Posts by dl2kp


    I think my problem is solved. After a new construction the result is acceptable.

    The physical dimensions of the small loop were wrong. This was the reason, why it was not possible to get an unique match. Further more a critical point is the connection of the coax cable to the feed point.

    Vy73, Georg

    Hi Achim,

    the design is according to jcoppens with his calculator at "" with 2.5mm conductor. I made another one for 137MHz with 10mm conductor and got S11=22dB. But I addmit at that time I didn´t look for other resonant points at higher frequencies. The function of the QFH-Antenna is similar to the patch antenna to get a circular polarisation discussed in this forum.

    Vy 73 Georg

    Good Morning,

    I built a 403-QFH-Antenna for weather balloon reception. After taking some measurements, I wonder that the best ajustment is not at the design frequency, but at about 1.5 fr=630MHz. At the design frequency S11=-13,5dB and at 630MHz S11 is about -30dB. How can I get the best match at the design frequency?. What did I wrong?

    Who can help?



    could anybody make a recommendation for a laptop to be used in a mobile QO-100 trx station? Are there any basic points to be observed to buy a laptop for a ground station. Is a I7 cpu enough? Should I use win11, linux or win11 with WSL 2 and Ubuntu? Which display-size would be preferable?

    73, Georg



    in addition a picture of the actual version:

    I guess that one important matter is to utilize silver braze solder to reduce attenuation, too. To keep the distance of the disks in position, you need a ficture during soldering. It is out of aluminium otherwise it would be soldered, too.

    To be honest, it is difficult to construct the filter with a accuracy within 0.01mm. I don´t have a CNC-lathe, unfortunately.

    Have a nice Easter!



    Eureka, the third attempt hits the target, as I promised. Analysing the previous built filters, I assumed that the reason of the high attenuation was the so called missmatch loss. Therefore, I tried another approach. Manual computation of the filter parameters according to Matthaei is time consuming and boring. In the net you will find very detailed Exel sheets by Dominique, F1FRV. It turned out that they are not applicable for 2.4GHz because of the small dimensions of the construction. Another online calculation is offered by

    I realized that this calculator presents the same results as I calculated. As I don´t have access and money to use HFSS Electro Field Simulator, I grabed the old free of charge available RFsim99 Simulation program. Unfortunately it does not include parameter optimization. But with the well known try and error method you can get good results together with an available auto match function. Here is the simulation circuit:

    And here the simulated results:

    At the working frequency the simulation shows, that we get an input impedance of about -67dB and an attenuation of -770ndB.

    With these new parameters a new design was created:

    In contrast to my first construction, the brass disks were soldered to the inner conductor with butan burner at about 750 deg Celsius.

    And here my new filter for QO-100:

    An attenuation of 0.1dB at the working band and an input impedance of of -27dB is an acceptable result. It is not perfect as you can see it could be better, but it is not a tunable filter. Now, I will try to build another one to get it perfect, probably.

    Best regards


    Hi Matthias,

    thank you for your fast help. It is clear that my LibreVNA has a problem, or the operator in front of it or both. I have some ideas to improve the design, already. It is like artillery, the third shot will hit the target, hopefully.



    I got the impression that the high attenuation of my coax filter could be an issue of the calibration of my LibreVNA. It was calibrated with female Rosenberger parts from SDR-Kits. They stated to be valid up to 6 GHz. I am not sure, if it is guaranteed. Now I got the appropriate s1p files for LibreVNA standards. Unfortunately the filter is disposed already.

    Nevertheless, a new filter was designed:

    The order of this filter is 7 instead of 9 of the old one and the cut-off frequency was chosen to 3.5GHz.

    And here some pictures:

    As you can see the material is alu EN AW 6082 (AlMgSi1) which is not very good for milling. This is while the surface is not perfect.

    Aluminium has a much better conductivity than brass.

    The measured parameters are as follows:

    The attenuation in the QO100 frequency range is now -0.5dB but the imput impedance is not 50 Ohm. This is disappointing as I included the 50 Ohm section on both sides of the filter.

    I have two questions:

    - what is the reason that the S11 impadance swings over 0 dB?

    - does anyone have an idea to improve the design?



    Hi Matthias,

    you are absolutely right, but it would be a wonder if the result would be perfect at the first shot. Now I am going to optimize the filter. At these frequencies it would be necessary to silver-plate the filter to get the best performance.

    Vy 73,




    now I got my first measurement:

    Here is the result after assembling without any tuning:

    The attenuation is pretty high. I guess, this is due to the kind of construction. So far I din´t solder either the disks nor the connection to the connector. They are pressed on the center 4mm wire, only. Further more the cuttoff frequency is at about 2.6 GHz and was designed at 3 GHz. For my application it is ok, but I don´t know why it it so low. In the next step I will solder all parts together.

    Vy 73,



    Hi Matthias,

    it is similar to Kurt´s one. The difference is the cutoff frequency and I missed an addaptation to 50 Ohm for in- and output. Maybe he didn´t show it. Here now some pitures out of my workshop:

    Now I am curious about the measurement results.

    Vy 73, Georg


    P.S: Matthias, did you glue teflon, already?


    I designed a coaxial lowpass filter for 2.4 GHz. The cutoff frequency is 3 GHz, Order 7 according the book "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" by G. Matthaei, L. Young and E.M.T. Jones. You can see the design here:

    The most difficult part is to calculate the fringe capacities at points, where the diameter of the inner conductor changes. Fortunately, in the book you can find diagrams to estimate them.

