Posts by G0MJW

    Thanks Rasto


    That's really interesting. I will see if I can get this to work within the limits of the student edition. I did try a similar approach earlier but it was not successful as it tended to be more suited to a lower F/D. However, this was with a circular patch at 6mm spacing and before I realised how to get CP so maybe. I will look again. I looked at that book myself and concluded the same and also that the small squint probably didn't matter too much.


    It is more complex to make, but not excessively so and also maybe a retro-fit - someone (not me) needs to compare and contrast perhaps. If it turns out to be a worthwhile improvement, we can call it the "Patch Of Next Year" or PONY for short and sell it for £25. (only Brits of a certain age will get this joke)


    If you can tell me the dimensions, it would help.


    Enjoy Doha.


    Mike

    Interesting - I am not sure if your conclusion is the antenna is good or bad.


    Remember I do not have access to the fine mesh optimisation of CST and had to do this in the frequency domain under rather limited mesh size, so getting even close is not a bad result. I am not an expert in antenna design. This was developed from scratch over the course of a week over Christmas, with no previous experience of CST or any other simulator. We did a little tweaking later on but it is pretty much the first attempt. I did not expect so many people to copy it.


    Some observations on your comments:


    An efficiency of just over 50%, with all the losses included is not a bad result. Especially considering this is a dual band antenna with a thumping great hole in the middle.


    I think we only claimed 18 dB return loss so not that far out. I did not have the capability to include the connector in the simulation, the mesh limits again...


    The axial ratio, 3,6 dB is not so bad given the compromise of a single feed point and all the limitations in my simulation. Perhaps even a bit better than expected. If it is properly tuned, it might improve. I noted this parameter varies strongly with frequency. Do you see the loop if you switch to the frequency domain?


    Squint - yes, I saw that too but it wasn't too serious, well maybe with a very large dish. A penalty of the feed arrangement. I was more worried about overspill loss at the time.


    I aimed to get the phase centres close but was not too concerned about S-band for the reasons you set out. It is 4 times less critical and 2.4 GHz power is not so hard to come by. All this was done last year, well before the satellite opened to use so we had no idea it was so sensitive.


    The question is now, can you do better with access to the full software? Are there better dimensions we can aim for?


    Mike

    Hi Mike,

    It will be published in part 2 (like a soap opera :)). I am working on it.


    73, Rasto

    Looking forward to it.


    It is a very simple design and the circularity will not be perfect - I expect it to be worse that some other designs I have seen, but there is a trade off between simplicity and performance. The feed is easy to make but comes at a cost in bandwidth, illumination, return loss and axial ratio.


    A better way would be a horn with four (two) feedpoints which address all the above, but that adds complexity. There was a Czech design like that published recently. It looks great but is much more complex mechanically.


    Axial Ratio - If you are trying to get poarisation re-use as if often the case, the cross polarisation discrimination is very important. If you are not, it is less important. As long as it's good enough not to lose too many dB it will do.


    Power - the patch has a power limit, for EME powers, it would probably be unsuitable. This is due to the small spacing and high fields at the edges. This is not likely to be an issue for QO-100 as that much power is too much.


    Illumination - the patch over illuminates common dishes and the illumination across the surface is not as good as it could be. This leads to sidelobes and spillover loss. Sidelobes could cause interference to adjacent satellites and on receive, there will be more noise. As it is only used on transmit and there are no adjacent satellites in practice it is only an efficiency loss.


    I am not sure all what this means in practice, but as long as the overall efficiency is over 50% I am happy. IF it turns out worse than a dipole, I will get my coat.


    Mike

    I assumed everyone knew this but perhaps I am wrong. A lens has a different refractive index to air so it bends electromagnetic radiation, thereby shaping the beam. https://en.wikipedia.org/wiki/Lens_(optics)


    In my dual band feed it narrows the beamwidth compared to the open waveguide. It also transforms the impedance to provide a better match - the rear of the lens inside the waveguide is about matching, the bit outside is doing the beam shaping.. The narrower beam is needed to better match an antenna with a longer focal length. The open waveguide would be OK for a F/D of 0.3 - a deep dish, but with a typical satellite dish the F/D is 0.6 so much of the radiation would miss the reflector. This both reduces gain and picks up more ground noise.


    Lenses can also be used as antennas, the lens horn for example is acting just like a magnifying glass focusing the sun.


