Posts by dj0abr

    Hi Joshua,

    I have look for replacements, but non of the possible transistors are available, or extremely expensive. It looks like there is a shortage in production maybe due to Corona.

    The 30W option was originally developed for small camping dishes, like the dish we are using for our portable club station. If you are using a normal >=80cm dish, then the 10W amp is a good choice. It gives real 8W which is enough the trigger Leila.

    vy 73, Kurt, DJ0ABR

    during my work on the 3-helix-feed I made a lot of measurements to compare it to other feeds, including the POTY.

    I have build many Potys until now (for DH5RAE, DO1RX, DG6RCH, DG6FAX, DL3MX, DL1EV and myself), all worked OK, but had some drop in TX performance compared with a simple helix antenna. Something between -2 to -3dB compared with a helix (when monitoring my own signal via QO-100 in a spectrum monitor).

    Also I talked to many people on QO-100 who had similar experiences, mainly because it was very hard to tune the "two" virtual antennas to the correct frequencies and to see the two resonances in a VNA. This results in a poor circular (or even no circular) polarisation which explains the -2 to -3 dB drop.

    I have read the manual (written by G0MJW, PA3FYM, M0EYT) hundred times, but this week I focused on this remark on page-4 about the distance between the to copper plates:

    "Aim for 3.0mm, not about 3mm”.

    When building a POTY I use 3mm aluminium spacers while soldering the copper plates. But this always resulted in a distance of the finished (and cooled down) antenne of 3.1 to 3.2mm.

    This week I did it different. I sanded my 3mm spacers to exactly 2.9mm and used it to build the Poty. The distance was now exactly 3.0mm.

    What a difference !!!

    When connecting the antenna to my VNA, it was almost perfect without any adjustment. The two dips are almost exactly as shown on page 7 "modelled return loss". It tooks only a few minutes to squeeze the copper plates to a perfect match.

    Also I found that the way how the connector is mounted on the reflector has hugh influence on the tuning of the Poty. I do not use screws any more, instead I solder the connector directly to the reflector. Also the hole for the central wire must be small. If its too big, the few mm² of missing copper will disturb tuning.

    Today I mounted it to my Gilbertini offset dish (f/D 0.67) and the signal was almost identical to the 4-turn helix.

    I also did a few RX tests with different dielectric lenses because I found STL files on But none of these lenses worked for me. The only lense working perfectly is the cap of the Red Eagle Black Buster.

    Thanks to G0MJW, PA3FYM, M0EYT for this excellent POTY!

    the idea to build this feed was triggered by Peter, DB2OS in January 20, when he sent me some papers and calculations from Karl, DJ4ZC, to construct a QO-100 feed with 3 helix antennas around the LNB. Additional information came from Achim, DH2VA and Michael, DD5ER, who sent me dimensions for the antenne coupler and Mario who did final adjustments on the Amsat Helix antennas in Bochum.

    Building a feed with three helix antennas is a complex project, so it took three month, until April 20, to get the first feed up and running.

    The goal was to get best reception and not to shade the LNB, and to get the same good TX performance of a single helix feed.

    To measure the shading of the LNB I built the reflector and LNB only, without helix, measured the transponder noise and DATV-MER and then glued three dummy helix on the reflector and repeated the measurement.

    this is the reflector + LNB, with three dummy helix.

    Resulting transponder noise: a single helix costs about 1.5 - 2 dB. The three helix antennas cost only 0.5 dB. Also MER was about 1dB better.

    My first idea to simply paralleling three helix antennas was impossible, because I never measured the 140 ohms as stated in Wikipedia or Rothammel. Therefore I tuned each helix to 50 ohms and built an antenna coupler.

    2.4 GHz coupler made from brass pipes, inner diameter 20mm.

    Today the new 3-helix feed went into operation. It give the same good TX performance as my single helix feed, and the good RX signal of an unshaded patch feed. The only downside is that it's a lot of work, and a good workshop is needed to build all the components.

    the 3-helix feed in operation

    50 ohms match and holder made from polypropylen

    tuning at the the coupler input with all three helix antennas connected

    vy 73, Kurt, DJ0ABR

    Hi Luc,
    I have it running on a PI4 with 2GB ram.

