2.4GHZ Patch_Antenne tuning

    Hi,

    I couldn´t suffer that an 1.2m dish was unused in my garden.






    Therefore, I decided to construct my own QO-100 ground station. Here the feed was analysed with sonnet:






    Unfortunately, I was not able to shift the frequency down to 2.4GHz. Sonnet light allows to analyse the antenna, only. Optimization is obviously in the full version possible, only.


    Nevertheless, I started to construct the feed, looking forward what will happen.



    After everything was soldered the first measurement was taken:


    The first measurement after construction:


    I have no experience to interprete this S11 trace. My guess ist, that the circular polarisation is not very impressive. So I played around and soldered some copper to the longer edge. At the end everything was removed. Solder remained at the two edges with following result:



    I have no clue why the solder will change the trace. I have to admit that all the measurements were done in my shack. Maybe this is the result off all the reflections.


    Vy 73,

    George,

    dl2kp


    P.S.: In the meantime I know the reason. The solder changes the thickness of the patch and so far the er of the poty-antenna. Changing er results in a significant detuning.

    Hi,

    after reading through "SWR POTY verbessern" and the comment by DG9FC I tuned my Poty bending all 4 corners carefully. Now I got this result:


    Both resonancies vanished and one resonance dip with about -55dB remained. I am very curious about the S11 amount. Maybe my LibreVNA is the reason or/and the measurement surrounding in my shack. I have to remeasure it when mounted to the dish. An amount between -25 and -30dB would be reasonable.


    Vy 73,

    George

    DL2KP

    Nice build, but do you know what the design was ment to be?

    2 resonating freq about 100 mhz apart causing a circulair pattern. The poty is not ideal on the working frequency witch is why most high power datv stations use helix.

    What i do not identify is the point where the signal is supplied to the front plate.

    The location of this is causing left or right hand circular...

    The white lens i see in your picture is not working on prime focus with 0,3 D but is meant for offset dishes with 0,6 D...


    Goodluck and see you soon...


    Benno, PA3FBX.

    Hi George,

    I have done much in asking Mike and Remko how this POTY works, but never got a good answer.

    Than i took a more practical approach.

    I have test build over 10 pieces for just finding what works and what does not.

    I do not have expensive antenna simulation software as the guys who have designed this POTY but like you my VNA showed big problems...

    In the end i made a test setup in a 1M dish and 10mw signal with goonhilly as my receiving reporting system.

    Modifying until i got most signal across which is what we want in the end...

    I saw no relation with bad swr but huge difference moving the dips up and down and wider and narrower..

    In my test experience the distance between the plates need almost perfect as described and the 2 dips need 40 Mhz to 50 Mhz higher and lower as the used frequency, Than use the right cable lengt to get the optimum swr at the amplifier.

    Keep in mind that 4 cm longer will influence the swr...

    Even in cables over 10M but those cables are to long anyway...


    Benno

    if that dip is at the working frequency AND if that is at the top of the rollercoasterloop then it is perfectly tuned

    install it ...

    with such a high return loss you will see the return loss will go down a bit from your 50 or 55 peak cause you see the reflections from the dish back into the feed and any slight bending will detune that very deep notch ... a bit of distance change and the reflections come back in a different phase related to the tx energy and you see that deep notch again (will vary a few db depending on distance) ... if you have made your reflector from a not too thick copperplate then you should reinforce it a bit (i glued a HPL sheet at the back for a few feeds i made for cf dish)

    and ... how does that poty work?? in principle it is a too long and a too short dipole as a cross ...both fed with the same current

    now what does a too long or too short dipole represent?? a radiator with a coil and a radiator with a cap inline

    and what does a cap/coil do?? they shift phase

    now ... if proper designed and tuned one element shift phase plus 45 degrees and the other shifts - 45 degrees .. voila 90 degrees difference (but with equally currents!!) and we have the circular polarisation

    and we do see that if the difference between the upper and lower resonances are around plus minus 2-3% (around 50 to 70 megs or 25 to 35 megs above/below working frequency)

    means the mechanical dimensions should NOT be changed and the feeding point must be at the absolute exact position to feed the elements with the right phases and current (and have the 50 ohms on that spot also with not reactance added) ... not easy but it can be solved with ... build it EXACT ... and finetune in very small steps with looking on the vnas smith diagramm (not to mention use a short cable and tune the cable out with osl calibration etc)

    a hint .. in several poty i made i had to slightly get the long edge near the feedingpoint a tiny bit closer to the reflector .. about half of a tenth mm only or so

    greetz sigi dg9bfc

    ps yes if proper tuned the circular polarity is good! and a single resonance spot does NOT mean both elements are tuned to the same single resonance .. BOTH TOGETHER form a single resonance with circular polarisation (see explanation how the poty works at the beginning of this text)

    Hi Sigi,

    the Poty is a coupled system consisting out of two antennas with a phase-shift of 90deg. Where at resonance you get a perfect circular polarisation. This point is exactly at the top of the impedance loop. Resonance means that the imaginary part of the komplex impedance is zero. Therefore the location of the loop-top shall be in the middle of the smith chart at the operation frequency. As soon as the top moves out of the middle, you get two resonances and the polarisation starts to be elliptical. In the extreme it will end to be linear.

    I think, we got it!


    P.S.: To clarify the 45deg angle you should know that the impedance of the shorter antenna shall be Z=50 -j50Ohm and the other Z=50+j50Ohm. Both impedances summarizes to real 50Ohm. The operational frequence at 50Ohm is in the middle of the frequencies of the two antennas. This means that one antenna is operated below the operation frequence and the other above. According to your description above.

    I have now read 3 times exactly the same...

    But in the end.. theory and practica are rarely the same as we are not living in an ideal world. Hence my practical approach using goonhilly web monitor. I found in bigger offset dishes very little reflection problems but the coax cable and its weight on the Poty is doing very much.


    So mount the Poty in a dish and start testing and don't stair your Eye blind on a VNA...

    10 mw in a 1 meter dish is more than enough to see your signal on Goonhilly.

    Hi Matthias,


    you can see it here in the forum under antenna :

    "Mate LNB and Patchantenna".

    The LNB-clamp has a diameter of 50mm. Therefore a 50mm waste pipe slides over it easily. On the other side the pipe is glued to the POM-cover of the bigger part.

    At the end the patch will be covered by a 0.5mm teflon sheet glued to a removable ring.

    73,

    George

    Actually, I don´t see a major problem. What you need is to build up a chemical bridge between the raw material and the glue. This could be done through a so called primer linke LOCTITE 770. Subsequent teflon can be glued with epoxy resin like LOCTITE 406. The only disadvantage is the high price.


    Vy73,

    Georg

    dl2kp