes? Surely an LNB's innards are the same and the feedhorn or the C120 flange is the only difference? In the old days, LNB noise figures were high, the gain (amplification) was low and satellite transponder power was typically 20 Watts. Imagine trying to see a 20 Watt light bulb 24,000 miles away! (You'd have trouble seeing a 20W bulb at the end of a 24 yard corridor). So, an LNB and feedhorn had to be matched to the dish. The internal antenna of the LNB had to be at the exact focal point of the dish and the horn had to be flared in such a way that, with the LNB at the focal point, the . horn could "see" the exact circular area of the dish - no more and no less. If it was less then it wasn't collecting signal from the full area of the dish. If it was more, it was also collecting unwanted "noise" from any warm object (wall) or from the sky behind the dish. A good compromise was to take just part of a much larger dish and mount the LNB in an "offset" position. The curvature of this partial dish is such that the focal point is now much lower so the LNB and feedhorn no longer obscure the signal path as they would with a "prime focus" dish. Nowadays, satellite transponders can produce typically 50 or 60 Watts and LNBs have higher gain and lower noise figures. With these strong transmissions, you can get away with murder. People stick any old thing on the end of the boom arm - which rather explains why one man's 0.6dN LNB is another man's nightmare when the signal strength is not optimum! The Sky minidish, for example, is a compromise between size and performance. It's very important that the LNB matches the dish exactly. This is one good reason why the dish comes with its own LNB. If you "mix 'n' match" by picking a 60cm dish and a Universal LNB at random, the chances are that the performance could be no better than that of the Sky minidish. Just to prove the point, here is the "Universal" LNB used with a Sky "minidish". The minidish is oval in shape, being much wider than it is high. Inside that plastic rain cover is the actual LNB. Note the difference in scalar ring height (red arrows). The side projections allow the LNB to focus on a wide area in the horizontal plane, while the top and bottom projections are longer and focus the LNB on a narrower area in the vertical plane. This LNB is designed specificaly for an oval dish and is likely to give poor results with a dish that is roughly circular or a dish that is taller than it is wide. The manufacturers might "fudge" the issue if asked. After all, if they admit that their LNB works best with, say, an 80cm Lenson Heath dish and you just bought an 1 metre dish made by someone else, you might not be too happy. People keep asking me what sort of LNB they have. It's easy to find out: If it came with a Sky Digital system, it's a "Universal" LNB, meaning that it has a 9.75 GHz internal oscillator for "low band" use and a 10.6 GHz oscillator that is selected by feeding a 22kHz tone to it for "high band" use. Many LNBs actually have the word "Universal" printed on them. A Universal LNB requires a 22kHz tone at 0.5v p-p to switch its Local Oscillator to 10.6GHz ("high band"). Otherwise it uses its 9.75GHz oscillator. Polarity switching is controlled by dc voltage supplied by the receiver. 12.5v to 14.5v gives vertical and 15.5 to 18v gives horizontal polarisation. A higher voltage than that can damage the LNB. A voltage that is too low will prevent the LNB from working correctly. If you have an analogue system, tune to Sky news (vertical polarisation) Astra at 19.2'E. Set your receiver LNB installation to 10.0 GHz (not adjustable in older receivers). If Sky News is roughly 11.377 GHz then you have a standard 10.0 GHz LNB. If it's roughly 11.627 GHz then you have a 9.75 GHz LNB which might be "Enhanced" or "Universal". (Enhanced LNBs have no 10.6 GHz oscillator for high band.) A lot of motorised systems use a Universal LNB nowadays. This is good from the point of view of ease of installation and adjustment but it really is a compromise if you are dealing with any weak signals. Where a weak analogue signal might give a picture with terrible "sparklies", a weak digital signal can give a blank screen and no sound. You need to cut the losses as low as possible and provide an accurate adjustment for "skew" which is the rotational angle at which your dish receives the signal from the satellite. You can fit a C120 flange LNB with a magnetic (coil) polariser and this will allow fine adjustment of skew (if your receiver provides it). However, a magnetic polariser has losses and a mechanical polariser is better in this respect - again provided that your receiver can drive it. Even though we are looking at something like only 0.1dB improvement, this can make the difference between receiving a watchable digital transmission and receiving a bunch of coloured squares. Another problem in choosing an LNB is that most manufacturers quote an "average" or "typical" noise figure. The first point to note is that your particular LNB might have this noise figure at a particular frequency but could be significantly worse at other frequencies. Even if you get a print out of the actual noise figures and gain performance when you buy your LNB, you have to bear in mind that these measurements were made in a factory under ideal conditions with the LNB receiving (probably) 18 volts and feeding an exact impedance of 75 Ohms. Of course, your cable is *supposed* to be 75 Ohms and your satellite receiver tuner input is *supposed* to be 75 Ohms but are they? In fact they seldom present exactly 75 Ohm impedance at all frequencies. Just like the LNB, the figure is "typical". Worse, it can be affected quite drastically by kinks and joints in the cable and, of course, by anything you insert, such as an amplifier, switch or (aagh!) splitter. What happens if the impedance is not exactly matched? Well, I'm not going to launch into a highly technical explanation of Voltage Standing Wave Ratio (even if I could!) Suffice to say that part of the signal gets reflected back. This can have the effect of cancelling out part of the signal with inevitable results. So how do you choose your LNB? It's best to go on recommendations. If a particular type of LNB shows good results with a specific make and size of dish, try that combination. Buy the best quality cable you can afford. Avoid kinking it and keep the run as short as possible. Avoid joints if at all possible. If you have to use a switch, choose one by a good manufacturer such as Global Communications in the UK. Fit the 'F' connectors carefully, making sure that the copper braid screening is clamped evenly all round. (Avoid any sort of connector or wall plate that uses a screw to clamp the inner core. There is no way that this can be properly screened or maintain an impedance of 75 Ohms. It may work fine for UHF TV aerial signals but I wouldn't trust it with satellite TV.) Finally, remember that the dish size, cable quality and the overall matching of LNB, dish and cable has FAR more effect on a weak signal than the actual noise figure of the LNB. What sort of LNB ? People keep asking me what sort of LNB they have. It's easy to find out: Tune to Sky news (vertical polarisation) Astra at 19.2'E. Set your receiver LNB installation to 10.0 GHz (not adjustable in older receivers). If Sky News is roughly 11.377 GHz then you have a standard 10.0 GHz LNB. If it's roughly 11.627 GHz then you have a 9.75 GHz LNB. 1) Standard LNB 10.0 GHz L.O. Often called a "Marconi switching LNB". Works in one band. Noise Figure usually 1.0 dB or better but older "Blue cap" types can be much worse. Integral feed horn, usually with 40mm neck but flange type available to special order and other neck sizes have been made (especially 22.5mm). Marconi also made a "Bullet" shape LNB of this type that used a PTFE insert instead of a horn. Polarisation switching is controlled by dc voltage supplied by the receiver. 12.5v to 14.5v gives vertical and 15.5 to 18v gives horizontal polarisation. A higher voltage than that can damage the LNB. A voltage that is too low will prevent the LNB from working correctly. 2) "Enhanced" LNB 9.75 GHz L.O. Works 10.7-11.7 GHz. Noise Figure usually 1.0 dB or better.