LNB mysteries explained


Low Noise Block-downconverter (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.

People keep asking me what sort of LNB they have.
The only way to determine whether an LNB is "universal" is to supply it with a 22kHz "tone" and see if the picture disappears (it switches to "high band"). However, most Universal LNBs are clearly marked either "Universal" or "9.75/10.6".

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. Integral feed horn with 40mm neck.
Normally used with later type receivers that have a 2GHz tuner but no 22kHz tone generator. Designed specifically for Astra satellite reception from satellites 1A, 1B, 1C and 1D.

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.

3) "Universal" LNB 9.75 and 10.60 GHz L.O.
Works in 2 bands* 10.7-11.8 and 11.6 - 12.7 GHz. (22 kHz tone switched). Noise Figure usually 1.0 dB or better. Integral feed horn with 40mm neck but flange type available to special order.
*If your receiver tuning range is less than 2.15GHz you will have a gap between high and low bands. Refer to calculations, below. In effect, this is a "Quad Band" LNB.

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.

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.

4) "FSS" LNB 10.0 GHz L.O.
Normally bolted to separate polariser and feed horn. Works in one band: 10.9 - 11.7 GHz. Receiver with standard 0.95 - 1.75GHz tuner may be used. Noise figures vary. Very old ones can be 3.0 dB!

5) "DBS" LNB 10.75 GHz L.O.
Normally bolted to separate polariser and feed horn. Works in one band: 11.7 - 12.5 GHz. Receiver with standard 0.95 - 1.75GHz tuner may be used. Noise figures vary.

6) "Telecom" LNB 11.0 GHz L.O.
Normally bolted to separate polariser and feed horn. However, Marconi made a voltage-switching version with integral feed horn*. Works in one band: 11.95 - 12.75 GHz. Receiver with standard 0.95 - 1.75GHz tuner may be used. (* identified by a serial number label with a red corner, although some were incorrectly marked). Noise figures vary.

7) "Dual band" LNB
Normally bolted to separate polariser and feed horn. Works in 2 bands 10.9 - 11.7 and 11.7 - 12.5 GHz. Receiver with standard 0.95 - 1.75GHz tuner may be used. Band switching achieved by supply voltage of either 14 volts or 18 volts. Noise figures vary.

8) "Tripleband" LNB
Normally bolted to separate polariser and feed horn. Works in 2 bands 10.9-11.8 and 11.8-12.75 GHz. Receiver with 0.95 - 2.0 GHz tuner should be used. Noise figures vary.

9) "Quadband" LNB
Normally bolted to separate polariser and feed horn. Works in 2 bands 10.7-11.8 and 11.7-12.8 GHz. Receiver with 0.95 - 2.05 GHz tuner should be used. Noise figures vary.

10) "Twin output" LNB
Currently available in Standard, Enhanced and Universal form, the twin output LNB provides two outputs to feed two separate receivers. Each output can be switched by 13/17 volt input by the individual receiver to change polarisation.

11) "Dual output" LNB
Currently available in Standard, Enhanced and Universal form, the dual output LNB provides two outputs to feed two separate receivers. Each output has a fixed polarisation; one horizontal and one vertical. This type of LNB should be used with switching boxes such as the "Mini Magic" which will feed four separate receivers.

Explanation of (L.O.) Local Oscillator Frequency:

Suppose a signal comes from the satellite at a microwave frequency of 12 GHz but your typical receiver tunes up only to 1.75 GHz? (Also bear in mind that most cable will NOT happily pass frequencies much above 2GHz).

The function of the LNB is to reduce the frequency of the satellite signal. It does so by subtracting a frequency figure from the satellite signal frequency.

This figure is called the "Local Oscillator" frequency ("LO") of the LNB.

So an LNB with a LO of 10.25GHz will send a 12GHz satellite signal down the cable at 1.75GHz (just within range of your old receiver).

12GHz - 10.25GHz = 1.75GHz

Working in reverse, if your highest satellite frequency is 12.6 GHz then you will need an LNB with a LO of at least

12.6 - 1.75 = 10.85GHz

in order to reduce the satellite signal to a frequency that your receiver can "see" (1.75 GHz).

