Posts by OH2NLT



    Another old example of 1300MHz +/-45deg cross dipole. Here we have same principle as in the POTY patch plate. One leg is longer and other shorter than required resonance length. One leg is inductive (+j) and other capacitive (-j). In optimal case these are +/-45 degrees phase at the design frequency. In this example not exactly there but antenna is working as expected and SWR plot also shows clear double peak.



    Again sorry I do not know why pictures are not full size. Click the pictures to see them.



    Unfortunately DJ7GP patch does show only one resonance. Transmission tests supports this finding. If you listen the antenna with dipole or quad and rotate it very clear minimum and maximum are present at the transmission attenuation. In some other mail I have seen note: "First generation DJ7GP antennas are linear polarized". Are there already other designs available?



    POTY tuned with some dielectric material under "long resonator". You can sind two points where j=0 (resonance points). Transmission tests shows quite little variation in the transmission attenuation when RX antenna is totated. This is a sign of circular polarization.



    At 2.4GHz measurement setup is quite critical. A lot depends where you set (or can set) your calibration reference plane. This picture demonstrates what happens if for example one SMA I connector is inserted. This means about 160ps delay = some tens of degrees rotation at the Smith chart. Same tuning as in the previous picture looks very different.

    Yesterday I made some not so scientific measurements. I compared Bamatech DJ7GP and PE1CKK POTY 2.4GHz patch antennas. Already known that DJ7GP design is not circular polarized but POTY should be.

    I measured SWR for both antennas and transmission over short (less than 1m) distance. RX antenna was 2.4GHz quad.




    DJ7GP patch and RX quad.



    DJ7GP H / V polarization difference is more than 10dB in room environment with all of the room reflections. Anyway you can see clear H / V difference when rotatin RX antenna.



    One clear resonance. No sign of double peak. Same also in the Smith chart (no picture).





    Just assembled PE1CKK POTY in the test bench.




    Transmission loss vary only dB or two at 2400.175 test frequency. When RX antenna is rotated transmission under or above the test frequency is warying even up to 20dB but transmission at the test frequency stays almost constant. What I understand this is a good sign of circular polarization.



    However there are no clear sign of the double resonance around the test frequency. I did not find good explanation for this.



    I even did new calibration for the 8753C with no cable and conected the antenna directly to the test port. No noticeable change. Do anyone have good explanations for this. Also I am interesting to hear / see results if someone have done H / V transmission tests for this type antennas. (click picture to see full picture. For unknown reason full picture is not shown here)

    Some more measurements with generator and power meter. Bellow pdf graph of gain and linearity of the amplifier. Measurement setup (cables and connectors) attenuation at 2.4GHz is about 1.4dB what should be added to the results. 28dB gain is not bad at all. Only problem is the limited thermal conductivity of 1mm FR4 board. No matter how many vias you try to put under the very small chip. 0.3...0.5W is the max continous power with this design. Output start to drop when the chip gets hot. Design is anyway good enough for SSB use.


    OH2NLT_RFPA5218_linearity.pdf


    Measurement setup. hp 20dB attenuatoe and Anritsu MA2421A/ML2437A. Generator hpE4422B


    Full 1W reached but needs little more than 0dBm drive. 3-1.4 = about 1.6dBm

    I am building QO-100 station around Adalm-Pluto. I did not find exctly suitable exciter amplifier between Pluto and the TX PA. I have limited skills and experience in uwave design so I thought that this may me a good learning project for me.

    I set following spec for myself:

    - can be narrowband 2400MHz

    - should have enough gain and power to get Pluto 0dBM up to +30dBM(1W) level

    - spurious filtering

    - simple design

    - 12V powered


    Afrer searching what is available and what can be purchased somewhwre I ended up tu try with Qorvo RFPA5218 chip.


    Schematic OH2NLT_2400MHz_1W_Amp.pdf



    Only challenge with this chip is that it is 3x3mm leadless package. Thermal design needs also some attention.



    Some measurements with hp8753C



    Results looks better what I expected from my first design. PCB is ordinary 1mm FR4 material.