    The next part will be to energize my workshop.

    Vy 73,




    in the forum I found several times a reference to a Cech article (e.g

    Computer analysis of dual band dish feed (G0MJW, PA3FYM, M0EYT). Part 1 and 2 –S Band).

    Unfortunately no translation was found anywhere. Here a section out of the original text:

    I couldn´t find the complete original text. Nevertheless, the most interesting part is written here. Therefore I tried a translation into English and German as it could be of interest here in the forum. I am not a translator and my knowledge in Cech language is limited especially in microwave wording.

    English version:

    ... (Picture 4.4).

    Introducing irregularities to the form of a patch results in a degradation of the original mode. By this process it is possible

    to generate two modi (#1 and #2) which are orthogonal to each other (Pic. 4.5). The suitable irregular segment shall shift the

    frequency in such a way that at the operating frequency f0 both modi have equal amplitude, but a phase difference of

    90 degree. This way, the target will be met to generate circular polarization. With the frequency shift at the quiescent

    point f0, the relationship of the two axes deteriorates drastically, but the match remains acceptable, normally.

    Picture 4.4: Layout of the modified segments and the single feeder point of the patch including circular polarization:

    (a) round patch, (b) square patch

    Picture 4.5: Layout of the resonant modi and circular polarization

    4.3 Circular Polarization with a Round Patch

    With a round patch a circular polarization could be obtained, by generating two different resonant modi. This depends on

    the shape of the patch. Basically it is a question of the dimensions of the pach to generate a different resonant mode from the original one. At the working frequency between the two modi, a circular wave will come into existence.

    My interpretation of this text leads to the result that the optimum S11 curve will be one dip, only, at the working frequency f0. Both amplitudes are added to form one maximum. At this point circular polarization exists. Left and right of it elliptical polarization will be met.

    Please don´t hezitate to comment.

    German version:

    ... (Bild 4.4).

    Die Herstellung von Unregelmäßigkeiten in der Form eines Patchs führt zu einer Degeneration des originalen Modus.

    Daraus lassen sich zwei Modi (#1 und #2) herstellen, welche aufeinander orthogonal sind (Bild 4.5). Das passende Störungs-

    segment muß die Frequenz so verstimmen, daß auf der Arbeitsfrequenz f0 beide Modi die gleichen Amplituden besitzen,

    jedoch mit einer Phasenverschiebung von 90 Grad. Damit erfüllt sich die Bedingung für die Entstehung einer

    Zirkularpolarisation. Mit der Frequenzverschiebung im Arbeitspunkt f0, verschlechtert sich das Achsverhältnis wesentlich,

    aber die Anpassung bleibt normalerweise akzeptabel.

    Bild 4.4: Anordnung der Fehlersegmente und die Einpunkteinspeisung der Patches mit Zirkularpolarisation: (a) runder Patch

    (b) quadratischer Patch

    Bild 4.5: Layout der Resonanzmodi und Zirkularpolarisation

    4.3 Zirkularpolarisation beim runden Patch

    Beim runden Patch läßt sich die Zirkularpolarisation dadurch erreichen, daß zwei Resonanzmodi unter der Patchplatte entstehen.

    Solch eine Einstellung beruht auf der Form des Patchs. Grundsätzlich handelt es sich um die Schaffung von Abmessungen

    des Patches, auf dem sich ein anderer Resonanzmodus als ursprünglich ausbilded. Auf der Frequenz zwischen den beiden

    Resonanzmodi, entsteht ein zirkulare Welle.

    Aus dieser Beschreibung entnehme ich, daß die korrekte Abstimmung einer Patchantenne optimal möglichts auf einen Resonanzdip erfogen sollte, da sich in diesem Resonanzpunkt beide Wellen addieren und sich die beste Entkopplung ergibt.

    Nur hier ist eine zirkulare Welle. Links und rechts wird sie elliptisch.

    Was ist eure Meinung?

    Vy 73,




    Problem gelöst. Nach vielen Stunden und unter Zuhilfenahme eines anderen Rechners wurde der culprit gefunden:

    Meine Plutos hatten diesen Adapter beigepackt, den ich mein volles Vertrauen gab. Leider vergebens. Er ist für den Müll. Aus Versehen hatte ich ein anderes Kabel verwendet und siehe da, alles läuft wie geschmiert.




    seit Stunden versuche ich den Treiber für Adalm Pluto zu installieren. Ohne Erfolg trotz intensiver Suche im Netz.

    Meine Hardware ist ein ENC-H897 NUC Embedded Motherboard mit Intel Haswell I3 2GHz CPU

    Von Analog Devices wurde das Installationsprogram: "PlutoSDR-MK2-USB-Driver.exe" heruntergeladen und gestartet.

    Der Installations Wizard meldet:

    - Analog Devices Inc. (usbser) Ports, (Winusb) Universal Serial Bus Device ready to use

    - Analog Devices Inc. (Winusb) Universal Serial Bus Device ready to use

    - Analog Devices Inc. Net ready to use

    - Analog Devices Inc.(Winusb) Universal Serial Bus Device ready to use

    Aus dem Gerätemanager kann ich entnehmen, daß der Treiber nicht installiert wurde (Code 28). Das Gerät wurde entfernt.

    Jetzt bin ich ratlos. Es hat alles so einfach ausgesehen. ||

    Wer kann helfen ?