Smallest Dish Size

  • I have long wondered just how small a dish we might be able to go to (one of my other interests is in Emergency Comms)


    Since others have tried 50 and 60cm dishes, I thought that since I had a small 45cmx35cm dish available, I would see if I could receive the 11205 V Beacon (10706 I have numerous issues with)


    A back to back comparison with my 1.1M dish was carried out. Pointing may not have been identical, so take these figures for what they are - A Quick test on a Sunday Afternoon.


    That said, On my 1.1M dish I was receiving the beacon with 26dB S/N. The 45x35cm only received the beacon with a 10dB S/N margin, 16dB down on the 1.1M dish


    I've also been having limited success with my FCDPP and RTL dongles (A Mixture of gain settings and potentially frequency selection etc, however on my 1.1M dish the FCDPP (serial way higher than Peter's) I was receiving the 11205 beacon with an 18dB margin. My RTL dongle was far worse at just 6dB margin. It should be noted that there was a 2 way F connector splitter in line, so add at least 3dB to those margins (Probably more).


    I'm yet to get my Pluto to hear the beacon at all, although that is the next test. With the 16dB loss between the 45cm dish and 1.1M dish, 45cm may not be entirely out of the equation, for narrow band work, although of course we need to take into consideration that the 11205 beacon will be nothing like our transponders



    73s


    Iain

  • Hello Iain,


    it's a bit off topic - but do you really have to care about a few adapters or the cable length ? The LNB converts the 10GHz Signal to the IF and has here a very high signal level. In "normal" conditions (e.g up to 20m coax and a few adapters) the total signal (even the noise) drops, but in these conditions you do not really have a change in S/N. Or am I wrong ?


    If someone knows better then I appreciate every comment - please with explanation. (I'm a physician and not a HF-engeneer :-) ).


    73 de Johannes

  • I agree with Johannes. My 75 Ohm cables goes first from the LNB to the DisEQ antenna rotator, 3 x F connectors, than through several connectors and 25m of 50 Ohm cable to my radio shack.. without any 75 to 50 Ohm matching =O.. doesn't matter.. the LNB has more than 30dB gain and I see a huge noise floor when turning the LNB power On/Off.. :) as long as you can see the noise floor.. nothing to worry about S/N ratio...:thumbup:

    Peter Gülzow | DB2OS | AMSAT-DL President | member since 1983 | JO42VG

  • That is indeed the case. I have here 40m Coax, a 4-way splitter with 12 dB attenuation. In addition, then an attenuator with 20 dB is required to have the signal for a SSB receiver in the correct level ... :-) The fat 65 dB gain from the LNB must first be adjusted ...

  • More important than the attenuation of the coax is the quality of the shielding!

    There are usually other services on the frequencies used for IF in the system, and you may get interference from that.

    E.g. when using normal satellite TV there is interference from "cordless telephone" DECT devices. When you put the base station of the phone near the settopbox or the cable towards it, you may have interference on some channels.

    When using an unmodified LNB (9750 MHz LO) the narrowband transponder IF will be 739.xxx MHz. This used to be in the UHF TV band (channel 54) but this band is being repurposed as a 5G mobile band. So there may be very strong signals or they could appear later.

    So make sure you used good quality coax with foil+braid shielding and apply the connectors in the correct way.


    W.r.t. the dish size: we don't know yet, as our objective is not to receive the beacon but to receive the amateur transponder. When it was on for testing, it was very strong. But we would have to see that in operation.

    Also, now that the satellite is in position, the situation is worse then when it was at 24e. There is noise from other satellites at the 10.706 beacon. Similarly this could affect the amateur transponder.

    So it may be that you require 60cm or 80cm just for the directivity, not as much for the gain. To attenuate the signal from Astra3 at 28.2e.

    This depends on your location. In the UK the situation is of course much worse than in southern Europe.


  • One possible reason for the lower than expected signal with your smaller dish may be a higher than expected background noise from transponders on nearby satellites. Probably not worth making too many conclusions yet - best wait for 'our' transponder and see how that pans out.





    73





    Charlie

  • Has anyone already measured the broadband nature of the LNB? The ZF of 739 MHz is already clearly out of specification ... Is it possible that the transponder signals then turn out to be significantly weaker than the beacon on 10706?


    73 Robert

  • DD4YR : Not measured it tbh, but was able to receive signals well when 'our' transponder was tested on 23rd December. Unfortunately I didn't record signal levels.


    Can confirm 10706 is the 4G/Mobile phone issue for me (and switching to 10600 LO drops the frequency below what my SBX can do)


    G3WDG : Totally agree we need to wait to draw final conclusions, I was just comparing delta of the signals received on each dish, and only quoted the margin. Noise with the 45cm dish was -121dB, signal at -111dB. The "Big Dish" [which I haven't named Arthur - Yet!] had noise floor at -116dB and signal at -90dB


    And Johannes and Peter are of course correct with regards to the splitter's impact, I was overly thinking things with the splitter inline :)


    The Pluto has now seen signals on the 1.1M dish, with -90dB noise floor, and -80dB signal. Interestingly, the FCDPP is now showing -88dB noise floor, and -71dB signal on the 1.1M. But then again, the night has come, and it has got very windy here (which means the small dish has been blown off the garden table, being only table top at present!)


    All figures are as shown by gnuradio QT Frequency sink and are relative...

  • DD4YR wrote: "Is it possible that the transponder signals then turn out to be significantly weaker than the beacon on 10706?"


    While receiving the tests on (iirc) December 23th last year, I saw the transponder noise around 5 - 10 dB above the 'normal' noise with my experimental setup (and

    at 739 MHz IF)


    Taking the transponder noise as an emperical starting point, I reckon the sensitivity of my receiving chain might be sufficient. So, although the LNB works out of spec I think receiving the (NB) transponder might not be an issue.

  • DJ7AL

    IK0EQJ

    thanks for the info.


    Then it probably makes sens to reduce the PLL crystal frequeny so far that the transponder (10,5 GHz) is converted to 1200 MHz?

    It is done easily as well as increasing the frequency OL on 10058 Ghz obtain IF 432 Mhz. Or reduce the OL frequency around 9194 to get a IF 1296 Mhz. So with due patience is a good instrumentation can be done.


    73

  • As today is raining and I can´t realign&fix my 80cm dish in the roof, I decided to test an old camping suitcase kit with a "plastic" 35cm antenna .Using a Darko (OE7DBH) modified twin LNB Octagon... a cheap chinese RTL-SDR New Gen TXCO 0.5ppm (green).Surprised when 10706 signal was on the display ..also the 11205 a bit stronger.

    Would like to test again in a future when NB is on and/or non-amateur 11205 -10706 are stronger.

    73,s Antonio


    s