Is there any point in using a large dish on RX?

  • Hi all,

    I am currently using a TV sat 80 cm dish, an unmodified LNB and an RTL SDR.

    When monitoring the NB transponder in SSB, SDR Console shows a noise floor at about - 90 dBm and the upper beacon some 30 dB above the noise floor.

    Is there any point in using a larger dish?

    Signals will certainly be stronger, but so will the noise floor too, with the s/n ratio still at 30 dB..


    What do you think?


    73s

  • HB9SLV Try if you can distinguish the tpx noisefloor and your system noise floor with a wider span (2 MHz or so).


    If you can distinguish, how much is it?

    Here is what SDR console says about the Noise floor:

    On 10.495 GHz: -94 to -97 dBm

    On 10.489 GHz: -94 to -96 dBm

    On 10.483 GHz: -95 to -97 dBm


    The high frequency beacon : -73 to -78 dBm.


    I think I got the point. The noise floor I am seeing is the same within or outside of the tpx band. This means that this noise floor that I am seeing is produced by my LNB.

    With a larger dish, the noise floor will remain at the same value but the beacon will be stronger.


    Does someone know what the s/n ratio is at the output of the transponder? I started thinking about this when I read on this forum that a few months ago, it was decided to lower the noise floor at the output of the transponder to save energy on the sat.

  • Guys,

    the noise figure of the receiver (system noise) is defined only by the LNB's noise not by the gain. The dish must be large enough to see the noise of the transponder. This can easily be checked by turning the dish to the cold sky. If the noise increases when pointing the dish back to the satellite, all is fine. Reducing the transponder gain requires larger receiving antennas but gives more dynamic range.


    @HB9LSV, the signal to noise ratio at the transponder output depends on your uplink power.
    It is helpful to operate duplex and watch the output spectrum.


    Take care not to overdrive the receiver behind the LNB. Use an attenuator. No matter, where the absolut level of the received signals is. The new S-Meter of the 3.0.13-release of the SDR-Console can show signal to noise ratios which allows useful reports.
    73s
    Andreas

  • @HB9LSV, the signal to noise ratio at the transponder output depends on your uplink power.

    Hello Andreas and thanks for your answer.

    You are right, of course. What I should have asked is : " what is the signal to noise ratio of the high frequency beacon?"

    Because if it is 20 dB at the satellite, as I see it here with my system, there is no hope I can make it better with a larger dish.


    But if a station sends a weak signal to the satellite, let's say 100 mW and a 60 cm dish.

    The transponder will output it with a small signal/noire ratio. Let's say 5 dB,

    Will I be able to decode this better with a larger dish?


    /3s

    • Official Post

    But if a station sends a weak signal to the satellite, let's say 100 mW and a 60 cm dish.

    The transponder will output it with a small signal/noire ratio. Let's say 5 dB,

    Will I be able to decode this better with a larger dish?

    no because the received signal will be uplink-limited and not downlink-limited.. Nothing you can improve here even with EME-type antenna on downlink.

  • I'm glad I came here to ask this question.

    I tried asking the same question on the Facebook group and someone answered that he had tried one 60 cm dish, an 80 cm one, a 1 meter and a 1.5 meter dishes and the bigger the dish, the better.

    Thanks again!

    Vy 73

    • Official Post

    The key phrase is here:

    for NB, a 60cm dish with optimal alignment and suitable feed

    we have seen many instances where a botched setup (bad feed, misalignment, ...) lead to bad performance which people believe that a big dish is required. Or even worse, they complain about downlink not working and just use any WebSDR.

    On the other hand (and this pleases me a lot!) many OMs recognize that improvements are possible and tweak the living daylights out of their system. That's what hamradio is all about.

  • Hallo,

    to say it more clearly, don't care about the beacons and the user signals to design a receiving system.
    Take care about getting the transponder noise above the system noise of the LNB.
    This is the only criteria.

    By the way, it is important to have the right radiation pattern of the LNB corresponding to the
    f/D-ratio of the dish. Commercial LNBs and dishes should work. Adding a patch to a LNB the feed is no longer a horn but an open waveguide. With a rocket-lense to my open tube nothing changed.
    This confuses me a bit. Further research has to be done.

    73s
    Andreas

  • Maybe the benefit of a better dish illumination was canceled by signal absorption within the lens.

    I was planning to carve a lens to be fitted in front of my LNB, to improve efficiency. The material I was planning to use is wax. Candle wax. Wax being a good dielectric, losses should be low, I hoped.


    So I melted a few candles into a cylinder slab 2 cm thick. The first thing I measured was the attenuation brought by 2 cm of wax in front of the LNB. I measured 4 dB with SDR console. This is about what I was expecting to gain with a better dish illumination.

    Needless to say, I shelved the project.


    A better way ( no absorption losses) should be a secondary reflector. There was such a project in a recent DUBUS issue.

  • See my other post - a larger dish will almost always be better, it is just a matter of how much. The signal to noise ratio is a combination of the uplink and downlink SNR, it can never be greater than the worse SNR. To give you a practical example, the SNR on the wideband DATV beacon here is ~10.5 dB with a 2m dish and not very good LNB. With a 1.2m dish, with 6 dB less gain, it is ~9 dB. With a 30m dish with 22dB more gain (yes, 22 dB - think about that) it is ~11.5 dB.


    This is very old now from 2007 and perhaps over-simplified as it was for Msc students but http://www.mike-willis.com/Tutorial/PF13.htm


    I should address the question directly - for QO100 the law of diminished returns on receive applies. The benefit of a larger dish falls quickly above 1.2m and there is little point going above 3m. This does not apply to TX where a larger dish will always be better - as long as you can aim it accurately.


    Mike

  • Adding a patch to a LNB the feed is no longer a horn but an open waveguide. With a rocket-lense to my open tube nothing changed.

    This confuses me a bit. Further research has to be done.

    This depends on the f/d of the dish. Assuming a standard offset dish with 0.5 or 0.6 f/d the recket lens should make a difference but will move the focal plane of the feed forward a few cm. You may have to move the feed back a little to adjust for this. With a prime focus dish the f/d is lower and the open waveguide will be better. What is your dish?