QO-100 Station Build Journal

  • Continued with my science project. (I love science projects!)

    Today I attempted to compare the performance of the two LNBs that I have.

    I don't have suitable dish, and I certainly don't have "visibility" to the QO-100, so my testing is done just using test equipment in my garage.

    I set up each LNB one at a time on a tripod (no dish) pointing directly to a test antenna that was connected to a 10GHz signal generator.

    The exact test frequency was 10.489GHz.

    Then I used the spectrum analyzer to measure the phase noise, and a crude S/N of the down converted 739MHz signal at the LNB output.

    One of my LNBs is using the Rafael RT320M chip, and second one is using NXP TFF1014.

    The short summary is:

    1)My TFF1014 LNB provides about 2dB better SNR in this setup.

    2)The TFF1014 LNB has significantly more gain.

    (I almost wonder if there's something wrong with the RT320M LNB, but since the S/N is OK, I do think the LNB is undamaged)

    3) The RT320M based LNB has slightly better phase noise.

    First plot, RT320M S/N.
    In the plot you can see the noise floor of the spectrum analyzer, the pass-band of my 740MHz SAW filter, and the in-band noise floor of the LNB. S/N = 40.5dB (specific to these spectrum analyzer settings_)

    Next plot, same setup, same settings, TFF1014.

    S/N = 43.3dB

    Then the phase noise plots.

    The green "reference" in both PN plots is the PN of the signal generator measured directly at 10.489GHz.



    Happy New Year, from AB6RF

  • Today I modified my "NXP TFF1014" LNB to use an external 25MHz reference.

    I removed the internal crystal, and connected a small 1:1 transformer to the reference pins of the PLL chip.

    The "upper" PLL chip in this picture is now disabled, and the "lower" one is the active, and the crystal in the middle is replaced by a transformer.

    Looks like the modification was successful.

    1) With the internal crystal the frequency error was about 177kHz, with the external OCXO the frequency error is about 20kHz.

    (temperature in my garage is currently about 15°C)

    Also, with the OCXO there's no detectable frequency drift after about 10 minutes of initial warm-up time.

    2) With the external OCXO, also the phase noise is slightly better than with the internal crystal.

    PN with the internal crystal, measured at the 739MHz IF output.

    PN with the external OCXO

    Again, the green line is the PN of the 10.489GHz signal generator, measured directly with a straight connection from the generator into the analyzer.

    The good phase noise of the OCXO is one of the reasons why I didn't want to use a PLL based reference oscillator.

    If locked to the GPS, they can have a better long-term frequency stability, but often the phase noise is much worse.

    On a related note, I noticed that I had made a mistake when writing down the part numbers in this LNB.

    I thought the NXP chip is TFF1014. It's not, it's actually a TFF1013.

    Looking at the TFF1013 and TFF1014 data sheets, the newer TFF1014 has 2dB lower noise figure.

    But it doesn't really matter much because there are two LNA stages in front of the PLL chip.

    Doing some quick math, with TFF1013 (NF = 9dB) the cascade NF of the LNB is 1.4dB.

    With the TFF1014 (NF = 7dB) the cascade NF would be only marginally better at 1.3dB.


  • I finally had an opportunity test my POTY and Helix antennas in an antenna test chamber, here are some of the results, first the POTY results.

    I tuned this POTY just using the S11, no further tuning was done to optimize the radiation pattern.

    For a reference, here's my final POTY S11.

    Here's a picture of the POTY in the antenna chamber, and the coordinates for the plots.

    Efficiency = -0.5 dB

    Directivity = 10.3 dB

    Gain = 9.8 dBi

    -3 dB Beamwidth (E plane) = 56°

    -3 dB Beamwidth (H plane) = 64°

    Here are two 2D plots of the radiation pattern, one is in the X-plane and other is in Y-plane.

    (Total power, sum of "vertical" and "horizontal" polarizations)

    And here's an animation of the 3D pattern.

    POTY 3D Animation

    Another very interesting thing is to take a look at the two linear polarization fields created by the POTY.

    The theory is that it should create two fields that are equal in magnitude, but 90° out of phase.

    Here's a polarization plot of the main beam. Each plot is one polarization.

