Computer analysis of dual band dish feed (G0MJW, PA3FYM, M0EYT). Part 1 –X Band

  • In the latest DUBUS magazine (2/2019), I found a description of the dual-band feed for an offset reflector antenna, published by Mike Willis – G0MJW, Remco den Besten – PA3FYM and Paul Marsh-M0EYT. Since I am generally interested in reflector antenna and authors have not published more information about the performance of their antenna design, I performed a computer analysis of this feed. Maybe some members of this forum are also interested in a performance of this feed, so I am sharing my calculated results here.

    Thanks to RF spin s.r.o. company, I was able to use CST MW Studio software for modeling and calculation. For feed performance calculation, I used time domain solver approach. For the whole antenna performance calculation, I- solver was applied. The feed radiation patterns were calculated and generated in 5 deg. increments of angles Phi and Theta and inserted into a focus of the parabolic reflector as a far field, shifted with amount of the phase center position (23.6 mm). Subsequently, next calculation was done in I-solver For directivity calculation of whole antenna assembly, I used a model of prime focus parabolic reflector, since geometry of offset reflectors are not unified. The expected error with this simplification should be smaller than few percent.

    Conclusion for X band


    -         Simple design

    -         Symmetrical radiation patterns for both E and H planes - good efficiency for dishes with f/D ratio of about 0.5 – 0.65

    -         Good impedance match


    - Worsening of LNB noise figure (0.2 – 0.3 dB), due to use of lossy dielectric material (Nylon 6, dissipation factor 0.2)

    - None UV resistance

    - Difficult adjustment in offset parabolic reflectors for the best performance

  • 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

  • A75GR Rasto, thanks for the review.

    Like Mike said, Willi HB9PZK also had a positive second opinion.

    Concerning the 'cons', it all matters how you look at it.

    a. Lens: at the time of writing the article (I believe early February) 'amateur lens science' was not as developed as nowadays ; -) I now use another design which performs as good as my 'Rocket LNB' lens. Note: RL (S11) seems not the most important thing. See picture below, picture taken bij PE1CKK.

    b. UV resistance: ?? (why didn't you took 'colour' or 'shininess' as performance parameter? ; -)

    c. Phase point: in practice it's very easy to find the phase point (focal point) with (offset) dishes. Just tweak on engineering beacons, like millions of satellite viewers do. As a result the S-band part is also in focus (is 4x less critical).

    I am curious to watch the 2nd episode of the opera : -)

  • To Remco -PA3FYM,

    a. I modelled and calculated feed, as it was published in DUBUS magazine. I do not have any upgrade version of your design. I wrote about it in upper section.

    b. Properties of Nylon 6 from Internet:

    Nylon is hygroscopic, meaning that it attracts and absorbs water from the surrounding environment. The composition of the plastic is changed as water molecules are suspended between the molecules of the material. Over time Nylon 6 will swell considerably and begin to degrade.Nylon is hygroscopic, meaning that it attracts and absorbs water from the surrounding environment. The composition of the plastic is changed as water molecules are suspended between the molecules of the material. Over time Nylon 6 will swell considerably and begin to degrade.

    Nylon 6 is not UV resistant. When placed in sunlight over time the physical and mechanical properties of nylon will degrade. It will often turn yellow and become brittle.

    Nylon 6 is generally not chemically resistant. It performs poorly in acids and halogens such as fluorine and chlorine.

    c. Offset parabolic reflector has more variables to be set. See attachment. I did not write, that it is not possible, but it is more complicated.

    It seems, that analysis of hamradio design must be written without cons. So the next part will consist only from physical properties without any + and - comments:)

    73, Rasto

  • A75GR Yep, the properties of Nylon are known to me and the lens is not an 'integral' part of the publication, but merely, like Mike stated, an alternative when people don't have 'Rocket LNB lenses' and want to illuminate a dish with f/D > 0.3 - 0.4 like standard BC-dishes with an f/D = 0.6.

    I tried to find out (hitherto with no success . . ) from which material these Rocket LNB lenses are made. It must be very cheap material because the overall prices of these LNB's are very low.

    Ad c. Yep, that complicates the simulation because you don't know which part of the parabola is considered. I did my experiments with a Triax offset dish (f/D = 0.6) and found max SNR with lenses generating some under illumination (G = >11 dBi). The lens proposed in the article -with the knowledge at that time- was modeled for exact 90° -10 dB opening angle (for a f/D = 0.6 dish). In practice it doesn't perform that bad, but with todays knowledge there are better alternatives.

    Considering pros and cons, of course feel free to mention them, but also accept that people react ; -)

  • 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.