New LNB or stick with the old

Discussions about low-noise blocks, waveguides, horns and other components used at the prime focus of your reflector dish.
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Re: New LNB or stick with the old

Post by Snowman » Mon Aug 01, 2016 5:47 pm

Can a control for a corotor be purchased? Or would a person need to hand craft one?

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Re: New LNB or stick with the old

Post by jess76 » Mon Aug 01, 2016 11:25 pm

Check out the "Automatic polarity controller" over on Ricks site. They run about $50. Jess out in California.
9' mesh with c/ku feed horn,lnb's w/polorotor. Recievers hooked up: AZ prem hd., Pansat9500,and Manhattan. I have a few others in my colection including an old Satcruiser and a Captiveworks3000.

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Re: New LNB or stick with the old

Post by Snowman » Tue Aug 02, 2016 9:22 am

Thank you

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Re: New LNB or stick with the old

Post by DirtyHarry » Mon Aug 08, 2016 9:32 am

tek2000 wrote:On the basis of noise temperature alone, our LNBFs will outperform your Uniden LNB. Most LNA chips produced today have noise temperatures under 20K and dropping. The noise temperature for your Uniden LNB is 45K. Compare all this to the first c band LNA chips which were manufactured in 1978 and had noise temperatures exceeding 2500K!

As a general rule, if your LNB or LNBF is 5 years or older, you should definitely consider purchasing a new LNB or LNBF so you can take advantage of the improved silicon performance.
Would really like to know how you tested the lnb for noise temp. Who did the tests and how was it possible to get 13K and .1db results?

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Re: New LNB or stick with the old

Post by tek2000 » Mon Aug 08, 2016 5:00 pm

DirtyHarry wrote:As a general rule, if your LNB or LNBF is 5 years or older, you should definitely consider purchasing a new LNB or LNBF so you can take advantage of the improved silicon performance.
Would really like to know how you tested the lnb for noise temp. Who did the tests and how was it possible to get 13K and .1db results?
This LNBF was not designed or tested by my company, so I cannot provide you with any accurate specifications. However, based on years of experience, I can tell you that its noise figure is probably under 20K.

The major source of noise in any TVRO system today is going to be ground noise that enters the feed from the side/rear lobes. This will vary between 20K - 40K depending on whether the reflector is aimed at zenith or horizon respectively. So, at the very least, you want to use an LNB/LNBF with a noise figure under 20K.

To reduce ground noise even further, a well designed corrugated feed is necessary, along with a dish shroud. These will go a long way in suppressing side/rear lobes and cutting ground noise down even more. We expect to sell these items to our customers in a few months.
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Re: New LNB or stick with the old

Post by satdish » Mon Aug 08, 2016 7:57 pm

To add if your dish is warped, out of parabola or poorly designed you will not only get loss from sidelobes but also will experience more thermal noise when sighting birds with low elevation. Bottom line is if you dish is crap you can run all the exaggerated claim 15k lnb's and it still won't make a difference. Start with a good reflector, then add good electronics, that is the best prescription for success. You go cheap you get cheap.
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Re: New LNB or stick with the old

Post by tek2000 » Mon Aug 08, 2016 8:30 pm

Below is the idealized radiation plot for a 10ft reflector. If the reflector were being used to transmit power, the majority of the power would be transmitted along the optical axis (i.e 0 degrees). This does not mean power is not transmitted in other directions, because it is! At 3 degrees off boresight, significant energy is transmitted, although it is attenuated by 20dB or so. To find out how much power is transmitted towards a particular area (say between -1 degree and +1 degree), you would calculate the area under this region and compare it to the total area on the graph.

Everything works exactly the same in reverse when the antenna is receiving radiation. Although it will receive most of the energy along the optical axis, it will not receive it all because it must also couple in the energy coming from all those pesky side lobes. In theory, only about 84% of the energy along the optical axis can be coupled by the reflector. This is known as the theoretical maximum effieciency of a reflector.

In real life, a few problems arise. The above analysis assumes an infinite paraboloid surface where surface currents are not hindered by the edge of the reflector (diffraction) or the feed assembly and struts. In addition, perfect illumination is assumed over the entire surface of the reflector. Since none of this is possible, you end up with significant sidelobes (@ 90 degrees) and backlobes (@ 180 degrees) in the chart below.

The net result from all this is that most of you have tvros that are only 50-60% efficient (compared to the theoretical of 84%). The best designed reflectors with cassegrain or gregorian feeds can achieve nearly 80% efficiencies (used by astronomers). A properly designed, corrugated conical feed at prime focus should nudge us up to 77%. :chaplin



radiation_plot.jpg
radiation_plot.jpg (31.88 KiB) Viewed 15358 times
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Re: New LNB or stick with the old

Post by DirtyHarry » Tue Aug 09, 2016 5:53 pm

The lnb not being tested is the only reason for low noise numbers. China has been selling the same lnb for years,,,,,only changing the numbers and cases to fool people,,,,,, first .6db,,,,,then.3db,,,,,,now.1db which is not possible!
Older lnbs from 30+ years ago are still working on many dishes. No need to change lnb if it still works. Older lnbs were built better,,,,,,tis the reason they still work today.

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Re: New LNB or stick with the old

Post by elko » Tue Aug 09, 2016 8:03 pm

You can always check the manufacturer's datasheet for specs on the low-noise amplifier. I have seen figures as low as 0.5dB for the 3-6 GHz range.

See Macom

https://www.macom.com/LNA

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Re: New LNB or stick with the old

Post by elko » Tue Aug 09, 2016 8:11 pm

It should be possible to get even lower noise figures with very narrow band amplifiers. This is what foxcreek was trying to do with his BPF centered around 3980 MHz. :biggrin

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