Posts by A75GR

    Hi Mike,


    you can use Sun8) to cold sky measurement to get G/T of your Rx system. It is common technique from radio astronomy. It is also used often to measure EME setup. Then you can apply VK3UM EME calculator to get final data. However, when your transponder noise floor is 10 dB, you must have good Rx system with sufficient signal margin to cover a possible link deterioration (heavy rain, snow etc.):)


    73 & GL

    Rasto

    Since my OM6AA station is currently QRT due to TRSV failure, I worked out small paper with LNBs measurement. See attachment. I guess, that experimenters with small downlink antennas can find some usefull information in this paper.


    73 & GL

    Rasto A75GR/OM6AA

    I measured my directional couplers with 20 dB and 30 dB coupling between ports 2 and 3 today. The same setup as for previous measurement was used. Results are in attachments. Directivity for 20 dB coupler is 31 db @ 2409.8 MHz, for 30 dB coupler 25 dB @ 2409.8 MHz respectively.

    I measured similar directional couplers with 20 dB and 30 dB coupling today. A PICO VNA 106, analyzer with a Rosenberger calibration set were used for measurements. Unused connector was terminater with ANRITSU 50 Ohm load with impedance match better than 40 dB. Results are attached. I guess, that couplers are really good and they worth your money. With AD8317(8) detector can be built very cost effectiv power meter or VSWR meter.

    Hello Mathias,


    the goal to properly feed patch, is to excite proper dominant mode. Usually it is TM11. In circular polarization patch, two orhogonal dominant modes must be excited. Changing feeding point position, also higher modes could be excited (TM21, TM31 ...), resulting in radiation pattern deformation or unwanted side and backlobe radiation. The proper feeding point, could have impedance different than 50 Ohms. An impedance match must be subsequently performed. To find the feeding point for the best VSWR for 50 Ohms system, means various trade off for radiation pattern changes.


    I think, that it is not good idea to decrease reflector size. You increase backward radiation this way and increase main lobe width. You can use collar ring also for your smaller reflector, but you can expect higher impedance changes, since collar ring is closer and more affects the patch. The collar hight can be up to lambda/4. It can be 1 to 2 mm thick. It is not important value.


    73, Rasto

    Hello Mike,


    I am sending dimension of collar ring. Its thickness is 1mm or so. See attachment. I am adding some feeding tips from another book. Please, private use ! Disclosure, reproduction, copying, distribution, or other dissemination or use of this pages is strictly prohibited. TNX


    GL & 73

    Rasto

    To Mike,


    I tried to modify your design, just adding colar ring on reflector. See attachment. I achieve impedance match to be on the working frequency and below 20 dB, substantially suppressed side lobes, significantly improve axial ratio and shift phase center almost on the same position as it is on X band. However, feed still suffer with squint of main lobe. I think, that it is due to asymmetric position of the feeding point. I am attaching some samples from antenna book. I did not study an influance of colar ring on X-band radiation pattern and I did not calculated efficiency in dish. It is job for one day. So it is the way how you can improve your design. Play with reflector size, size of colar ring (It can be also with conical shape. I saw similar design at University) and feeding point.. I am leaving Eu for Duha now. No more time for antennas... Maybe for some band activity.

    73 & BR

    Rasto

    The same methodology was used to calculate feed parameter for S-band. Time domain solver approach was applied to calculate feed radiation pattern and impedance match. I –solver (MLFMM) was applied to evaluate the whole antenna assembly. 3D radiation patterns and their appropriate 2D cuts are shown in the attachment. The main lobe of the pattern exhibits some asymmetry and squint angle from Z axis. It can be evidently observed in polar plot. Axial ratio (AR) diagrams for angels Phi = 0, 45 and 90 deg. are shown in the following pictures, respectively. The established industry standard for circular polarization antennas is less than 3dB. However, I was curious about the changes of AR after reflection of the energy from parabolic reflector. The result is shown in the next diagram, from which it is obvious that reflector has not changed this parameter. Just a small deterioration occurred. Despite the authors report of very good impedance match, I was not able to get better value than 16 dB at 2.42 GHz. I got only one dip, just a little bit higher above the working frequency. I used very fine mesh setting with the mesh refinement around the edges. I have used this software setup in many of the projects, resulting in a very good agreement between the calculated and measured values. It is possible, that I used different N-style connector for modelling than the authors. See the attachment. For the impedance match adjustment, authors recommend bend patch to – from feed reflector. This technique may cause another deformation of the radiation pattern. However, it is better to sacrifice a few percent of the antenna efficiency than loose power amplifier or transventer. I guess, that better option is to insert some sheet of low loss dielectric material between the patch and its reflector.

    Remarks: The established industry standard of the impedance match (or returned loss RL) for a UHF fixed narrow band Tx antennas is around 20 dB (for example. GSM antennas have VSWR warning threshold 1.4, i.e.RL= 15.5dB). I measured a “professional“ product – helical feed for 2.4 GHz for EsHail 2 with VSWR =3, RL= 5.8dB recently).

    The phase center of the S band section lays 11.3 mm in front of the feed reflector plane. For X band it is 23.6 mm. Since, I consider more important to achieve better efficiency for X- band than for S-band, I calculated the feed optimally positioned for the X band. This introduces axial defocusing error of about 0.1λ. The efficiency calculation is involving axial defocusing error, X - polarization losses, diffraction losses, illumination losses, phase error losses, spillover losses, and aperture distribution losses. The efficiency curve is not pure concave shaped, it has also a convex part. I intentionally did not smooth the efficiency curve in the MATLAB for better detection of this behavior. The feed on S-band operates on the edge of its farfield region. For close reflectors, the field emanating from the feed has no point source properties. resultnig in phase error losses.More can be find in my article at: http://www.om6aa.eu/Loop_Feed_with_enhanced_performance.pdf This is not this desing‘s fault but a general problem of small reflector antennas.

    Conclusion

    Engineers of Mitshubishi electronics did an excellent job. Sensitivity of the transponder despite twice lowering its gain is amazing. It opens a gate for hamradio experiments on UHF/SHF bands. There are reports of sucessful work even with 8 dBi Tx antennas. RF antenna designer can try and verify their design easily using an SDR technology or web-site receivers. Possible deficiencies in the design can be easily compensated by increasing the transmitter power. This simplicity thus enables utilization of the transponder not only for common amateur radio applications but also for education or emergency purposes. Behalf of QARS I wish you to enjoy the EsHail 2 satellite!

    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

    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


    Pros:

    -         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


    Cons:

    - 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

    Hi Ed, we have 40 deg. C these days in Doha. In summers, temperature rises up to 50 deg. C. A cover can not be transparent, due to greenhouse effect. Requirements on cover are - good thermal stability, excellent UV resistance, RF transparency, white color (to avoid thermal absorption) and low weight. Possible materials are PTFE (Teflon), PVDF (Kynar) and Kapton. The best shoud be a Teflon bag, but it was not available at my size, Kapton is too dark. So Kynar bag with teflon foil beneath is doing this job. :)

    I am sending some pictures of my antenna in Qatar. Aperture is protected with PTFE foil. Feed and LNB are protected from dust, sand and moisture with PVDF bag. More info about feed is available at http://www.om6aa.eu

    Looking forward to hearing you on QO100! 73, Rasto A75GR - OM6AA