    This Twitter post shows it well https://twitter.com/HHolsink/status/1131596061703909376


    Mike

    This is great - I do not have access to the full CST so the design was difficult to do in the student version. I did my best but the limitations mean there are likely to be small errors which will show up in matching. The performance has not been tested - we are amateurs and do not have access to test facilities. All we can do is compare against other antennas and that's not easy to do well.


    The nylon lens is not part of my design, I proposed a rocket LNB lens, these are designed for the job and I find them very good considering what they are. The biconical nylon lens is proposed by Remco, PA3FYM, I did what I could to optimise but that was rather limited due to the constraints of the simulator. Time domain can't be used at all and the mesh size has to be constrained. This lens was for those who cannot get hold of one of these rocket lens and always described as a work in progress. Your simulation result is very encouraging!


    However, since then Willi HB9PZK has come up with a new and better lens design in Rexolite and it looks good. He has access to CST and has also done an analysis of the design. Interesting to compare against your full simulation.


    By the way, it had to be simple and reproducible with hand tools. If you have access to the full CST, perhaps you can improve on it?


    Have you looked at the S-band performance?


    Mike G0MJW

    If you want a weak source the 1N4148 is fine. A more exotic diode will be better, but it's not necessary at 10 GHz which is a relatively low frequency these days. Don't feed it with a Baofeng, you won't know which spurii is which.


    For a strong source, find a packaged mixer with a highish LO/RF/IF capability, split the source and feed it into the IF and LO ports at high level but being careful not to exceed the ratings. E.g. 1296 MHz x8 gives 10368 MHz. Seems to work.


    Mike

    I'm afraid you can not create a general receive list.
    My experiments have shown that the receiver always evaluates the service name in the SI tables. Only when it matches can a stored station be recalled repeatedly ...


    any other experiences?


    73 de Robert

    I am not so sure - It would be good to have a channel list to import. I have come back to find new testcards on my screen without my tuning them, same modulation and PIDs.


    I am quite impressed by this little RX for under £100. It may be hackable, I am using the standard firmware supplied, OPEN ATV 6.2. So far I have received signals at 250k, 333k, 1Ms and 2Ms and I haven't even taken the lid off.


    One of these streaming to the web tuned to one of the RB channels might be useful reverse beacon. E.g. The upper channel and RXs set to 333K or 250K. Perhaps with some software the webserver could detect the rate and push the appropriate virtual button via the network interface.


    My main problem is going to be the YAM requirement, (Yet Another Monitor). I will be looking for a small, inexpensive HD monitor.


    Mike

    Hi Ole


    I see - I thought you had the dish. I would think it is more likely to be related to the matching than losses in the dielectric. Let us know how you get on with the dish.


    Mike

    Ole this is very useful information What is the F/D of your dish - I am surprised that no lens in better than with lens. This would be true for a dish with an F/D under 0.4 ish, but not for a longer focal length.


    It's also not a lot to lose but nice if we can get it back with a better matched lens.


    Mike


    Mike

    Just to be clear - I was joking about the polishing - however over time brass and copper tarnish and this might impact losses a little, so it's worth taking steps to protect from the weather. A light coat of clear lacquer is an alternative. It won't impact performance though it might move the match a little.


    Let us know haw you get on.


    Mike

    Kai,


    I think it is more likely the problem is RF on the 2.4 GHz coax shield getting into the LNB coax because of the poor match. Have you got the two cables close together?


    Mike

    It is only possible to get interference if there is a strong harmonic content - 2.4 GHz signal will not propagate down the waveguide. The 3mm spacing is critical, going to 1.5mm does not work so something is wrong with the patch. You should see 16-17 dB return loss.



    Edit - I just looked at the photo. The patch is built incorrectly. The feed is in the wrong place.

    Experiments with helix element diameter


    The 10GHz element of the feed is a 22mm diameter water pipe squeezed in a vice to fit inside a WG16 (WR90) flange (to connect to my DU3BC preamp).

    I used a MK1 Birmingham screwdriver to get 22mm water pipe into a WG16 flange for a dual mode 10 GHz feed. It worked very well. Your dish is a bit shallow for the patch but it would be interesting to know if it impacts the 10 GHz at all. I think it might a little but nowhere near as much as a helix.

    For SSB Class AB is quite sufficient. No need for nearly Class A.

    I agree but that is not an answer that is universal. SSB is not the only mode being used via OQ-100 so it depends. If you are doing DATV, which is where you need the larger powers this amplifier can generate, you might need more bias, or not, there is a sweet spot between current and spectral regrowth.