db rating on LNBs

hansov

Something I have been wondering about for some time. So I thought that I would ask the experts! What does the db rating of an LNB mean and is lower better than higher?

Zaphod

http://www.satcure.co.uk/tech/lnb2.htm

The lower the dB figure i.e. less noise, the better.

watty

Zaphod's answer is perfectly correct...

Goat stuff

BUT...

1) The overall noise figure (NF) is what counts, it turns out that below 0.6dB to 0.8dB the system NF is not much improved.

2) It is dubtfull if most of the LNBs "better" than 0.6dB (i.e. much more expensive 0.3dB NF) really are better.

2a: The flatness of frequency reponse is more important. Esp. a wealk transponder in a group of strong ones.

2b: Typically a 0.3dB NF is an AVERAGE over the band. An 0.6dB "worst case" LNB may be lower noise.

3) A next size up dish will give typically 3dB to 6dB improvement in overall S/N ratio (which is what counts for getting the signal). Going from 0.6dB NF LNB costing 10 Euro to a 0.3dB LNB costing 80 Euro may only give an overall 0.2dB improvement in S/N ratio.

Since dB is a log scale 0.2dB is a tiny, tiny change. 6dB is twice the voltage, a huge change, 0.2dB is a 1.02 times change, almost no effect at all.

Conclusion:
Decent brand 0.6dB LNBs at budget price are fine. If the signal is marginal you need a bigger dish. If the signal vanishes in rain with 0.6dB €10 LNB it will still vanish even with an €80 0.3dB NF LNB.

An old 1dB LNB on 80cm Dish out performs a Sky Minidish with 0.7dB Grundig.

The 0.6dB NF LNBs have been found to often outperform expensive 0.3dB NF LNBs in Gibralter on Astra 2D.

On the SAME 80cm dish the 0.6dB LNB does improve marginal barely working channels compared with the 1dB old LNB. But no difference on "good" signals.

Maybe 10 years or 15 years agoi when some LNBs might have had a 4dB or higher NF, the LNB NF was important. Nowadays it is not as even the cheapest LNB is typically 0.6dB. Really the only important thing nowadays is a big enough dish.

Large dishes need custom "feed horns" and use "flanged" LNBs which are much more expensive. Typically up to 1.4m a regular "universal" LNB with integral horn and focus rings is used.

A Sky mini-dish may or may not need a specific LNB apart from the mounting issue. The Grundig LNB I have for Sky mini-dish and for a regular neck mount look very similar. However that is a separate issue to the NF in dB, it is the F/D ratio of the built in feed horn to suit the geometry of the dish (NOT its size, but ratio of diameter to focal length, which can't be measured off an offset dish, manufacturers data is required.

hansov

Now that's why I call ye guys experts! What a thorough reply! Thanks again.

Tony

hansov wrote:

Now that's why I call ye guys experts! What a thorough reply! Thanks again.

Having said that heres what a customer of mine said :
" Hi Tony,
I thought I should email you - the 0.3db LNB I got from your works great.
I wasn't able to pick up SkyDSL on Telecom 2D with an 80cm orbital dish
with a 0.6db LNB but with the 0.3db LNB I can get it fine. There wasn't a
problem with the 0.6db LNB as my father-in-law has it now but it just must
have been that I needed a small bit more signal or a slightly bigger dish
but the LNB did the trick."

Zaphod

Telecom 2D, and SkyDSL TP, are a bit strange in that the skew should be more or less zero in Ireland for that sat. In fact, by skewing the LNBF as you would for Hotbird 13E, you can increase the signal quality by more than 30%.
http://board.satbook.de/wbb/thread.php?threadid=754&threadview=0&hilight=reisat&hilightuser=0&page=3

As for LNB noise figures, I often find the local oscillator stability numbers to be a better guide than dB. For example, a 0.7dB Invacom LNB with +/-450KHz drift outperformed a 0.5dB Precision LNB with +/-2MHz drift, across the entire Ku band.

Tony

Zaphod wrote:

Telecom 2D, and SkyDSL TP, are a bit strange in that the skew should be more or less zero in Ireland for that sat. In fact, by skewing the LNBF as you would for Hotbird 13E, you can increase the signal quality by more than 30%.
http://board.satbook.de/wbb/thread.php?threadid=754&threadview=0&hilight=reisat&hilightuser=0&page=3

As for LNB noise figures, I often find the local oscillator stability numbers to be a better guide than dB. For example, a 0.7dB Invacom LNB with +/-450KHz drift outperformed a 0.5dB Precision LNB with +/-2MHz drift, across the entire Ku band.

Yeah that could well explain it. I agree with you about freq stability, it tends to get ignored and is crucial for good operation

watty

Yes I find different band LNB the skew is not the same. So that may be why. Many beginners/DIY don't realise to adjust the skew. It can be even more critical on some digital channels where there is an overlap in frequency on the H&V polarities.

On very cold morning on one LNB the low SR channel DADx at SR1500 needs retuned.

The "AFC" range though varies from receiver to receiver. The Palcom DSL4 is much better than the PC Card 9I have experimented with how far +/- the frequency can be wrong and still lock. On low SR SCPC transponders the accuracy of the frequency setting can vary channel change lock time from about 0.5s to nearly 4s.

watty

One advantange of of a separate polariser on a motorised system is per satellite skew adjustment. Most receivers can't control this though. With regular box and motor you have to have a Compromise skew, it is not perfect for all satellites.