LNB <----> RX Level adjustment

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    Vy 73 de OE7DBH

    Darko

    • Official Post

    Ich empfehle hier auch gerne die Friis-Formel:

    https://de.wikipedia.org/wiki/Friis-Formel


    Würde aber trotzdem empfehlen das Dämpfungslied nicht hinter dem LNA, sondern hinter dem Konverter bzw. direkt vor den RX einzufügen... auch nicht durch das S-Meter oder Audio täuschen lassen, es geht um's SNR. Die Zunahme des Grundrauschens sollte man immer noch spürbar vernehmen können..


    Gruß Peter

    • Official Post

    Die Dämpfung des ATTenuator im TS-711E ist 20db. Da konnte ich am S-Meter bei Empfangstests/QSO's feststellen, dass 20dB Dämpfung zu viel sind, die Signale wurden deutlich leiser und QRP-Stationen unverständlicher. Mit dem 10dB-Dämpfungsglied, wie oben abgebildet, zwischen RX und Konverter (1. Version von AMSAT-DL) bin ich bestens bedient. Natürlich spielen auch der LNB und die Länge des Sat-Kabels eine Rolle. Bei mir sind es 20 Meter.


    Darko gab einen guten Denkanstoss, aber bei jeder Anlage verhält es sich anders.

  • Hallo,

    um Übersteuerungen grundsätzlich zu vermeiden, wäre auch ein Minimalsystem gut :)


    Wie man sieht, ist der LNB (mit Quarz, kein TCXO) ca. 100 KHz daneben. Die Bake ist gut sicht- und hörbar. Für Telegrafie würde es reichen. Mit dem nicht ganz ernst gemeinten Foto gestatte ich mir nochmals darauf hinzuweisen, Rapporte mit absoluten Pegelangaben sind nicht sinnvoll, es geht immer nur um die Differenz zum Rauschsockel des Umsetzers, welcher hinreichend über dem Eigenrauschen des Empfängers (LNB) sein sollte. Das wurde schon oft dargestellt, ist aber immer wieder in fälschlicher Form zu hören.

    73
    Andreas

  • Hallo Achim,
    thanks, this is an interesting idea. In my courtyard there is much noise. Turning a dish away from the Satellite, noise changes unexpected. A house and trees and so on. Guess, I don't have cold sky conditions. May be this is the reason, all trials to receive the beacon DL0SHF went wrong. With a piece of metal it's no longer handsome stuff :-(.
    But always good for further tests and playing around.
    By the way, the receiver as a chinese clone (I and so many others believed there is an agreement between the russian developers and the chinese manufacturers) didn't work at all at higher frequencies. The reason was a bad clock oscillator. The heart of the radio is a chip from MIRICS as a wideband receiver. At least it is good to check LNBs and adjust dishes.
    73
    Andreas

  • Würde aber trotzdem empfehlen das Dämpfungslied nicht hinter dem LNA, sondern hinter dem Konverter bzw. direkt vor den RX einzufügen...

    Wouldn't this cause unnecessary reciprocal mixing between the noise floor and the LO of the converter? The LNB defines the noise figure, and any attenuation in an IF frequency should not matter unless the attenuation is so high that the noise floor cannot be detected anymore..

  • Hallo,

    from my little important point of view one should take care not to overdrive the converters mixer. There are LNBs with different gain. But it is much to much so the resulting noise figure of the receiver as a whole is defined by the LNB always. The required dynamic range is small. That means, the gain of the receiver chain can be adjusted to see the transponder noise clearly at the lower edge and not to overload the receiver (mixers are sophisticated always) at the higher edge. In the above example I had to disable the preamp in the Malahit to see the small peak of the beacon. Finally, a lot of things are possible right behind the LNBs, long cables, IF-filtering, attenuation and so on.
    For testing, switch the LNBs feeding voltage on and off. The noise floor changes rapidly.
    The complete rx-chain including the dish has to be adjusted to receive the transponder noise. It was discussed ealier, it can be helpful to use a wide range spectrum window watching the "noise mountain" transmitted by the satellite beside the cold sky. With nearly two years of experience I can say, use a RTL-Stick V3 after the LNB and SDR# to mount or to check the dish. With a transceivers analog S-Meter it is more difficult to do. With 160 KHz max. the spectrum window of the little Malahit isn't wide enough.
    Of course all this well known and I am repeating basics. But probably it can be helpful for beginners.
    73
    Andreas

  • Hallo,

    88 Dollars for a little box and some cheap components? Is it hamlike still?
    It is very easy to assemble a bias feed, a choke and a capacitor. If an attenuator is necessary use a cheap one from 75 Ohm tv cable systems. Max. attenuation is 20 dB. In many cases it is possible to operate LNBs with less then 12V. This reduces dissipation and increases stability.
    As I remember, any kind of advertisement is unwanted here.

    By the way, this is a little current protection circuit. When experimenting and feeding LNBs with supply voltage and connect and disconnect the F-plugs shortcuts can happen. To avoid smoke, the circuit switches of, if a specific current is reached. R3 opens Q1and the FF is set. Q gets high and the FET is off. An additinal feature is, the LNB can be switched on and off to check the increase of the noise. D2 limits output spikes.

    Happy Easter

    73
    Andreas

  • Hallo,

    an overcurrent protection is well suitable for any kind of experiments. PTCs do not protect in case of just overriding a specific limit. They get hot but don't switch off. This is good for hard short cuts only, assuming the power supply can deliver enough current.
    Playing around with sophisticated electronics a switch can save live. When developing circuits, measuring and soldering it is nessecary to disconnect the feed voltage even with every change. Probably a fast switch beside the test object can help.

    73
    Andreas

    • Official Post

    Mit einem Relais aus der Autobranche und einem Widerstand schütze ich mein Rig vor Kurzschluss, Verpolung und Überspannung. Tritt einer dieser Fälle ein, unterbricht das Relais die 13.8V vom Netzgerät.


    Nun weiss ich aber nicht, was dies noch alles mit dem Titel dieses Threads zu tun hat ?

  • DL5CN: you wrote "When experimenting and feeding LNBs with supply voltage and connect and disconnect the F-plugs shortcuts can happen"


    ... and that is exactly a "hard short" you are writing about in your next post.


    So why not use a polyfuse here? For protecting the LNB/Supply it is a easy/reliable/cheap solution.


    For other applications (e.g. settable Itrip&CC / OVP) a polyswitch is clearly not the best option (without additional electronics) .


    P.S. sorry to @oe7dbh for going a little bit offtopic here :)


    73s..