    (Theta and Phi, in ham radio we would call them Vertical and Horizontal)

    Very equal, and almost exactly 90° out of phase!

    In other words, on the bore axis of the main beam, the axial ratio is very low.

    But further off-axis, the pattern gets more messy, and the axial ratio gets larger.

    Integrated over the (almost)entire pattern, the average axial ratio is about 3 dB.


    I should explain the coordinate system.

    Phi angle is in the X-Y plane, with 0° being towards the positive X-axis. Similar to what’s usually called the Azimuth.

    Theta is in Z-Y plane, with 0° towards the positive Z axis (= straight up), could be called the Elevation.


  • Yes, run out of time yesterday, but here are the Helix numbers.

    About 4.2 turns, 42mm diameter, 2mm copper wire. S11 @ 2400MHz = -30dB.

    Efficiency = -0.5 dB (= 89%)

    Directivity = 11.2 dB

    Gain = 10.7 dBi

    -3 dB Beamwidth (E plane) = 47°

    -3 dB Beamwidth (H plane) = 50°

    2D radiation pattern in X- and Y-plane.


  • Hello Robert,

    The test chamber software does not report the -10dB beamwidth, but I can calculate it from the raw data.

    Give me few days, I need to get access again to the test chamber, I saved the raw data but it's in binary format, I need export it into some other format for analysis.

  • I analyzed the raw antenna test data in Excel. The Theta angle measurement was done every 10°, so for a -10dB beamwidth I had to interpolate between the measurement points. So these are not absolute accurate.

    POTY -10 dB beamwidth = 110°

    4-turn Helix -10 dB beamwidth = 86°

    (if you include the impedance matching vane, it's closer to a 4.5 turns total)

    This is calculated for the total power. (Sum of each linear polarization)

    And also the calculated axial ratio integrated over the entire pattern.

    POTY axial ratio 3 dB

    Helix axial ratio 1.4 dB

  • I'm not an antenna expert either, just an amateur :-)

    Ideally, yes a perfect circular polarization has an axial ratio of zero.
    Anything greater than zero, it's an elliptical polarization.

    In some ways, pure linear polarization and perfect circular polarization are special cases of the elliptical polarization.
    (axial ratio becomes either infinity, or zero)

    But just like with my "obsession" with the phase noise, it's very interesting to measure and try to optimize, but in reality it doesn't matter that much. It just needs to be "good enough". We're not doing EME here.

    Let's look at the magnitude of the polarization loss vs. axial ratio.

    This table shows the power loss (second line from the left), as a function of the TX axial ration (left), and RX antenna axial ration (third from the left).

    I added three examples in there.

    Green line: 3 dB TX antenna axial ratio (my POTY), 1 dB RX antenna axial ratio -> loss = 0.2 dB

    Purple line: 3 dB TX antenna axial ratio (my POTY), 0 dB RX antenna axial ratio -> loss = 0.1 dB

    Red line: 20 dB TX antenna axial ratio (= almost perfect linear polarization), 0 dB RX antenna axial ratio -> loss = 2.2 dB

    So a 3 dB axial ratio TX antenna causes a negligible loss (would be a concern for a EME station, but not here).

    But yes, I also agree, based on the S11 I have with this POTY, the "double dip" is there, and the hump is exactly at 2.4GHz, I was expecting, or guessing, that the resulting polarization would be more ideal.

    Last thing I want to point out, the axial ration is better in the bore axis, and becomes higher when you go off-axis. The 3 dB result is integral over the (almost)entire radiation pattern.


  • To visualize the radiation patter, and specifically the -10 dB beamwidth, I have two more plots to share.
    Here again the Theta angle is from the Z-axiz (zero degrees is straight up), and Phi angle is from the X-axis.

    Each plot shows four "slices", at 0°, 45°, 90° and 135° Phi.


    4-turn Helix


  • Hi Mikko,

    why you don`t tune S11 to one peek, only? Here in the forum are different opinions. Is it possible that the circularity will be lost?

    Could you be so kind to measure your poty tuned to one dip, only? This could explain the words by Mike Willis,G0MJW, that one dip will not produce a good circularity. I don´t belief it.

    Which dish are you going to use?