LNB questions

[delorehal]

I have a Telefuken labelled as a "twin LNB for one satellite and two receivers".
I would like to know if the two outputs are independent, ie if one can receive 10.7GHz and the other 12.75GHz

What about the noise figure, some claim 0.2dB, others 0.1dB, are these numbers only advertisements ?

I would like to understand noise and gain numbers :
Can a little dish with a high gain lnb and low noise figure be equivalent to a bigger dish using a lower gain and worse noise figure lnb ?

thanks

 

[RimaNTSS]

Can a little dish with a high gain lnb and low noise figure be equivalent to a bigger dish using a lower gain and worse noise figure lnb ?

Forget about that! Bigger dish is always better and does not really matter what kind of LNB is used.

 

[satelliteman]

The ports will work independently.

 

[rolfw]

Twin would normally suggest two identical outputs, both universal with voltage switched horizontal and vertical bands and tone switched high and low bands.

 

[delorehal]

The ports will work indepently (for exemple one high/vertical and the other low/horizontal) or not ?
Previous posts seem different

 

[Lazarus]

Quoted noise figure almost certainly reflects the best performance over a small part of the Band ... the NF vs Frequency plot most certainly won't be flat.

Most of us treat the headline figure with a pinch of salt and rely more upon recommendations from other users when considering acquisition of LNBs. Or, in the case where you've already got one or more, then experimenting to see which performs best in practice for your own needs.

Same caveat applies to Gain .... This will vary across the band.

 

[s-band]

What about the noise figure, some claim 0.2dB, others 0.1dB, are these numbers only advertisements ?

I would like to understand noise and gain numbers :

The 0.2dB NFs quoted are impossible with the design used in normal LNBs. (maybe one day trading standards will look at claims, or not) The active devices themselves, without losses in the circuitry and feed, have noise figures of 0.3-0.4dB at best. The Octagon range quote 0.2dB or so and have been measured at 0.6dB (not by me) at best but, as has been said, the NF will not be flat across the band. The performance of sub-£10 LNBs is actually very good for the money. Unless you get a bad one, it will probably be ~1dB across the band, which is fine.

As long as the gain is enough to overcome cable losses and the noise in the indoor box, it doesn't matter much. If the noise, measured by the indoor box, increases by >10dB, or so, the gain is "enough". If you have a long cable run, go for a high gain LNB. Gain flatness isn't really a problem unless the ripple is very high and varies too much across a single channel.

The bottom line is:

Forget about that! Bigger dish is always better and does not really matter what kind of LNB is used.

Unless the LNB is broken.

 

[Terryl]

I have a Telefuken labelled as a "twin LNB for one satellite and two receivers".
I would like to know if the two outputs are independent, ie if one can receive 10.7GHz and the other 12.75GHz

What about the noise figure, some claim 0.2dB, others 0.1dB, are these numbers only advertisements ?

I would like to understand noise and gain numbers :
Can a little dish with a high gain lnb and low noise figure be equivalent to a bigger dish using a lower gain and worse noise figure lnb ?

thanks

Not with that LNB, you would need a universal LNB, one capable to cover that big a spread.

Something like this one......Twin LNB (LNBF) | Universal Low Noise Block-Downconverter

 

[delorehal]

Why this Chaparral LNB differs from mine ?
Does the term universal lnb mean that both outpouts are independent ?

 

[Terryl]

The LNB I posted will cover the 10.7 GHz to 12.7 GHz range by using different LO* frequency's, (selected at the receiver in it's menu) and yes both outputs are independent from each other, each can run separate receivers.

For low band transponder channels (10.7 GHz to 11.7 GHz) the LO would be 9.75 GHz, for the high band (11.7 GHz to 12.75 GHz) it would be 10.6 GHz.

Thus the terminology of a "Universal LNB"

*LO= Local Oscillator, the LO is used to down convert the incoming frequency from the satellite transponder, this is done so you can use standard RG-6 coax to send the signal to the satellite receiver.

To do this you take the LO frequency and subtract it from the transponder frequency.

So for a satellite transponder frequency of 11.345 GHz you would use the 9.75 GHz LO frequency, this would give you an IF (Intermediate Frequency) frequency of 1.595 GHz, this lower frequency IF can be sent down standard coax.

This is done because the 11.345 GHz signal coming down from the satellite would need to use a wave guide to get this signal to a receiver, a wave guide is very expensive.

 

[delorehal]

Thanks Terryl, but in what this Chapparral LNB differs from the Telefunken I have which also cover 10.7/12.75 GHz band ?

How about my initial question about dish size vs lnb noise/gain.
In the real world I understand that lnb are close in characteristics so there is no replacement for size, but in theory what count is the signal/noise ratio and a better lnb should be equivalent to a greater dish. I'm correct or not ?

 

[s-band]

Assume you change from 1dB NF to 0.5dB NF LNB that's roughly 78K to 36K noise temperature. Add 30K for noise temp of dish to each gives noise temp totals of 108K to 66K which equates to an improvement of about 2.3dB. 2.3dB is roughly equivalent to changing from a 1m to a 1.3m dish.

All figures "back of envelope" and theoretical, not taking real world errors (and mine!) into account.

Good tutorial here: LNB mysteries explained - part 1

 

[delorehal]

Great,
many thanks

 

[Terryl]

Thanks Terryl, but in what this Chapparral LNB differs from the Telefunken I have which also cover 10.7/12.75 GHz band ?

How about my initial question about dish size vs lnb noise/gain.
In the real world I understand that lnb are close in characteristics so there is no replacement for size, but in theory what count is the signal/noise ratio and a better lnb should be equivalent to a greater dish. I'm correct or not ?

I could not find your brand of LNB in the "Universal" line up, the "Chaparral" LNB was only used as an example, if your LNB is a true universal then the LO specifications should be correct, all you need to do is specify the correct LO frequency and LNB type when setting up the transponders on the satellite(s) your trying to view.

And thanks to S-Band for the good info.

 

[delorehal]

OK thanks Terryl