POTY and return loss plot

    Hello all POTY users,

    i just ready built my POTY and take yesterday some measurements. It was not so easy to come to a <-20dB matching in S-band. With new soldering and changing the distance from 3mm to 1,5mm i got ~ -17dB max.

    First transmit test gave same results as before with my Helix solution mounted above the Pollin LNB (out of focus). The receive part with Pollin LNB gave exact the same signal level as the standard feedhorn mounted before.


    The biggest disadvantage is the fact that the uplink signal totally interfeering with the X-band downlink. The isolation between up- and downlink seems really bad in my case.


    But i know that alot of station do not have this kind of problem - maybe it's the Pollin LNB or someone have an idea why it is so extremly in compare to my Helix setup.


    Duplex is not possible anymore so i have to go back to the Helix solution which gave me less mechanical stability for portable use.


    I am intersting if someone can report the same behaviour or better have an idea how to solve this issue? Is someone doing the test POTY-POLLIN LNB ?


    Best Regards

    Kai DH0SK


    *** Aahh shit - just saw that i solder the patch on the wrong side :( hmm is this impacting the LHCP polarization ?

    Edited 3 times, last by DH0SK: Aahh shit just saw that i solder the patch on the wrong side :-( hmm is this impacting the LHCP polarization? ().

    Quote from PA3FYM

    Yes, that impacts LHCP pol, so mounted in front of a dish it has to be LHCP, so the dish reflects RHCP. See also HB9PZK's independent analysis of the POTY.

    Yes i know :) your right i did it in a hurry on the weekend without looking to the PDF. But i wondering my signalstrenght was the same as the left hand Helix. I expected a much lower signal. Its a nice experiment to check the polarisation. I will change at the weekend.

    Maybe this will solve the problem with the blocking of the RX while transmitting..

    73 de DH0SK

    Solving the problem with duplex, I don't think so. Others use this antenna e.g. with ATV on the WB tpx in duplex mode with success.


    Anyway, I am curious what your perceived performance is when the 'Erreger' makes LHCP (into a dish).


    Simulations and practice show that the antenna makes CP with a good axial ratio,

    Quote from PA3FYM

    Solving the problem with duplex, I don't think so. Others use this antenna e.g. with ATV on the WB tpx in duplex mode with success.


    Anyway, I am curious what your perceived performance is when the 'Erreger' makes LHCP (into a dish).


    Simulations and practice show that the antenna makes CP with a good axial ratio,

    I will let you know at the weekend - if it has a good crosspol xpd i would assume i will be to strong...😉

    But when i understood correct it depends a little bit of the tuning of both resonances to get the correct phase shifting? Maybe i was more linear than CP. I expect at least 10-12dB practical xpd for that antenna design and focal lenghts.


    Lets use this mistake as a good practical justification for your simulation.


    73 de Kai

    Kai,
    if you reduce the distance between reflector and patch, you will have less circular polarisation. So in your case, it doesn't matter, if you have mounted the patch the wrong way. With almost 20dB match, it is linear.

    I am using a Spectrian PA with approx. 70 watts and don't have any influence to the X-band. So the problem belongs to the Pollin-LNB itself. With either a Rocket-LNB, or a modified Octagon OQSLG, all works well.

    vy73,
    Wilm

    It is only possible to get interference if there is a strong harmonic content - 2.4 GHz signal will not propagate down the waveguide. The 3mm spacing is critical, going to 1.5mm does not work so something is wrong with the patch. You should see 16-17 dB return loss.



    Edit - I just looked at the photo. The patch is built incorrectly. The feed is in the wrong place.

    Quote from G0MJW

    It is only possible to get interference if there is a strong harmonic content - 2.4 GHz signal will not propagate down the waveguide. The 3mm spacing is critical, going to 1.5mm does not work so something is wrong with the patch. You should see 16-17 dB return loss.



    Edit - I just looked at the photo. The patch is built incorrectly. The feed is in the wrong place.

    Yes, this is what i expected from the WG. I havn't had the blocking problem with the Helical antenna used before.

    Maybe the hormonic rejection is better with the Helical antenna.

    At the weekend I will change the Patch and add a S-band bandpath to avoid harmonics. Maybe the problem is solved with that effect.

    I will report.


    Kai

    Measurements of one POTY I have made. SWR is about 1,16 at 2,4GHz. The second picture is the same antenna with some dielectric material inserted between patch and reflector. It gave the interesting result.


    I tried to keep the dimensions in a few tenths of mm. But I still had to bend some parts to tune it :)


    The tube is made of copper, the other parts are of 1mm brass. I didn't have thicker material.




    ok1phu Fully agree with G0MJW (also concerning polishing the antenna ; -)


    Serious now: plots look good. Issue is that nowadays people have (relatively cheap) VNA's and are able to see details that only 'super' professionals were able to see 10 - 20 years ago.


    In the uWave HAM-scene there was a golden rule for decades: "When you've 10 dB RL it's enough"


    Now we're able to measure RL with 0.1 dB resolution and 1 Hz precision.


    IDEALLY the 'bult' between the dips has to coincide with 2400 MHz but in practice this is hard to accomplish. When you've a RL in the order of 15 - 20 dB @2400 MHz it is ok.


    Yes, it is not the theoretical optimum.

    Yes, you might loose axial ratio performance of a few cB's (centibels)

    Yes, you may not sleep with this 'loss'

    Yes, you feel uncomfortable


    but ..


    Yes, people who just do it [ (c) Nike ], get on the air, have fab signals, make a lot of QSO's, have a lot of fun and .... have nice dreams afterwards ; -)