Matching with lumped constants

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N3DT, Jan 18, 2021.

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  1. N3DT

    N3DT Ham Member QRZ Page

    OK, my 80M antenna shows 50+j93 at 5.357. Can't I make a simple L network with an inductor and capacitor to match that to my TS2000? Since we're limited to 50W at the antenna on that band, it shouldn't have to be large stuff. The internal tuner will not match it. I'm willing to accept the SWR loss to the antenna. Easier than throwing up a 60M dipole. It makes my head hurt looking at the Smith Chart instructions. I even have to go look in the trig book anymore to figure out trig functions. I managed to make a stub match with RG6, but it's a mess of cables, if I could just use lumped constants it would be easier, it's only one frequency too.
  2. WB3BEL

    WB3BEL Ham Member QRZ Page

    You just need series capacitor of - j93 to bring the match to 50 Ohms
    Xc = 1/(2*PI*f*C) or
    C = 1/(2*PI*f*Xc)
    Capacitor is about 319pF
    K2CAJ likes this.
  3. N3DT

    N3DT Ham Member QRZ Page

    Series being in line with the center conductor, not across it? That should be easy enough. Why am I fighting with all this coax? And the voltage rating of the cap? Given that the SWR on the 50+j93 is about 4:1 and 100W @ 50Ω is 200VP-P does that multiply the voltage rating by 4:1 or what? I figure I've got about an additional 4dB loss to the antenna at 4:1 SWR. Then there's the mismatch at the antenna.
    Last edited: Jan 18, 2021
  4. WB3BEL

    WB3BEL Ham Member QRZ Page

    Yes in series means in line with the center conductor.
    The voltage across the capacitor is about 2X the source voltage.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Interesting, I just found a bug in this on-line calculator. Bug goes away if you make the Real load 50.001 or 49.999 instead of just the integer 50.

    For 100W applied, I make the capacitor voltage 127Vrms @ 1.4Arms.
  6. N3DT

    N3DT Ham Member QRZ Page
    That's what I was looking for but could never find. Thanks, but where do I find a 320pf 400V cap? I have a bunch of silver micas I might be able to make one up.

    I was looking at that 50+j93 and wondering if I couldn't just get rid of the +j93 with -j93, but a regular trans line stub introduces other issues.

    I've also got a nice wide spaced 22-150pf adjustable.

    My 40M antenna shows 76-j120 would it be easier to come up with a +j120 inductor? 1.5:1 would be good enough for me.
    Actually using the calculator, 76-j120 requires a 3.08uH inductor in series with a 55pf in parallel. That might be easier to do, another on line calculator shows a 3.08uH coil to be 10 turns #14 close wound on a 1.12" form, ie. a piece of 1" pvc. I have all that stuff including the enameled #14.
    Last edited: Jan 18, 2021
  7. WB3BEL

    WB3BEL Ham Member QRZ Page

    A 330 pF is within a few ohms reactance. So a 500V or higher part would give some margin and a good match.
    Easy to short around it with a relay or switch when you want to use it on 80m.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    If your existing 80m feedline is Belden 8237 (solid dielectric RG-8), then if you install a coax tee at 29.0ft from the antenna feedpoint, and connect a 9.7ft long shorted stub (same coax), then that will cancel +j93 Ohms. To go back to 80m, just unscrew the stub. You could devise a remote relay to automate that...

    I will recompute the stub distance and stub length if you tell me what actual coax you are using.
    Last edited: Jan 18, 2021
  9. RW4HFN

    RW4HFN Ham Member QRZ Page


    Attached Files:

  10. N3DT

    N3DT Ham Member QRZ Page

    I already made up a stub match but with RG6. It had similar numbers, but the whole arrangement was a mess that I'm trying to get away from. It did work however. I'll see if I can make up a 320pf cap in a box. I use BNC RG58 jumpers from the LP100A to a relay (80/40) and then to a BNC panel for the antennas. The feedline to the 80M antenna is mostly 9086 so switching things around would be easy enough with a box with BNC connectors on it, be it the capacitor alone for the 80M or the choke and variable cap in parallel for the 40M. I'll play around with it and see what I can come up with.

    I probably should just go throw up a 60M antenna, I've got it cut and I added it as another coupled resonator to my 80/20/17 coupled resonator antenna, but it screwed up the 20 and 17 bands for some reason so I took it off. I've got several places I could pull the center up 60-70' but I need rope for the ends. It's just so cold.

    I actually have been using a length of RG6 to transform the 40M and it brings it down to 2.5:1 or so, usable, but not great. I've made plenty of contacts with that arrangement.

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