Posts by G0MJW

    No - that look OK. But need to see the next part about multicast really.


    Can I check you are using this software ? https://github.com/F5OEO/dvbsdr and you are running ./jeston_nano.sh from the scrips directory and not another script?


    Also you have:


    #Could be VIDEOSOURCE_PICAMERA, VIDEOSOURCE_USB_CAM , VIDEOSOURCE_IP

    VIDEOSOURCE=VIDEOSOURCE_IP

    VIDEOSOURCE_IP_ADRESS=239.255.42.42

    VIDEOSOURCE_IP_PORT=5004




    Mike

    Claudio I was asked that question before and I had thought I had made it clear that I do not measure the phase noise directly. I measure it by its effect. I am sorry but it is all I can manage at the moment without access to an RF test lab. There is a well known relationship between the phase noise and the error rate but there is also white noise present. To a very rough, no warranty given or implied, usual disclaimers apply, approximation you can assume the effect of phase noise on a QPSK demodulator is just like that of amplitude noise. This is so rough and wrong that many RF engineers will be at this point shouting "YOU CAN'T DO THAT". Which to be fair is true but the approximation close enough for telling if something is adequate, bad or really bad. The constellation is also useful as if the points look randomly distributed around the ideal phase noise probably is not the problem but if they are in arcs around the points it probably is.


         


    However, you can measure Phase noise directly by analysing the baseband spectrum of a pure carrier that the LNB has down-converted to IF. The problem is you end up measuring the weakest link, and from the satellite, the signals are just not strong enough to get the dynamic range needed to differentiate between the oscillator phase noise and the white noise (unless the LNB is really bad). Therefore a local very pure reference signal is needed, e.g. a good crystal oscillator multiplied up to 10.5 GHz. Even then, with an IF around 1 GHz an SDR is probably not good enough so this IF would need to be mixed down to baseband with another good oscillator. Then look at the output amplitude and phase with a direct sampling RX like a CloudSDR, SDRIQ etc. Spectravue would be a good tool I think.


    This is complicated and needs careful calibration and attention to detail to make the measurement setup as clean as possible.


    Fortunately, there is an easier way to infer the phase noise degradation, measure it and minimise it. Basically, if the MER does not meet the expectations of the SNR, the difference is probably the phase noise.

    G0MJW Mike, changing the LO to 9.5 GHz results in a higher IF, lying more into the spec band width of the chip. Couldn't that be the 1.6 dB MER difference ?

    No I don't think so as I also found other things that worked, even just a few Hz from 25 MHz made a difference, which makes me suspect it is related the Leo Bodnar reference division ratios. I also tried 24 MHz but that was not very good. The PLL multiple is 390 for 9.75 GHz (424 for 10.6 GHz) so if you wanted a 430MHz If you would want about 25.79MHz. I tried 23.59 MHz which puts the transponder on 23cms but it wasn't very good. Best for me seems to be 9.5 GHz.


    Other LNBs may be and probably are different.


    Mike

    dg0opk That's very helpful Michael - thanks. My experience (August 2019) is that Digikey are supplying the D75F (25MHz and 27MHz) with date codes 2018 and onwards. So, that supplier seems to be ok now. 73, Russ.

    Too late now - Connor Winfield have lost my trust and I do not recommend them. As soon as the company knew bad chips had made it into the supply chain they should have immediately withdrawn them and initiated a recall. Instead it was left to users to find out and complain and seek redress individually.


    Meanwhile, there are other possibilities but I think in the long run it is better to have a stable low phase noise source in the shack than out on the LNB.


    Phase noise is not a single number and it's impact depends on the bandwidth of the signal. You can have oscillators that sound bad in CW but work well with DATV and ones that sound excellent in CW but are near useless for DATV. An approximation to the performance is an integration over the signal bandwidth, so close in phase noise is very important at low bandwidths and far out phase noise increasingly important at wide bandwidths. A good test is being able to receive both 33ks and 27500ks with an MER that is realistic with respect to the SNR.


    I was using a modified Octagon LNB on my 1.2m dish and it's performance on Badir BBC was several dB worse than an unmodified identical LNB. That was driven by 25MHz from a Leo Bodnar GPS locked source. It turned out the phase nose was dependent on the precise reference frequency, presumably due to the combined reference phase noise and PLL phase noise. With a 25 MHz reference, this LNB could only manage an MER of 9dB on the beacon with my 2.4m dish. Changing the reference to give a 9.5 GHz LO improved the MER to 10.6 dB. I am sure each case if different but it's worth spending some time tuning. It might be best with a good quality 100MHz Ovanised reference divided by 4. Such references are often available in otherwise obsolete test equipment and because they are often 30 years+ old, their stability is excellent.


    If using the single cable solution with the reference injector board mentioned above, adding a low pass filter to the reference is also a good idea, especially if you are using a low IF.


    Mike

    Yes - except it depends on the way the antenna is designed. Isoflux is a concept of a radiation pattern, not a single antenna. The designs I have seen are all for satellites but there is no reason at all why one can't be designed one with a lower radiation angle, except it usually needs simulation software that's not available at amateur prices.


    There is also the question of why circular polarisation is required. If the satellite is circular that's understandable but quite possibly it's linear but of unknown polarisation angle so the axial ratio isn't as important.


    SDR receivers are cheap so it's sometimes worth just having several antennas looking in different directions each driving it's own receiver. I have done this in Gnuradio for other applications.


    The QFH you mentioned is a simple to make antenna and this is a nice summary: http://orbanmicrowave.com/wp-c…drifilarHelixAntennas.pdf


    Mike

    Ideally yes, practically no. The isoflux antenna is designed to provide a uniform flux on the visible earth beneath a satellite taking account of distance. Putting one on the earth would give the characteristics asked about - well almost as they don't radiate towards the horizon but from about 20 degrees and up. The concept remains and not seeing the horizon is often good for noise rejection.


    This https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6239565 is a nice paper and proposing a meta-surface made from small patches. This sort of thing is perhaps too complex for amateur use, but it's something that could be made for satellites.


    A long quadrifilar helix antenna is possibly a good option.


    Mike

    It's a sensible rule - there are many rules that can't be enforced. For example you are not supposed to transmit more than the license allows yet you can buy a 1kW amplifier. You are not allowed to exceed 70mph in the UK but nearly every car can exceed this. The responsibility is put on citizens to comply with the regulations, with penalties for infringements.


    There are other users at 2.4 GHz, including some amateur satellites and if we are lucky one day we might get another transponder. Then it will matter how tight our beams are.


    Mike

    How can I make a lossless connection between SMA and N ??? In first step it is for indoor through an open window.

    Tnx

    You can't, that would violate physics but you can make a very low loss connection with a quality SMA-N adapter and a slightly higher loss with coax. As long as it's short the loss will be negligible.


    I am making an assumption this antenna is for QO-100 TX? Bear in Mind the power handling won't be that high. Quite likely to be high enough for a 10W SSB uplink though.


    Please let us know how well it works.


    Mike

    I found the phase noise was poor on my Octagon quad with a 24 MHz reference (normally 25MHz) so I investigated good frequencies. Setting a 9.5 GHz LO with a 24.35897436 MHz reference worked well. The maths is easier too. Now I can use an Octagon SF8008 to receive DATV as the IF is above 950 MHz.


    Mike