Now let's reverse the process again:

A "standard" LNB has a LO of 10.0GHz. So the highest satellite program frequency that your standard receiver can *see* is

1.75 + 10.0 = 11.75GHz

An "enhanced" LNB has a LO of 9.75 GHz

1.75 + 9.75 = 11.50GHz

A "universal LNB has TWO LOs. One is 9,75 (same as "enhanced") The other is 10.6 which is selected if it "hears" a 22kHz (just above audio) tone.

1.75 + 10.6 = 12.35GHz

Of course, if your receiver can accept signals up to 2.0 GHz then the highest acceptable signal frequency becomes

2.00 + 10.6 = 12.60GHz

And a receiver with a tuner that extends to 2.15GHz achieves

2.15 + 10.6 = 12.75GHz (which happens to be the top of the "Telecom" band!)

Now, you are still puzzled about the DBS LNB

This has a LO of (typically) 10.75GHz

So an old 1.75GHz receiver will get up to

1.75 + 10.75 = 12.50GHz

A final consideration has to be the LOWER limit on tuning:

Most old receivers can tune no lower than 0950 MHz (= 0.95GHz) whereas later ones might go down to 0.70GHz.

Check out the above calculations with these lower tuning range limits to see the overall tuning bandwidth for any receiver.

This is all very basic "sums" - nothing complex - so once you have a "picture" of what is happening, you can sketch little band plans for any combination of receiver annd LNB.

Once you know the value(s) of the LO(s) in the LNB and of the upper and lower tuning limits of the receiver in question, you can quickly figure out what can be received.

NOTE:
Older receivers *expect" an LNB with a 10.0 LO and the frequency display is arranged just for this. However, you can use an LNB with a different value LO. It just means that the *displayed* frequency will be incorrect.

Transponder  - LNB Local  = tuner frequency
Frequency      Oscillator
 
 
12750 MHz
|        
|          
|  Telecom/Astra 1F
|     
|     
|  DBS/Astra 1E
|     
11700 MHz  - 10000 MHz = 1700
|                         |
|  Astra 1B               |
|                         |
|  Astra 1A     Receiver tuning range without ADX
|                         |
|  Astra 1C               |
|                        950  + 500 = 1450
|  Astra 1D                              |
|                               Tuning range with ADX
|                                        |
10700 MHz  - 10000 MHz = 700  + 500 = 1200


An old standard receiver usually tunes from 950 to 1700 MHz.
The map above shows the limited tuning range of an old standard receiver with an old standard 10.0 GHz LNB. The addition of an ADX Channel Expander moves the Astra 1D frequencies up by 500 MHz into the tuning range of the receiver.

To receive all Astra channels from satellites D to B without using an ADX, a receiver would need a tuning range of 700 to 1700 MHz.


Transponder    LNB Local
Frequency      Oscillator
 
 
12750 MHz  - 10600 MHz = 2150
|                         |
|                         |
|  Telecom/Astra 1F       |
|             Receiver tuning range for Hi band (22kHz on)
|                         |
|  DBS/Astra 1E           |
|          - 10600 MHz = 1100
11700 MHz  -  9750 MHz = 1950
|                         |
|  Astra 1B               |
|                         |
|  Astra 1A               |
|              Receiver tuning range for Lo band (22kHz off)
|  Astra 1C               |
|                         |
|  Astra 1D               |
|                         |
10700 MHz  -  9750 MHz = 950


An Enhanced LNB has a local oscillator frequency of 9750 MHz.
The receiver now needs a tuning range of 950 to 1950 MHz as shown in the map above.

If a Universal LNB is used, its local oscillator can be switched from 9750 to 10600 MHz by sending a 22kHz tone up the cable. Some receivers have this facility built inside. Some will need an external tone-inserter box connected into the cable.

If the receiver has a range of 950 to 2150, it will be able to receive programmes on both Hi and Lo band.



Another possibility is to use an ADX-Plus. This has an internal switch which, when moved across, makes the ADX-Plus move the frequencies DOWN by 500 MHz instead of up.