    DL3MX has it running on a PI3 with 1GB (I didnt know that a PI3 is fast enough, but dl3mx is happy and showed me that it is working).

    The complete Setup is now done in a SETUP page, no more programming is required to adapt it to the own system.

    There is a new update available covering the new band plan and also has a new beacon lock.

    vy 73, Kurt

    @PA1PAS: the GPS reference frequency goes through an extremely slow PLL ( a couple of seconds). This is required because the GPS module output has a very precise frequency, but has a huge jitter.

    The PLL is too slow to compensate a short term drift caused by a temperature change of the TCXOs.

    A good enclosure helps to solve that, and additionally Robert's hint to put a piece of foam over the TCXOs also helps a lot.

    All frequencies, including the LNB reference are derived from this PLL.

    73, Kurt

    in our local club U02 we are actively helping hams to install QO100 stations. Until now we have build 6 complete stations, for the cost of material only, of course. And did a huge number of LNB modifications.

    While most people can install a dish without problems, one of the biggest issues for most Hams is the modification of LNBs or SDRs, soldering a POTY or adjusting a helix because special soldering equipment (and skills) and analysers are required.

    I have modified an uncounted number of LNBs in my work shop and a couple of Plutos, but it simply takes too much time.

    I think it would help a lot if somebody (i.e. Amsat) offers ready built feeds including a modified LNB, ready to use.

    vy 73, Kurt, DJ0ABR


    Ich hab das hinter mir, irgendwo im Forum hab ich auch Bilder mit Wärmebildkamera gepostet, da kommt man ins schwitzen wenn mann die Kabel anschaut ...

    sehr interessant die Bilder. Während der N Stecker rechts relativ kühl ist, glüht der SMA links förmlich.

    0,5dB Verlust sind schnell erreicht, das sind schon 10% der Leistung.

    Bei N-Buchse im Poty muss man auf den Platz schauen, bei mir kommt sich der mit der Halterung in die Quere.

    vy 73, Kurt

    After having some Beer and Obstler I finished the Helix and did some comparisons in different configurations, here are the results:

    TX-signal (received with a separate dish):

    Poty: +25dB

    Helix: +28dB

    Transponder Noise:

    Poty: 6.1dB

    Helix: 4.3dB

    DVB-S2 MER (measured with Minitiouner):

    Poty: 7.8dB

    Helix: 6.6dB

    As expected with the TV offset dish, the TX signal of the Helix is almost 3dB stronger.

    However, the helix shades the LNB by 1 or 2 dB, which is not relevant for SSB, but represents a significant attenuation for a DATV signal.

    So what to do? Building a PA with 3 dB more power? Or buy a larger dish to compensate for the loss of MER? Or just living with these results the way they are. I think I'll do the latter.

    vy 73, Kurt, DJ0ABR

    Hi Renauld,

    Running the Downconverter V3d with an unmodified LNB may cause an unspecified frequency shift, maybe out of band.

    For the calculation of the output frequency (6 ... 144,55 MHz) the converter needs to "know" the LNB frequency, which is not the case if you don't use the clock delivered from the downconverter.

    By default the downconverter operates with 24 MHz clock. Your LNB may work with a 25 MHz TCXO, so you need to jumper the downconverter to 25 MHz by putting one jumper as shown in the documentation. Then you should get a signal on 144,55 MHz, but with some shift caused by the different frequencies of your TCXO and the downconverter's clock.

    If that works you should consider to modify the LNB to get a more precise output frequency.

    vy 73, Kurt, DJ0ABR

    Version 2.0 was released today with a lot of improvements.

    * now it is a "real" websdr, supporting up to 20 users. Each user can tune and hear his own signal. More users are configurable if the computer is fast enough.

    * automatic transponder noise measurement: the RX level on abt. 10489,250 and abt.10489,540 is measured (TX carriers are suppressed). Then it calculates the difference which is the transponder noise. Nice feature to compare dishes and receivers.

    * prepared for the new upcoming bandplan. As soon as the new bandplan is released this websdr can be adapted within seconds.

    a new feature for DATV is described in the WB forum

    vy 73, Kurt, DJ0ABR

    Github link as usual:

    Hallo Oliver,

    9360 stimmt, also 09360000 um genau zu sein.

