Posts by A75GR

    Dear OMs,


    I performed S/N ratio measurement on 29/07/2021 using a R&S FSP38 Spectrum Analyzer. I have a 1.2m dish with dual-band coaxial feed fitted with a SPCR5300 LNB (measured noise figure 0.9 dB). An AOR AR5000A receiver with AGC off was used for these measurements. I measured on Rx's IF output – 10.7 MHz.


    The channel power measurement for 2500 Hz bandwidth (SSB) was performed. As a signal source I used a middle PSK beacon. For noise power density measurement, I used clear free frequency, 10 kHz below mid beacon. I used an RMS detector, since there is no correlation between the peak value of the video voltage and noise power. The resulting S/N ratio was 16.49 dB with fluctuation a few tenths of dB. For the different bandwidth (BW), for example 1 Hz, the result must be corrected by 10 · log (ChanBW), i.e. 10.log(2500) = 33.98 dB. So, my measured result for 1 Hz bandwidth is giving S/N ratio 50.47 dB. The achieved results very well correspond with Goonhilly and IS0GRB Earth Station S/N values.


    As was formerly written by Achim, Peter and others, when transponder noise is dominant over the Rx noise, then there is no room to improve S/N ratio and the above described test is becoming a hearing test or operator capability test, not an equipment test.


    73 & BR, Rasto OM6AA

    Ahoj Bill,



    a H155 coaxial cable is not good choice for Tx use, especially for higher power. This cable, thanks to its structure, produces a lot of intermodulation ( tested in our contest station OM6A). You can get any good quality cables + connectors in Slovakia. See www.ges.cz, www.gme.sk  www.tme.sk etc. Even you can order professional cable assembly for reasonable price here: https://konektor-kabel.cz/. Here, you can select good quality stranded or hard-line cables + high end connectors (Rosenberger, Andrew, Spinner). I do not recommend you to use no name shiny connectors. These connectors are working with acceptable impedance match (20 dB) maximum up to 70 cm. They have impedance match less than 13 dB on 13 cm.


    Good luck with your satellite project.



    See you on transponder,


    Rasto OM6AA-A75GR

    Hi Peter-DJ7GP,


    nice experimental work. Could you kindly describe the last polar diagram in more details and in English please? I guess, that it is a polarization envelope diagram, isn`t? What are the magnitude units (dB)?

    Thank you.


    73, Rasto

    Dear Alex,


    As Achim and Mike wrote, the problems of signal propagation from the ground to satellite and vice versa is quite complex. In general, there are three types of effects on the signal path.


    1. The atmospheric effect (causes depolarization, attenuation, rain, snow, fog etc)


    2. The ionosphere effect (scintillation – causes short term attenuation)


    3. The depolarization effect (Faraday rotation – magnetosphere + ionosphere effect)


    More about this topic you can find also here:


    https://www.itu.int/dms_pubrec…31-12-201309-S!!PDF-E.pdf


    If you want to measure polarization properties of your signal path, you must be sure, that your Tx antenna has the same pattern for E and H plane, when you change polarization. Also, you should be able to switch between polarization in short time. Unfortunately, I'm afraid that the Yagi antenna as a feed is not very suitable for this measurement. I guess, that a waveguide feed (coffee can), fitted two probes (perpendicular to each other) would be better solution. If you want to measure circular polarization, a septum feed should be the best option.


    73, Rasto

    Dear Gerhard,

    I added Example 3 into my paper. I performed many calculations with actual radiation patterns of helical feeds. Results are published in edition 2 of my paper “Nomographs for Parabolic Reflector Antennas“ You can find the answer on your question regarding of helical turns there.

    I try to prepare some simple explanation on your second question. Pls. give me some time, since I was calculating 3 days...


    73 & GL,

    Rasto A75GR/OM6AA

    Dear Gerhard,


    Thank you for your inquiry. The offset parabolic reflector has asymmetric structure with different added edge taper for upper and lower rim (it depends on the offset height - H). This added edge taper must be compensated by the feed. But how to compensate it, when upper AET is higher than AET on lower rim? To use asymmetric radiation pattern? Or by taking mean AET value for particular reflector size and optimize design of the feed this way? Or by changing feed angle? Or mix all these procedures together?

    t is obvious that each design of the feed for particular offset reflector becomes unique. The good compromise seems to be technique as I described in my paper. It is not my invention, it has been described many times in the professional literature. However, this concept works well if the radiation pattern is similar to the cos^2N (θ) function. As example of application this technique, I am attaching some results from my project “Low noise antenna for QO100 downlink“ The feed dual-mode horn with gain of 14.5 dB was used associated with offset reflector with f/D = 0.66 and diameter 40 cm (Gibertini OP40E). The efficiency of 73 % was achieve with very low antenna noise temperature. See attachments.

    However, radiation pattern of helical feed has different pattern than cos^2N (θ) function. I will recalculate Example 2 from my paper for the actual helical feed with 5-10 turns to figure out the optimal turns more precisely. It takes me some time, since one calculation takes about 30 – 40 minutes.


    TNX & 73, Rasto

    I have prepared several nomographs for antenna experimenters, during my mandatory corona-virus quarantine. Nomographs are plotted for parabolic reflector antennas and their feeds, with focus on QO100 operational frequencies. Nomographs could be helpful for quick orientation in selection of a suitable parabolic reflector and its feed.


    73, Rasto - A75GR/OM6AA:)

    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