20m BPF Elsie Design vs homebrew - need help?

Discussion in 'General Technical Questions and Answers' started by S21RC, Jun 12, 2021.

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  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    You should also take a close look at the permitted flux density in the cores.

    100 W into a parallel circuit using T80-6 cores in the inductor will make the peak flux to exceed the advisable level with a factor 2 about.

    This heats the cores and may also create distorsion.

    Cores in the T1o6 size have more margin.

    S21RC likes this.
  2. S21RC

    S21RC Ham Member QRZ Page

    Thanks a lot Kerry for simulating the circuit.
    While I am waiting for the Silver-Mica capacitor to arrive from China, I found some mica capacitors (CDM CM05ED/ +/- 5%) in my junk on the LPF board of PRC1099 HF manpack.

    I have now adjusted the Elsie parameters (adjusted the capacitor values (520pf (510) - 39pf(36) - 520pf(510) ) as per my real capacitor, set L Q = 500 and C Q=200). I am seeing the BPF in nanoVNA now showing some improvement over the previous build, 1db loss compared to 1.5db. Still far away from the simulated 0.3db loss. The VNA is calibrated with the blue patch cable. The only additional mechanical connection is the two SMA female on the board.

    I am wondering the following:
    1. Can the SMA female on the board cause the 0.7db loss?
    2. There is a sharp null at 24Mhz which was not in the elsie simulation!!

    So, is there any improvement I can do?

    Here is the plot of both Elsie and VNA (click to enlarge pls):
    BPF-Mica-Elsie.JPG BPF-Mica-VNA.JPG
    image0 (5).jpeg
    Last edited: Jun 18, 2021
  3. S21RC

    S21RC Ham Member QRZ Page

    The -3db BW for the whole BPF comes as 4.62MHz in the vna reading.
  4. S21RC

    S21RC Ham Member QRZ Page

    Thanks Karl for advise. So far for the fun of homebrewing two BPF the following I have already procured :( 60$ worth of silver mica on its way. seems getting new knowledge is always expensive.

    Positive side is the long wishing toys (VNA and LC meter) I have now for playing around during the lockdown.

    I have followed a 1988 design from QST which suggested for 100w T50-6/T80-6, also instead of Mica other ceramic capacitor - surely it was long ago and not all data was available that time.

    Please advise, If I stack 2 units T80-6 cores for each inductor for the 100w ssb voice operation, can it handle 100w?

    Thanks in advance, 73 S21RC bpf-parts.jpg
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    In the original article, feeling the cores for heat was suggested for ensuring they are not
    overloaded. I, for one, prefer to calculate the flux before choosing a core dimension.

    You can of course send 100 W through the filter for some time and then touch the cores.

    By using two cores, you are doubling the core area, which in turn approximately halves the flux density, and you should then be more on the safe side.

    The deep null at 21 MHz is caused from a spurious resonance somewhere, most likely a parallel resonance in the series resonant LC circuit.

    And, two SMA connectors do not create 0.7 dB loss on HF.

    Last edited: Jun 18, 2021
    S21RC likes this.
  6. K8DO

    K8DO Ham Member QRZ Page

    While theoretical mathematics may be fun I might suggest returning to basics. High Q caps (as discussed) and high Q inductors. Remove the garbage wrapped around a piece of ceramic (not conductive) and wind tightly to improve the coupling to the core. Might need to reduce the wire gauge to keep the wire against the core.
    S21RC likes this.
  7. SWL37632

    SWL37632 QRZ Member

    The diameter of the wire on the toroids is too small. The leads of the capacitors are also problematic.....for less loss I have learned that using classic mica trimmers that have large surface areas for the leads as well as the active capacitance area will not only increase Q but will decrease insertion loss. As Karl-Arne has already pointed out, the Elsie program that you have modeled does not include series resistance and parallel capacitance of the individual components. Another factor to consider is the thickness of the PCB copper.
    S21RC likes this.
  8. S21RC

    S21RC Ham Member QRZ Page

    Thank you. Could you please clarify "wrapped around a piece of ceramic (not conductive)" I didn't understand. Do you mean some kind of ceramic material (I am not familiar with) wrapped around the toroid core? 73 S21rc
  9. K8DO

    K8DO Ham Member QRZ Page

    The toroid is not conductive. Test it yourself with an ohmmeter. It has been fired in a kiln. For all practical purposes it is piece of ceramic.
    Any insulator you wrap it with is adding a dielectric between the core and the RF winding. All dielectrics change the velocity factor and increase losses in an RF field.
    This is why a dipole (for example) using bare wire has to be longer than if made with the same gauge wire with insulation on it.
    On the comment above about increasing the wire gauge I don't have an opinion other than to note that the thicker the wire the more difficult it is to get it to lay against the toroid core.
    Good luck with your project.
    S21RC likes this.
  10. S21RC

    S21RC Ham Member QRZ Page

    Ohh I get it, you mean to wrap the wire around the core tightly without any tape/garbage. I tried to wrap some teflon tape as somewhere I saw it helps to reduce stray capacitance, but with repetitive adjustment the tape was damaged. The wire is a #21 SWG. I will buy some thinner guage and try again to see if it improves. Thanks again. 73 s21rc

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