    Ich empfange mit Minitiouner-Express genau so wie du es beschreibst.

    Vielleicht misst du mal die Spannung am WBin, ob es auch wirklich 18V sind.

    Wenns dann immer noch nicht geht, schau dir die paar SMD Bauteile in der Nähe des WBout Steckers unter der Lupe an. Beim EInlöten des Steckers braucht man kräftig Hitze, vielleicht hat sich einer der Winzlinge dadurch gelöst.

    Weiß übrigens jemand wie man die 9360 in der Minitiouner Software fest einstellen kann, dass man sie nicht jedes mal neu eingeben muss?

    vy 73,Kurt

    Danke für die vielen Infos,

    das scheint also schon einiges auszumachen. Ich bin auf der Suche nach fehlendem MER der DATV Bake. 6,0 in einem 120cm Spiegel ist schon arg wenig.

    Heute habe ich einen neuen, unmodifizierten Octagon Green HQ PLL reingeschraubt (nur LNB, ohne TX Antenne) und kam auf MER Werte zwischen 6,9 wenn der LNB ganz vorne oder ganz hinten geschoben war, oder 7.7 wenn er in der Mitte ist. Das ist immer noch nicht berauschend, aber immerhin. Ein Skew Winkel hatte fast gar keinen Einfluss, vielleicht ein zehntel.

    In diesem Thread


    schreibt dg0opk von MER 9,4 bei einem 120er Spiegel.

    So langsam hab ich den Verdacht dass mein SAB Spiegel vielleicht krumm ist und keinen vernünftigen Focus hat (war auch eher billig), optimal ausgerichtet ist er jedenfalls.

    vy73, Kurt

    Hallo Oliver,

    das hört sich so an, als ob der Downconverter die seriellen Daten nur ausgibt wenn das Oled Display angesteckt ist. Das könnte gut sein. Die Firmware, welche die Daten auch ohne Display ausgibt, wurde erst vor kurzem freigegeben und hat es evt. nicht mehr auf die Boards geschafft. Das war allerdings die einzige Änderung, ansonsten ist dein Downconverter auf dem aktuellen Stand. Da du aber ein Oled Display hast, wird das wohl kein Problem sein.

    vy 73, Kurt


    So, regardless of the diameter of your dish, you still need a -10 dB 90 degree opening angle for a f/D=0.6 dish --> 6 windings .. because of mathematics/goniometry ...

    thanks for the answers,

    where does the -10dB at 90deg come from?

    Another "secret" number (at least for me :-) ) is the turn spacing which is usually 0.25, but why 0.25?

    If I play with the turn spacing, a 4 turn helix can get the same opening angle as a 6 turn helix.

    My goal is to make as few turns as possible so as not to shade the LNB.

    Currently I use a poty feed, but want to try a helix, just for comparison.

    vy 73, Kurt, DJ0ABR

    Hallo Bert,

    im Prinzip geht das auf einem schnellen Odroid (N2) oder auf dem Raspi-4.

    Für WB muss dann ein SDRplay benutzt werden und für NB ein RTLsdr.

    In der Software (in main() ) ist eine Abfrage für doppelten Start drin, diese muss man deaktivieren.

    Für diese Anwendung würde ich den Odroid N2 empfehlen. Ein Raspi-4 wird bei der hohen Last extrem heiß werden und nur mit perfekter Kühlung funktionieren, während ein Odroid N2 kaum mehr als handwarm wird.

    vy 73, Kurt


    To illuminate a dish with a helix properly a rule of thumb is that you need one (1) winding per f/D-unit. So f/D = 0.6 needs 6 windings. With smaller f/D's less windings are needed, however, with less windings the axial ratio becomes worse.


    a question to this older post,

    this rule of thumb "1 winding per f/D unit", is this still valid?

    Most people use f/D=0,6 offset dishes, but most use 3.5 windungs and some people 5 windings.

    I am currently building a helix for a 150cm offset TV dish, f/D=0.6 and want to use the best number of turns.

    vy 73, Kurt