So a channel at 12750 MHz is moved down like this:

12750 - 10600 - 500 = 1650 MHz (with an ADX-Plus)

which is well within the tuning range of an old standard receiver.

And the lowest channel receivable will be:

950 + 500 + 10600 = 12050 MHz (with tone ON and ADX-Plus)

By fiddling with the ADX-Plus switch and the tone inserter you can probably receive the full range of channels if you have a Universal LNB and a standard receiver!


Note: MHz (MegaHertz) = GHz (GigaHertz) x 1000
So 9750 MHz = 9.75 GHz
Please let me know if you have any other thoughts on this and maybe we can add it to the FAQs.


>Martin,
>
>Many thanks, I took your advice and sat down with a pen and post-it
>note, and went through your LNB FAQ. Very informative thanks.
>
>I think I've got it sussed! If you don't mind, I'd just like to clarify
>my understanding:

Sure. That's the idea.

>Having establish that I've got a "Universal" LNB it has two L.O. frequencies -
>9750 MHZ and 10600 Mhz. I now understand that these values represent the
>degree to which the received frequencies are shifted by (reduced by).

Yes.

>The tuning frequency of my receiver with it being an old Amstrad SRD400
>is probably: 950 Mhz to 1700 Mhz a range of 750Mhz.

Yes :o)

>Astra broadcast frequencies range from 10700 Mhz (Astra 1D) through to
>11700 Mhz (Astra 1B). So I need to calculate which of these frequencies
>each of the two L.O.s on the LNB in conjunction with the tuning range
>of the receiver, I can receive, thus:
>
>L.O. 1 of 9750 Mhz
>
>950 Mhz (lower tuning capabilities of receiver ) + 9750 Mhz (L.O. 1)
>= 10700 Mhz
>1700 Mhz (upper tuning cap. of receiver) + 9750 Mhz (L.O. 1) = 11450
>Mhz.
>
>Hence able to receive frequencies of 10700 Mhz to 11450 Mhz.
>
>L.O. 2 of 10600 Mhz
>
>950 Mhz (lower tuning capabilities of receiver ) + 10600 Mhz (L.O. 2)
>= 11550 Mhz
>1700 Mhz (upper tuning cap. of receiver) + 10600 Mhz (L.O. 2) = 12300
>Mhz.
>
>Hence able to receive frequencies of 11550 Mhz to 12300 Mhz.
>
>Being an old crappy Amstrad SRD400, I presume my receiver isn't able to
>signal the LNB to switch to the upper of the two L.O. frequencies, so I
>can't access anything above 11450 Mhz. With a tone inserter I should
>therefore be able to access 11550Mhz - 12300Mhz?

You are doing OK so far.

>However, this leaves a gap in the middle between 11450 and 11550 of
>frequencies which I can't access! Presumably with an ADX (and tone
>inserter) I can shift those frequencies into tunable ranges?
>
>Presumably, all I need now is an ADX and a tone inserter in order to
>receive all Astra frequencies (and then some ... )? Oh yes, and a Sky
>subscription :-)

You don't really need a tone inserter.

>Would I be better off buying an ADX plus, so that I shift the
>frequencies down rather than up? So as to ensure my receiver is happier
>about it?

Yes, I would think so. There's nothing on the higher frequencies for your Amstrad to receive from Astra at 19.2 degrees East.
Sky Sports 3 would be the highest useful channel, with a couple of foreign stations just above that.


>How much would I expect to pay for an ADX and tone inserter?

If you have a Sky subscription they will post you an ADX-Plus for just £9.99.
Otherwise you can buy one for £12.95 upwards.


>Many thanks for all your help. It all seems so much simpler now. Or have
>I missed something?

Nope. I told you it was simple. People become frightened because they think it's too technical. The truth is that you simply need to be able to add and subtract - roughly to primary school level or lower! You don't need *any* technical knowledge. Just realise that there are various ways to add or subtract the frequencies. Specifically: there are various fixed values that you can use - Typically 10.00 GHz, 9.75 Ghz, 10.6 Ghz etc. for an LNB. Also +0.5GHz and -0.5GHz for an ADX-Plus Channel Expander, dependent on its internal switch position.

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