High Gain LNB ??

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OldskoolPirate
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High Gain LNB ??

Post by OldskoolPirate » Wed May 17, 2017 11:24 am

Anyone know if high gain or sensitive lnbs exist ? Or will make any improvement with micro link. Or what's the best lnb to use for this ?
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Albert H
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Re: High Gain LNB ??

Post by Albert H » Thu May 18, 2017 3:22 am

There are some that are marginally more sensitive (or have a better noise figure), but to be honest, they're all much the same. The largest part of the antenna gain comes from the focussing effect of a dish when they're receiving satellites. Like most consumer electronics, they're built down to a price, not up to a specification. The modern LNB seldom has much of an RF amplifier stage - there are usually two identical stages, for the two polarisations, which are switched as required, then they go through a crude bandpass filter straight into a GAsFET mixer, then through a single buffer amplifier stage (to match to the 75Ω coax). The local oscillator is a temperature-compensated SAW type, and is remarkably stable despite the environment it's working in and the range of temperature it experiences.

The most sophisticated ones (at sensible prices) that I've found are the Humax jobs - cheaply available from CPC - and I've always had good results with them. The circuit of one is attached:
LNB.png
They are actually quite remarkable feats of engineering when you consider that they have to work at 10 - 12 GHz and cost almost nothing!

One thing you can do to improve your receive end of your link is to add a dish. Obviously it has to be tipped forward a lot, so that it's looking at a terrestrial source, but the gain provided by even an 80cm dish can make your link range quite enormous. My record for a simple 10 GHz link was 48 km, and that was thrown together in an afternoon!
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Maximus
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Re: High Gain LNB ??

Post by Maximus » Thu May 18, 2017 8:00 am

You could try an inline amp. I haven't had much luck on the digital side with them but worth a try at £4. If all else fails, use a minidish:

http://www.systemsat.co.uk/amplifiers/m ... GwodLHoFKQ


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Re: High Gain LNB ??

Post by thewisepranker » Thu May 18, 2017 12:06 pm

An inline amplifier is just going to amplify the noise and add even more noise. If there are problems already it will only add to them. A dish is the way to go.

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Re: High Gain LNB ??

Post by Albert H » Thu May 18, 2017 2:26 pm

A dish is the only real solution. It doesn't have to be a big dish (though the bigger, the higher the antenna gain). I've used dishes at both ends many times.

If you're not having much success with microwaves, you could move down to the wide-open spaces on Band IV or V. There's lots of room between the digital TV multiplexes, and you can get huge, high gain TV Yagis for a few quid. A TV aerial never looks out of place on a rooftop, and you can get spectacular range out of relatively low transmitter power.

The transmitter strip I usually use is a TSA5511 PLL controlling a varicap-tuned Colpitts oscillator at the output frequency. The oscillator is usually a dual-gate FET (BF961 usually), followed by two BFR96 stages to get up to around 150mW output into 75Ω. I use F-type connectors for the coax (less loss than the Belling-Lee connectors I used to use!).

The receiver is slightly trickier, but if you use a standard TV front-end (ideally one like the UV616 with a TSA5511 for tuning), you'll get a 35MHz first IF. I put this through a ceramic filter (cheap enough from TV spares places), and then mix with a 24 MHz crystal and filter off a 180kHz-wide second IF at 10.7MHz. This is amplified by a transistor, further filtered in a standard ceramic filter and fed to a CA3189 IF strip. The 3189 is configured for PLL-demodulation (I can recover signals that sound like white noise on other receivers!) and the whole receiver is astonishingly sensitive and selective. In the old days of analogue TV, this receiver was selective enough to be used over short ranges in the vestigial sideband of one of the big TV transmitters at Crystal Palace! [Gotts & Co never sussed the way that link worked!]

TV coax is actually pretty low-loss, so as long as your TX and RX are designed to match 75Ω and you use 75Ω connectors (BNC, F-type, Belling-Lee or whatever) you'll get good results. I've linked in fully noise-quietening stereo (with RDS) over 35km with just 200mW of link TX power.
"Why is my rig humming?"
"Because it doesn't know the words!"
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